Evolution of Fungal Pathogens in Domestic Environments?

f u n g a l b i o l o g y 1 1 5 ( 2 0 1 1 ) 1 0 0 8 e1 0 1 8

journal homepage: www.elsevier.com/locate/funbio

Evolution of Fungal Pathogens in Domestic Environments?
a,*, Martin GRUBEb, Nina GUNDE-CIMERMANa,c Cene GOSTINCAR a

Centre of Excellence for Integrated Approaches in Chemistry and Biology of Proteins (CIPKeBiP), Jamova 39, SI-1000 Ljubljana, Slovenia Institute of Plant Sciences, Holteigasse 6, 8010 Graz, Austria c University of Ljubljana, Biotechnical Faculty, Department of Biology, Ve
cna pot 111, SI-1000 Ljubljana, Slovenia b

article info

abstract

Article history:

Specific indoor environments select for certain stress-tolerant fungi and can drive their

Received 20 October 2010

evolution towards acquiring medically important traits. Here we review the current knowl-

Received in revised form

edge in this area of research, focussing on the so-called black yeasts. Many of these melan-

7 March 2011

ised stress-tolerant organisms originate in unusual ecological niches in nature, and they

Accepted 8 March 2011

have a number of preadaptations that make them particularly suited for growth on hu-

Available online 15 March 2011

man-made surfaces and substrates. Several pathogenic species have been isolated recently

Corresponding Editor: Sybren de Hoog

from various domestic habitats. We argue that in addition to enriching for e potentially e pathogenic species, the selection pressure and stress acting on microorganisms in indoor

Keywords:

environments are driving their evolution towards acquiring the missing virulence factors

Black fungi

and further enhancing their stress tolerance and pathogenic potential. Some of the polyex-

Domestic environment

tremotolerant fungi are particularly problematic: they can grow at elevated temperatures,

Evolution

and so they have a higher potential to colonise warm-blooded organisms. As several spe-

Pathogen

cies of black fungi are already implicated in health problems of various kinds, their selec-

Polyextremophilic

tion and possible evolution in human environments are of concern.

Polyextremotolerant

ª 2011 British Mycological Society. Published by Elsevier Ltd. All rights reserved.

Introduction The recent years have witnessed alarming news relating to microbial growth in indoor environments. Topics such as sick-building syndrome, indoor allergens caused by microbes, and emerging pathogens have contributed to a wide-spread perception of microbes as a threat to human health. When coupled with the extensive marketing of antimicrobial products, this has created a desire to make our homes ‘safe’ and ‘germ-free’. Good hygiene practices are clearly important for public health, but in most cases, no clear benefits have been demonstrated for the abundant (and still rising) use of antimicrobials (Levy 2001). On the contrary, we argue that under certain conditions their application can even select for undesired species and traits. Anthropogenic changes in the environment

can influence our interactions with the bacteria in a unpredictable and possibly dangerous ways (Baquero 2009). This probably holds true for microfungi as well. Human pathogens have to overcome several different defence mechanisms of the host. A known fungal pathogen Cryptococcus neoformans, for example, copes efficiently with several stresses during infection: pH fluctuations, anoxia and nutrient deprivation, and reactive oxidative, nitrosative and chlorinating species. However, it is considered unlikely that its stress response pathways were developed specifically for survival in a mammalian host; they are more likely to be the result of the stress that the fungus encounters in its primary ecological niche (Brown et al. 2007). Certain generalist species, such as the black yeasts, are particularly good at adapting to a variety of different stressful

* Corresponding author. Tel.: þ386 1 3203385; fax: þ386 1 2573390. E-mail addresses: [email protected], [email protected], [email protected], nina.gunde-cimerman@ bf.uni-lj.si 1878-6146/$ e see front matter ª 2011 British Mycological Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.funbio.2011.03.004

Evolution of Fungal Pathogens in Domestic Environments?

environments. In addition to their often wide-spread distribution in nature, they can also successfully colonise various domestic stressful environments. Some of them are already regarded as emerging pathogens, although the factors behind their increasing medical importance are still largely unknown. We believe that the conditions they encounter in domestic environments can select for traits that serve as preadaptations for the evolution of their pathogenicity. We discuss here the possibility and mechanisms of such a scenario.

Humanemicrobe interactions in anthropogenic environments Indoor environments offer numerous different habitats that can be occupied by microorganisms. Although so far mostly limited to bacteria, published studies have shown that kitchens and bathrooms are the most contaminated parts of the house (Beumer & Kusumaningrum 2003; Ojima et al. 2002). The occurrence of pathogenic species is not uncommon in these environments. For example, bathrooms can be a reservoir of mycobacteria (Feazel et al. 2009; Nishiuchi et al. 2009). Non-tuberculous mycobacteria, Legionella spp., and Pseudomonas aeruginosa can be isolated from domestic tap water (von Baum et al. 2010), and even methicillin-resistant Staphylococcus aureus can be found on inanimate domestic surfaces (Scott et al. 2008). With respect to fungi, however, research has mainly focused on the indoor air or on contaminated food. A large share of the oligotrophic black fungi encountered in humid indoor environments has significant potential to cause human infection (Lian & de Hoog 2010). The most widely known case is of dermatophytic fungi in swimming pools (Hilmarsdottir et al. 2005). Pathogens can gain access by skin maceration during showering, as has been suggested for infections by oligotrophic thermophilic black fungi in domestic bathrooms (Lian & de Hoog 2010). Additionally, fungi from indoor environments are involved in sick-building syndrome (Straus 2009; Thrasher & Crawley 2009). Most building materials support fungal growth at sufficient humidity (Gorny et al. 2001; Shirakawa et al. 2003). However, some so far largely neglected findings are particularly problematic: those of pathogenic species on ordinary household objects and in water. Examinations of bathwater and sludge in drainpipes that are warmed daily to over 42  C have identified several species of the medically important genus Exophiala (Nishimura et al. 1987). Several species from this genus are known to cause various diseases, which include fatal systemic and brain infections. The most prevalent among these is Exophiala dermatitidis (Li et al. in press; Zeng et al. 2007), which has also been isolated in large numbers from steam baths (Matos et al. 2002), sink drains (Hamada & Abe 2010), and drinking water (Hageskal et al. 2006). Biofilm-forming Exophiala mesophila has been isolated from chlorine-dioxide-treated dental unit waterlines. It was suggested that treatment with continuous-use chemicals can select for the growth of fungi in biofilms (Porteous et al. 2003). A new and unexpected indoor habitat of E. dermatitidis and Exophiala phaeomuriformis is reported by Zalar et al. (2011). These two species have consistently been found to form a stable community in dishwashers, together with some other human pathogens belonging to the

1009

genera Aspergillus, Candida, Dipodascus, Fusarium, Penicillium, Pichia and Rhodotorula. The possibility of a microorganism developing a relationship with an organism will increase along with the frequency of their encounters. All potential introductions between novel species and increasing contact and interactions will increase the probability that new microorganisms will develop into human pathogens (van Baarlen et al. 2007). Growth of fungi in indoor environments, together with the increasing confinement of humans in these environments, obviously increases the number of contacts. There may not be any apparent novelty in that: humans and microbes have always co-existed. However, through dramatically changed conditions in indoor environments due to fast economic growth and increased housing standards during the last decades, we are increasingly surrounding ourselves with species selected in new ways. Baquero (2009) recently summarised 12 major anthropogenic factors that change the environment and that might be important for interactions between humans and pathogenic bacteria. At least five of these might also be important for interactions between humans and microfungi in indoor environments: (i) Sanitation and hygienic measures reduce exposure of humans to microorganisms, but also increase access of selected species that can survive these measures (such as Exophiala spp. and Cladosporium spp. in bathrooms). (ii) Chemical pollution alters microbial biodiversity. Apart from pharmaceuticals and cleaning products that create stressful environments (possibly shaping the evolution of indoor microorganisms), indoor environments contain an abundance of unusual substrates that can select for certain species, such as biofilm-forming fungi growing on silicones (Wallstrom & Karlsson 2004), or black fungi that degrade volatile organic compounds (Lian & de Hoog 2010 1584). (iii) Demographic changes, including increased mobility, can change and increase host contact with different microorganisms. (iv) Global environmental changes lead to changes in geographical localisation of pathogens. It has been proposed that global warming might increase the impact of fungal pathogens due to increased environmental temperatures (Robert & Casadevall 2009). The presence of only decades old high-temperature habitats in our homes, such as dishwashers, will also select for thermotolerance, a trait that is crucial for human pathogenesis that most fungi lack. (v) Increases in the number of hypersusceptible hosts for infections. Immunocompromised individuals are more likely to be restricted to indoor environments, which further increases the number of encounters with potentially pathogenic species present there.

Selection pressures indoors Habitats in indoor environments are diverse and are characterised by different selection pressures. A metagenomic study of indoor air organisms from two sites located 6.7 km apart from each other suggested that indoor air microbial communities share organisms and genetic features. It appeared that the major stresses encountered by air microbiota include iron limitation, oxidative damage and desiccation, all of which can reasonably be expected in indoor air. This study implied that the conditions of the indoor air enrich for certain

1010

organisms, and it showed that sequences suggestive of opportunistic pathogens as well as virulence-associated genes are common in the air DNA. It was suggested that resident microbes in the indoor atmosphere have been selected for an indoor life cycle, part of which is spent in the air (Tringe et al. 2008). Inanimate surfaces can offer numerous opportunities for microbial growth, especially in environments with the occasional presence of large quantities of liquid water and high humidity (such as bathrooms and kitchens). However, even the humid habitats in bathrooms and kitchens usually dry out for at least short periods of time, creating a strongly fluctuating environment that can also be characterised by low water activity. The colonisation abilities of microorganisms present in indoor environments are determined by the physical and chemical characteristics of the building materials, and especially by the nutritional substances that can be derived from these building materials, together with the moisture content in the substrate (Gorny 2004). Certain fungal species are so common that they can be regarded as indicator organisms for contamination at different water activities (Samson et al. 2004). Selection pressures vary substantially between different indoor habitats, although in most cases, they result in limited species diversity. For example, a recent study showed that the showerhead environment can be enriched in microbes that are known to form biofilms in water systems, possibly due to their mechanic resilience and the chlorine resistance of the enriched species (Feazel et al. 2009). Microbial growth in dishwashers, on the other hand, is determined by fluctuations in pH, temperature, salinity and humidity (Zalar et al. 2011). Apparently, these conditions select for traits connected to high virulence: although all Exophiala dermatitidis genotypes are thermotolerant, the majority of isolates in dishwashers belong to the most virulent genotype A (Zalar et al. 2011). Similar selection of a few species of eurytolerant generalists and a decline in normal and harmless skin bacteria numbers has been observed when using antimicrobial textiles (N.G-C., unpubl. data). The presence of disinfectants and other chemicals that are distributed by regular cleaning practices in our homes provides an additional stress. As remarked by Levy (2001), it is now possible to outfit whole bathrooms and bedrooms with products containing triclosan (a common antibacterial agent), including pillows, sheets, towels and slippers. No added benefit of such use in the households of healthy people has been demonstrated. Instead significant concerns have been raised, such as cross-resistance to antibiotics, and pollution of the environment (Fang et al. 2010; Levy 2001). Triclosan is an emerging contaminant of the environment, and it can be found in drinking water, surface water, wastewater, and environmental sediments, in the bile of wild fish, in human breast milk, and in urine in Europe and USA (reviewed in Fang et al. 2010). Household cleaners often also contain oxidative chemicals, such as bleach or hydrogen peroxide, thus adding to the secondary oxidative stress microbes encounter due to other stress factors. The presence of aromatic pollutants, detergents, biocides, and other household chemicals might expose the cells to water stress, which can be mediated by their chaotropicity (Hallsworth et al. 2003) or hydrophobicity (Bhaganna


ar et al. C. Gostinc

et al. 2010). Both types of compounds trigger similar cellular responses, which are also used to counteract the consequences of other types of water stress. Besides stressful physicochemical parameters, indoor environments are characterised by an abundance of unusual synthetic materials and chemicals, such as plastics and aromatic pollutants. Although these substances are not present in nature, they are prone to microbial growth. Degradation of synthetic polymers is tightly linked to the formation of biofilms on the surface of the infected material. For instance, it has been shown that E. dermatitidis can colonise the rubber seals in dishwashers (Zalar et al. 2011). Nutrients can be acquired from outside or by degradation of the polymer by biologically generated free radicals or enzymes (Wallstrom & Karlsson 2004). Aromatic pollutants can be metabolised by several extremotolerant fungi from the group of black yeasts. It has been suggested that there might be physiological connections between aromatic hydrocarbon assimilation and certain patterns of mammalian infection (PrenafetaBoldu et al. 2006). Neurochemistry features a distinctive array of phenolic and aliphatic compounds that are related to molecules involved in the metabolism of aromatic hydrocarbons. Many of these volatile-hydrocarbon-degrading strains are closely related to, or in some cases clearly conspecific with, human pathogenic fungal species that can cause severe mycoses in immunocompetent people, especially neurological infections. While they may be very different at first sight, indoor habitats are in many ways similar to those found in natural outdoor extreme environments. Coastal hypersaline environments, Arctic glacial ice, and bathroom surfaces are all characterised by periods of extremely low availability of liquid water. As a consequence, there is an overlap of microbial diversity between these habitats: the black fungi Cladosporium halotolerans and Aureobasidium pullulans, for example, are found in all of these habitats (Gunde-Cimerman et al. 2000; Zalar et al. 2008b; Zalar et al. 2007). In addition, A. pullulans occurs in the phyllosphere (Andrews et al. 2002), in polluted water (Vadkertiova & Slavikova 1995), in salt-preserved food (Nisiotou et al. 2010), in aviation fuel tanks (Rauch et al. 2006), on various indoor habitats (Summerbell et al. 1992), on synthetic polymers (Cappitelli & Sorlini 2008), and on degrading polyurethane and PVC plastics (Shah et al. 2008). Aureobasidium pullulans has also been reported to cause a variety of localised infections, as well as rare systemic infections (reviewed in Hawkes et al. 2005). Its increased indoor concentrations have also been correlated to various health symptoms (Su et al. 1992). Similarly, the stresstolerant yeasts Rhodotorula mucilaginosa, Debaryomyces hansenii and Pichia guilliermondii have all been isolated from glacial ice (Butinar et al. 2007), hypersaline water (Butinar et al. 2005), and salted meat products (Gardini et al. 2001; SaldanhaDaGama et al. 1997), while only R. mucilaginosa and P. guilliermondii have also been found in dishwashers (Zalar et al. 2011). Furthermore, R. mucilaginosa is found in poplars (Xin et al. 2009), copper contaminated wastewater sediment (Villegas et al. 2005), nitrobenzene-contaminated active sludge (Zheng et al. 2008), acidic water (Russo et al. 2008), and was isolated from dairy plant surfaces after disinfection (Bore & Langsrud 2005). All three of these species have been described in human infections, and at least R. mucilaginosa is considered

Evolution of Fungal Pathogens in Domestic Environments?

to be an emerging opportunistic pathogen (Desnos-Ollivier et al. 2008; Tuon & Costa 2008).

The co-evolution of stress tolerance and pathogenicity The detection of variations in microbial systems as a result of anthropogenic or natural changes is critical to both the detection and assessment of the risks, and to the management of the potential damage (Baquero 2009). Large population sizes and short generation times enable microbes to evolve over short time-scales. Changes in habitats provoke stress, and Baquero (2009) suggested that this would probably alter the local evolutionary time by changing microbial evolvability (the possibilities of microbes to evolve). This trait is successfully exploited by evolutionary engineering (Patnaik 2008), while it is also the cause of e from a human perspective e less desirable adaptations, such as the ability to invade a human body or resistance to antimicrobials. It has been shown that sub-inhibitory concentrations of commonly used biocides, such as benzalkonium chloride, and even stress, can lead to a significant decrease in sensitivity to biocides as well as antibiotics. This is achieved through several mechanisms (Braoudaki & Hilton 2004; Langsrud et al. 2004; Mc Cay et al. 2010; Russell 2002); for example, by changes in the expression or structure of cellular efflux pumps (Mc Cay et al. 2010), or by biofilm formation (Szomolay et al. 2005). In addition to tolerance to antimicrobials, biofilms can also confer tolerance to multiple stresses. Salmonella biofilms that are adapted to benzalkonium chloride show upregulation of enzymes involved in the coldshock response and the stress response, and in detoxification (Mangalappalli-Illathu & Korber 2006), clearly showing the importance of the biofilm life form in evolution (Hall-Stoodley et al. 2004). In a previous review, we proposed a hypothesis that can explain the evolution of extremotolerance in fungi
ar et al. 2010). We argued that generalistic species (Gostinc serve as a genetic reservoir of potential candidates for the evolution of extremophiles. Their remarkable phenotypic plasticity and robust genotypes allow them to persist across varied environments without obligate adaptation to local conditions. Fragmentation of permissive habitats and local extinctions at their ecological edge create small populations that are exposed to severe selection pressures due to extreme physicochemical parameters, founder effects and genetic drift, and possibly even to stress-induced increases in mutation rates. Fixation of beneficial plastic phenotypic traits and further adaptation of local populations can lead to adaptive radiation and subsequent specialisation for only one or for a few very similar extreme environments. An unexpectedly rich biodiversity is encountered in rather stressful environments
ar et al. 2010; Zalar et al. 2008a; Zalar et al. 2007), fur(Gostinc ther increasing the pool that is available for the evolution of extremotolerance. As an analogy, opportunistic pathogens thrive on a wide range of organic substrates, and they generally show low virulence towards a broad array of living hosts (although not immunocompetent hosts) (van Baarlen et al. 2007). Similar to the proposed evolution of extremophiles from generalists

1011


ar et al. 2010), it has been speculated that obligate (Gostinc and facultative pathogens (which can both infect immunocompetent individuals, with the first also able to survive outside the host) have evolved from microbial lineages that originally caused only opportunistic infections (van Baarlen et al. 2007; Scheffer 1991) (Fig 1). Adaptation to (extreme) indoor environments and human hosts can be facilitated by several factors, such as the high evolvability of certain groups of organisms (Draghi et al. 2010), horizontal gene transfer (which occurs more frequently under stressful conditions [reviewed by Baquero 2009] and is enhanced in biofilms [Sorensen et al. 2005]), high expression noise (Zhang et al. 2009), phenotypic plasticity (WestEberhard 2005), ecological fitting (Agosta & Klemens 2008), and even elevated mutation rates triggered by stress (Rosenberg & Hastings 2004). Increases in mutation rates have previously been proposed to fuel the evolution of microbial pathogenesis and antibiotic resistance, both of which occur under stress (Galhardo et al. 2007). Studies with Pseudomonas aeruginosa, for example, have shown very high prevalence of mutator strains isolated from patients with chronic lung infections, which are linked to high antibiotic resistance rates (Henrichfreise et al. 2007; Macia et al. 2006). Fluctuating conditions can contribute to the evolution of stress-induced mutagenesis mechanisms, which appear to be under shortterm environment-specific selection. They are probably selected for in changing environments, and lost in static ones, and thus repeatedly evolve (Galhardo et al. 2007). Periodic high temperatures, desiccation, and the presence of cleaning chemicals in dishwashers are good examples of such conditions. Dormancy is another beneficial trait in a fluctuating environment (Lennon & Jones 2011), and it has also been suggested to be an important factor in Cryptococcus neoformans infections (Moranova et al. 2009). Phenotypic traits that arise under one set of conditions can be useful adaptations under other sets of conditions. Mutations that are responsible for survival under a stress that would have been lethal to the ancestral lineage have probably appeared and spread during previous exposure to severe, but sub-lethal, stress (Samani & Bell 2010). The same principle might hold true when the origin of stress is different: reactive oxygen species (ROS) are part of plant and animal immune responses (Nappi & Ottaviani 2000), and are also generated during various abiotic stresses (Daly et al. 2010; Garre et al.
2006). Pathogenicity mechanisms can thus be 2010; Petrovic selected for in both biotic and abiotic environments (van Baarlen et al. 2007). Fungi associated with human diseases do not depend on the human host for survival and have been found to live in the most unexpected and extreme of natural habitats, such as the Antarctic desert soil (Bridge & Newsham 2009). Many of the virulence factors identified for human pathogenic fungi appear to allow both the establishment of the microbe in a mammalian host, and its survival in the environment, e.g. by preventing predation by amoeba, slime molds, and nematodes (reviewed in Casadevall 2007). Traits that enable pathogenesis, such as thermotolerance, need not have evolved primarily as traits selected during the development of pathogenicity, but can be considered to be preexisting adaptations to relatively extreme environmental conditions outside the natural host (van Burik & Magee 2001).

1012


ar et al. C. Gostinc

Fig 1 e Polyextremotolerant fungal species might represent a reservoir for evolution of extremophiles and pathogens. Schematic representation of the possible evolution from polyextremotolerant species towards extremophiles and pathogens (curved arrows). Different rows represent different conditions: three different stress factors (generalised as A, B and C), and two types of hosts (immunocompromised and immunocompetent). Columns represent organisms with different lifestyles (polyextremophilic, (mono)extremophilic, polyextremotolerant, opportunistic pathogen, pathogen, and obligate pathogen; as indicated). Thickness of the vertical lines represent their ability to cope with individual conditions. Polyextremotolerant species can resist the unfavourable physicochemical parameters such as elevated temperatures, low water content, oxidative stress, and others. All of these factors are encountered in many indoor environments and during infection. Due to their wide stress tolerance, these species can shift between a wide variety of environments, including extreme ones, and they can colonise and invade a human host. By further specialisation they can evolve into extremophiles (specialised for one or several stress factors) or pathogens (either facultative or obligate). Polyextremophily and obligate animal pathogenesis are rare in fungi.

When conditions become less favourable, evolutionary rescue is more likely when the stress factors change only in quantity, and not in quality (Samani & Bell 2010). As we will show, the correlations between certain indoor environments and the mammalian body are surprisingly large. The rate of deterioration of conditions must also be slow enough to allow the time for beneficial mutations to spread (Samani & Bell 2010). This is possible by succession from immunocompromised towards immunocompetent hosts. Several extreme conditions in natural environments might be important in the evolution of preadaptations for pathogenesis. It has been proposed previously that the human pathogenicity of fungi is associated with moderate osmotolerance at the order level (de Hoog et al. 2005). According to phylogenetic studies, the ancestors of Chaetothyriales black yeasts were rock-inhabiting fungi, and the traits that they evolved for life in this extreme environment are the basis of the numerous independent shifts to pathogenicity that have occurred in this group of fungi (Gueidan et al. 2008). These possibilities further increase the importance of frequent occurrence and enrichment of black fungi in human environments.

Traits that increase stress tolerance and virulence: the case of black fungi Stress tolerance is crucial for survival in many indoor environments, and also for infecting mammalian hosts (Brown et al. 2007). Several genes involved in stress responses are differentially expressed during fungal infection (Cairns et al. 2010). Not surprising, many of the stress-tolerance traits of black fungi

can also have a role in survival in the challenging environment of the human host, as follows: (a) Temperatures in a warm-blooded animal body are above those that are favoured by most fungi, but black fungi are known to cope with this efficiently, as they can readily tolerate temperature extremes (Sterflinger 1998; Zalar et al. 2011). It has been shown that for every 1  C gain in body temperature in the range of 30  Ce42  C, approximately 6 % of the fungal species are excluded as potential pathogens (Robert & Casadevall 2009). The ability to grow at 37  C appears to be the decisive factor for pathogenesis. However, it does not necessarily mean that this temperature is also encountered in the natural environment of the species. Several opportunistic pathogens, such as Trichophyton and Malassezia have been found in extremely cold polar and alpine environments (Branda et al. 2010; Bridge & Newsham 2009; Fell et al. 2006). (b) The skin limits colonisation by its low water content, acidic pH, resident (normal) microbiota, and antimicrobial lipids (Elias 2007). Growth at low pH is characteristic of fungi, but a shift to a slightly basic pH occurs when entering the host (Davis 2009), requiring rapid adaptation. Black fungi can tolerate both high (Nishimura et al. 1987; Shiomi € lker et al. et al. 2004; Zalar et al. 2011) and low pH values (Ho 2004; Zalar et al. 2011). They are also resistant to low water activities, caused by either desiccation (Gorbushina et al. 2008) or high salt concentrations (Gunde-Cimerman et al. 2000).

Evolution of Fungal Pathogens in Domestic Environments?

(c) The immune system kills fungi (among other things) by an oxidative burst or by cell lysis through defensin or the neutrophil cationic peptides that target and disrupt fungal cell membranes (van Baarlen et al. 2007; Hamad 2008). The exploitation of ROS as a weapon for fighting invading microbes represents a core and evolutionary ancient innate immune response among both animals and plants (Nappi & Ottaviani 2000). Oxidative stress is also a general secondary stress that has been shown to accompany various abiotic stressors like high salt concentra
2006) and dehydration (Daly et al. 2010; tions (Petrovic Garre et al. 2010), and at least in prokaryotes, also ionising radiation, UV radiation, and some xenobiotics (Daly et al. 2010). Mechanisms to deal with the consequences of oxidative stress, such as shielding proteins from ROS-induced damage (Daly et al. 2010), contribute to stress tolerance. Candida albicans upregulates oxidative stress response genes when co-cultured with neutrophils (Kumamoto 2008). Fungi can significantly adapt their membrane composition to many stress factors (reviewed
ar et al. 2009), possibly altering their susceptibilin Gostinc ity to membrane disruption, especially if antimicrobial compounds interact specifically with membrane sphingolipid targets, as is the case with at least some of the defensins (Thevissen et al. 2004). (d) Mediators released by phagocytes restrict fungal growth and minimise fungal infectivity through iron sequestration, inhibition of dimorphism, and resistance to phenotype switching (Hamad 2008). The ability to grow in oligotrophic environments can help cells to survive despite the limiting amounts of iron and other nutrients, for example by producing siderophores (Atkin et al. 1970; Cairns et al. 2010; Holzberg & Artis 1983; Wang et al. 2009). Not only the responses, but also the stress response pathways are interlinked: the mitogen-activated protein kinase pathways that are involved in responses to several abiotic stresses, also have central roles in oxidative stress responses in many fungal pathogens (reviewed in Brown et al. 2009). The activity of calcineurin, a highly conserved protein that is involved in fungal stress responses, strongly correlates with pathogenicity of C. albicans (Blankenship et al. 2003). Heterogeneity of the mammalian body represents an additional challenge. Niche-specific gene expression of C. albicans shows that microenvironments in different locations within a single host, and even within a single tissue, are different. To survive effectively within its host, a pathogen must be highly adaptable (Kumamoto 2008). As well as the general stress tolerance of black fungi, their great adaptability is likely to be a crucial factor in their ability to evolve into future pathogens. Mechanisms for dealing with specific stresses are extremely diverse. However, stress tolerance is also a result of some general traits, many of which can be found in a very successful group of black yeasts. Their characteristic black colour is the result of their thick and heavily melanised cell walls. As reviewed by van Baarlen et al. (2007), melanin shields against adverse conditions and environmental stresses of various kinds, and even against the activity of clinically used antifungal agents.

1013

Many black fungal species can grow meristematically and form microcolonies, which enhances their survival under numerous different abiotic conditions (Kogej et al. 2007; Selbmann et al. 2005). Recolonisation of old cells by new ones in microcolonies has been reported, resulting in multilayered cell walls and thus additional protection for the newly formed cells (Gorbushina et al. 2003). The production of melanin and meristematic growth can also confer invasive abilities (Feng et al. 2001; Schnitzler et al. 1999). Melanin is a scavenger of ROS that are produced during host defence reactions. Melanised cells are more resistant to lysis, and unlike their unmelanised equivalents, they can prevent phagocytosis. The roles of phenotypic plasticity and dimorphism have also been extensively studied in connection with virulence (reviewed in Slepecky & Starmer 2009). Black fungi can often shift between filamentous and yeast-like growth (Slepecky & Starmer 2009), a trait that is present in all true fungal pathogens. It was previously suggested that these factors might have primarily evolved in response to extreme environments (Haase et al. 1999; de Hoog 1993; Prenafeta-Boldu et al. 2006). This has been confirmed by comprehensive phylogenetic studies of black fungi (Gueidan et al. 2008, 2011; Ruibal et al. 2009), which place stress-tolerant rockinhabiting fungi at the base of the phylogenetic lineages of fungi with other lifestyles, including lichens and pathogens. Black fungi often produce protective extracellular polysaccharides (Selbmann et al. 2005) and many are oligotrophic (Gueidan et al. 2008; Lian & de Hoog 2010). The general stress-protecting extracellular polysaccharides (Selbmann et al. 2005) can be important virulence factors, as is the case of the Cryptococcus species (Zaragoza et al. 2009). Biofilms formed by the production of extracellular polysaccharides are multicellular communities that enable adherence to abiotic and living surfaces. They protect from adverse environmental conditions, including immune system responses, antimicrobial agents, and other environmental stressors (van Baarlen et al. 2007; Porteous et al. 2003; Szomolay et al. 2005), and they are thus involved in pathogenesis (d’Enfert 2009). They may also be important for survival of Exophiala species, and for their adherence to surfaces of dishwashers (Zalar et al. 2011). When nutrient conditions become limiting, cells released from the biofilm enter a free-living phase and can then spread to colonise new habitats (van Baarlen et al. 2007). All these traits enable individual black fungi species to tolerate a variety of stressful conditions (Fig 2), from extreme temperatures (Sterflinger 1998), desiccation (Gorbushina et al. 2008; Gueidan et al. 2008), and high salt concentrations (Gunde-Cimerman et al. 2000), to variations in pH, nutrient deficiency (Selbmann et al. 2005), and UV and ionising radiation (Dadachova et al. 2007). We suggest the term polyextremotolerant to describe the remarkable ability of these fungi (e.g. numerous species of black yeasts) to colonise such a variety of different environments and to endure a broad range of ecological conditions. In an extension of the suggested evolutionary path towards extrem
ar et al. (2010), most polyextremotolerant ophily of Gostinc fungi with a mesophilic optimum have great potential to become (mono)extremophiles, although some may become polyextremophiles, as suggested by Bowers et al. (2009) for

1014


ar et al. C. Gostinc

Fig 2 e Adaptations of black yeasts that are responsible for their polyextremotolerant character. Thick melanised walls, the ability to grow meristematically and also switch between yeast-like and filamentous forms, production of extracellular polysaccharides (EPS), and biofilms enable black yeasts to tolerate a wide variety of stressful conditions, from extreme temperatures, desiccation and high salt concentrations, to variations in pH, nutrient deficiency and UV and ionising radiation.

Archaea. We further suggest that this high evolutionary potential also includes the possibility to switch from abiotic to biotic habitats and to acquire a pathogenic potential (Fig 1). The previously mentioned ubiquitous black yeast Aureobasidium pullulans is a good example of a polyextremotolerant species and a possible emerging pathogen (Hawkes et al. 2005). Its ability to survive in hypersaline (Gunde-Cimerman et al. 2000), acidic and basic (Ranta 1990; Shiomi et al. 2004), and cold and oligotrophic (Onofri 1999) environments is possible because of
ar et al. 2008; Kogej et al. several molecular adaptations (Gostinc 2006; Kogej et al. 2005; Turk et al. 2007) and of their rapid dimorphic switching from small colourless yeast cells to thick-walled, heavily melanised, meristematic forms (Bermejo et al. 1981). Due to its traits, A. pullulans has recently been proposed as a model for investigating fungal phenotypic plasticity (Slepecky & Starmer 2009). Aureobasidium pullulans produces a large number of extracellular hydrolytic enzymes (Chi et al. 2009). Proteases, lipases and phospholipases are important virulence factors. These enzymes not only have roles in nutrition, but also in tissue damage, dissemination within the human organism, iron acquisition, and the overcoming of the host immune system, which all strongly contribute to fungal pathogenicity (KarkowskaKuleta et al. 2009), as reported for Aspergillus fumigatus and C. albicans (van Baarlen et al. 2007). These are all reasons why A. pullulans and other similar polyextremotolerant fungi in our homes should be carefully monitored in the future.

Conclusions Industrialisation has created environments that can support the growth of only a few adapted microbial species, and thus select them for traits that might also be important for

human pathogenesis. This is the case with polyextremotolerant black fungi, as their general stress-tolerant features appear to serve as preadaptation for further evolution towards either extremophily or pathogenicity. This and other similar groups represent a pool of species with potentially medically important features that might enable them to switch from environmental niches to human bodies (e.g. Li et al. 2008). Evolution in indoor environments is driven by periods of extremely unfavourable abiotic conditions (e.g. low water activity, high temperature) and the presence of unusual substrates and chemicals, such as silicone rubber and disinfectants. This selects for traits that (i) confer tolerance to multiple (and fluctuating) abiotic stress factors, and (ii) can act as pathogenicity factors: melanisation, meristematic growth, production of extracellular polysaccharides, biofilm formation, phenotypic plasticity, and probably others. The numerous opportunities of the selected/evolved species for contact with e and infection of e humans, the ageing human population, the high numbers of immunocompromised individuals, and the lifestyle that is increasingly confined to indoor environments might all contribute to an increase in the risk of emergence of novel fungal pathogens. We have made our indoor environments prohibitive for the growth of the majority of microbes. However, the most resistant (and as such, possibly the most dangerous) ones have succeeded in surviving and adapting. While for some traits (e.g. antibiotic resistance) this is already widely accepted, we are still not aware of the other consequences of this evolution that is going on around us. We might have turned our homes into microcosms for the experimental evolution of the most resilient of microbial species, the adaptability of which might enable them to find new niches in the human body.

Evolution of Fungal Pathogens in Domestic Environments?

Acknowledgements We wish to thank Prof. Børge Diderichsen for critical reading of the manuscript. The scientific work was partly financed via the ‘Centre of excellence for integrated approaches in chemistry and biology of proteins’ number OP13.1.1.2.02.0005, financed by European Regional Development Fund (85 % share of financing) and by the Slovenian Ministry of Higher Education, Science and Technology (15 % share of financing).

Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.funbio.2011.03.004.

references

Agosta SJ, Klemens JA, 2008. Ecological fitting by phenotypically flexible genotypes: implications for species associations, community assembly and evolution. Ecology Letters 11: 1123e1134. Andrews JH, Spear RN, Nordheim EV, 2002. Population biology of Aureobasidium pullulans on apple leaf surfaces. Canadian Journal of Microbiology 48: 500e513. Atkin CL, Neilands JB, Phaff HJ, 1970. Rhodotorulic acid from species of Leucosporidium, Rhodosporidium, Rhodotorula, Sporidiobolus, and Sporobolomyces, and a new alanine-containing ferrichrome from Cryptococcus melibiosum. Journal of Bacteriology 103: 722e733. van Baarlen P, van Belkum A, Summerbell RC, Crous PW, Thomma BPHJ, 2007. Molecular mechanisms of pathogenicity: how do pathogenic microorganisms develop cross-kingdom host jumps? FEMS Microbiology Reviews 31: 239e277. Baquero F, 2009. Environmental stress and evolvability in microbial systems. Clinical Microbiology and Infection 15: 5e10. von Baum H, Bommer M, Forke A, Holz J, Frenz P, Wellinghausen N, 2010. Is domestic tap water a risk for infections in neutropenic patients? Infection 38: 181e186. Bermejo JM, Dominguez JB, Goni FM, Uruburu F, 1981. Influence of pH on the transition from yeast-like cells to chlamydospores in Aureobasidium pullulans. Antonie van Leeuwenhoek 47: 385e392. Beumer RR, Kusumaningrum H, 2003. Kitchen hygiene in daily life. International Biodeterioration & Biodegradation 51: 299e302. Bhaganna P, Volkers RJM, Bell ANW, Kluge K, Timson DJ, McGrath JW, Ruijssenaars HJ, Hallsworth JE, 2010. Hydrophobic substances induce water stress in microbial cells. Microbial Biotechnology 3: 1751e7915. Blankenship JR, Wormley FL, Boyce MK, Schell WA, Filler SG, Perfect JR, Heitman J, 2003. Calcineurin is essential for Candida albicans survival in serum and virulence. Eukaryotic Cell 2: 422e430. Bore E, Langsrud S, 2005. Characterization of micro-organisms isolated from dairy industry after cleaning and fogging disinfection with alkyl amine and peracetic acid. Journal of Applied Microbiology 98: 96e105. Bowers KJ, Mesbah NM, Wiegel J, 2009. Biodiversity of poly-extremophilic bacteria: does combining the extremes of high salt, alkaline pH and elevated temperature approach a physico-chemical boundary for life? Saline Systems 5: 9.

1015

Branda E, Turchetti B, Diolaiuti G, Pecci M, Smiraglia C, Buzzini P, 2010. Yeast and yeast-like diversity in the southernmost glacier of Europe (Calderone Glacier, Apennines, Italy). FEMS Microbiology Ecology 72: 354e369. Braoudaki M, Hilton AC, 2004. Adaptive resistance to biocides in Salmonella enterica and Escherichia coli O157 and cross-resistance to antimicrobial agents. Journal of Clinical Microbiology 42: 73e78. Bridge PD, Newsham KK, 2009. Soil fungal community composition at Mars Oasis, a southern maritime Antarctic site, assessed by PCR amplification and cloning. Fungal Ecology 2: 66e74. Brown AJP, Haynes K, Quinn J, 2009. Nitrosative and oxidative stress responses in fungal pathogenicity. Current Opinion in Microbiology 12: 384e391. Brown SM, Campbell LT, Lodge JK, 2007. Cryptococcus neoformans, a fungus under stress. Current Opinion in Microbiology 10: 320e325. van Burik JAH, Magee PT, 2001. Aspects of fungal pathogenesis in humans. Annual Review of Microbiology 55: 743e772. Butinar L, Santos S, Spencer-Martins I, Oren A, GundeCimerman N, 2005. Yeast diversity in hypersaline habitats. FEMS Microbiology Letters 244: 229e234. Butinar L, Spencer-Martins I, Gunde-Cimerman N, 2007. Yeasts in high Arctic glaciers: the discovery of a new habitat for eukaryotic microorganisms. Antonie van Leeuwenhoek 91: 277e289. Cairns T, Minuzzi F, Bignell E, 2010. The host-infecting fungal transcriptome. FEMS Microbiology Letters 307: 1e11. Cappitelli F, Sorlini C, 2008. Microorganisms attack synthetic polymers in items representing our cultural heritage. Applied and Environmental Microbiology 74: 564e569. Casadevall A, 2007. Determinants of virulence in the pathogenic fungi. Fungal Biology Reviews 21: 130e132. Chi Z, Wang F, Yue L, Liu G, Zhang T, 2009. Bioproducts from Aureobasidium pullulans, a biotechnologically important yeast. Applied Microbiology & Biotechnology 82: 793e804. Dadachova E, Bryan RA, Huang X, Moadel T, Schweitzer AD, Aisen P, Nosanchuk JD, Casadevall A, 2007. Ionizing radiation changes the electronic properties of melanin and enhances the growth of melanized fungi. PLoS ONE 2: e457. Daly MJ, Gaidamakova EK, Matrosova VY, Kiang JG, Fukumoto R, Lee DY, Wehr NB, Viteri GA, Berlett BS, Levine RL, 2010. Smallmolecule antioxidant proteome-shields in Deinococcus radiodurans. PLoS ONE 5. Davis DA, 2009. How human pathogenic fungi sense and adapt to pH: the link to virulence. Current Opinion in Microbiology 12: 365e370. Desnos-Ollivier M, Ragon M, Robert V, Raoux D, Gantier JC, Dromer F, 2008. Debaryomyces hansenii (Candida famata), a rare human fungal pathogen often misidentified as Pichia guilliermondii (Candida guilliermondii). Journal of Clinical Microbiology 46: 3237e3242. Draghi JA, Parsons TL, Wagner GP, Plotkin JB, 2010. Mutational robustness can facilitate adaptation. Nature 463: 353e355. d’Enfert C, 2009. Hidden killers: persistence of opportunistic fungal pathogens in the human host. Current Opinion in Microbiology 12: 358e364. Elias PM, 2007. The skin barrier as an innate immune element. Seminars in Immunopathology 29: 3e14. Fang JL, Stingley RL, Beland FA, Harrouk W, Lumpkins DL, Howard P, 2010. Occurrence, efficacy, metabolism, and toxicity of triclosan. Journal of Environmental Science and Health - Part C: Environmental Carcinogenesis & Ecotoxicology Reviews 28: 147e171. Feazel LM, Baumgartner LK, Peterson KL, Frank DN, Harris JK, Pace NR, 2009. Opportunistic pathogens enriched in showerhead biofilms. Proceedings of the National Academy of Sciences of the United States of America 106: 16393e16398.

1016

Fell JW, Scorzetti G, Connell L, Craig S, 2006. Biodiversity of microeukaryotes in Antarctic dry valley soils with <5% soil moisture. Soil Biology & Biochemistry 38: 3107e3119. Feng B, Wang X, Hauser M, Kaufmann S, Jentsch S, Haase G, Becker JM, Szaniszlo PJ, 2001. Molecular cloning and characterization of WdPKS1, a gene involved in dihydroxynaphthalene melanin biosynthesis and virulence in Wangiella (Exophiala) dermatitidis. Infection and Immunity 69: 1781e1794. Galhardo RS, Hastings PJ, Rosenberg SM, 2007. Mutation as a stress response and the regulation of evolvability. Critical Reviews in Biochemistry and Molecular Biology 42: 399e435. Gardini F, Suzzi G, Lombardi A, Galgano F, Crudele MA, Andrighetto C, Schirone M, Tofalo R, 2001. A survey of yeasts in traditional sausages of southern Italy. FEMS Yeast Research 1: 161e167. Garre E, Raginel F, Palacios A, Julien A, Matallana E, 2010. Oxidative stress responses and lipid peroxidation damage are induced during dehydration in the production of dry active wine yeasts. International Journal of Food Microbiology 136: 295e303. Gorbushina AA, Kotlova ER, Sherstneva OA, 2008. Cellular responses of microcolonial rock fungi to long-term desiccation and subsequent rehydration. Studies in Mycology 61: 91e97. Gorbushina AA, Whitehead K, Dornieden T, Niesse A, Schulte A, Hedges J, 2003. Black fungal colonies as units of survival: hyphal mycosporines synthesized by rock dwelling microcolonial fungi. Canadian Journal of Botany/Revue Canadienne de Botanique 2: 131e138. Gorny RL, 2004. Filamentous microorganisms and their fragments in indoor air - a review. Annals of Agricultural and Environmental Medicine 11: 185e197. Gorny RL, Reponen T, Grinshpun SA, Willeke K, 2001. Source strength of fungal spore aerosolization from moldy building material. Atmospheric Environment 35: 4853e4862.
ar C, Grube M, de Hoog GS, Zalar P, Gunde-Cimerman N, Gostinc 2010. Extremotolerance in fungi: evolution on the edge. FEMS Microbiology Ecology 71: 2e11.
ar C, Turk M, Gunde-Cimerman N, 2009. Environmental Gostinc impacts on fatty acid composition of fungal membranes. In: Misra JK, Deshmukh SK (eds), Fungi from Different Environments Science Publishers, Enfield, NH, USA, pp. 278e327.
ar C, Turk M, Trbuha T, Vaupotic
T, Plemenita
s A, GundeGostinc Cimerman N, 2008. Expression of fatty-acid-modifying enzymes in the halotolerant black yeast Aureobasidium pullulans (de Bary) G. Arnaud under salt stress. Studies in Mycology 61: 51e59. Gueidan C, Villasenor CR, de Hoog GS, Gorbushina AA, Untereiner WA, Lutzoni F, 2008. A rock-inhabiting ancestor for mutualistic and pathogen-rich fungal lineages. Studies in Mycology 61: 111e119. Gueidan C, Ruibal C, de Hoog GS, Schneider H, 2011. Rock-inhabiting fungi originated during periods of dry climate in the late Devonian and middle Triassic. Fungal Biology 115: 987e996. Gunde-Cimerman N, Zalar P, de Hoog S, Plemenita
s A, 2000. Hypersaline waters in salterns - natural ecological niches for halophilic black yeasts. FEMS Microbiology Ecology 32: 235e240. Haase G, Sonntag L, Melzer-Krick B, de Hoog GS, 1999. Phylogenetic inference by SSU-gene analysis of members of the Herpotrichiellaceae with special reference to human pathogenic species. Studies in Mycology 43: 80e97. Hageskal G, Knutsen AK, Gaustad P, de Hoog GS, Skaar I, 2006. Diversity and significance of mold species in Norwegian drinking water. Applied and Environmental Microbiology 72: 7586e7593. Hall-Stoodley L, Costerton JW, Stoodley P, 2004. Bacterial biofilms: from the natural environment to infectious diseases. Nature Reviews Microbiology 2: 95e108. Hallsworth JE, Heim S, Timmis KN, 2003. Chaotropic solutes cause water stress in Pseudomonas putida. Environmental Microbiology 5: 1270e1280.


ar et al. C. Gostinc

Hamad M, 2008. Antifungal immunotherapy and immunomodulation: a double-hitter approach to deal with invasive fungal infections. Scandinavian Journal of Immunology 67: 533e543. Hamada N, Abe N, 2010. Comparison of fungi found in bathrooms and sinks. Biocontrol Science 15: 51e56. Hawkes M, Rennie R, Sand C, Vaudry W, 2005. Aureobasidium pullulans infection: fungemia in an infant and a review of human cases. Diagnostic Microbiology and Infectious Disease 51: 209e213. Henrichfreise B, Wiegand I, Pfister W, Wiedemann B, 2007. Resistance mechanisms of multiresistant Pseudomonas aeruginosa strains from Germany and correlation with hypermutation. Antimicrobial Agents and Chemotherapy 51: 4062e4070. Hilmarsdottir I, Haraldsson H, Sigurdardottir A, Sigurgeirsson B, 2005. Dermatophytes in a swimming pool facility: difference in dermatophyte load in men’s and women’s dressing rooms. Acta Dermato-Venereologica 85: 267e268. € lker U, Bend J, Pracht R, Tetsch L, Muller T, Hofer M, de Ho Hoog GS, 2004. Hortaea acidophila, a new acid-tolerant black yeast from lignite. Antonie van Leeuwenhoek 86: 287e294. Holzberg M, Artis WM, 1983. Hydroxamate siderophore production by opportunistic and systemic fungal pathogens. Infection and Immunity 40: 1134e1139. de Hoog GS, 1993. Evolution of black yeasts: possible adaptation to the human host. Antonie van Leeuwenhoek 63: 105e109. de Hoog GS, Zalar P, Gerrits van den Ende AHG, GundeCimerman N, 2005. Relation of halotolerance to humanpathogenicity in the fungal tree of life: an overview of ecology and evolution under stress. In: Adaptation to Life at High Salt Concentrations in Archaea, Bacteria, and Eukarya. Springer, Dordrecht, The Netherlands. Karkowska-Kuleta J, Rapala-Kozik M, Kozik A, 2009. Fungi pathogenic to humans: molecular bases of virulence of Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. Acta Biochimica Polonica 56: 211e224.
ar C, Volkmann M, Gorbushina AA, GundeKogej T, Gostinc Cimerman N, 2006. Mycosporines in extremophilic fungi d novel complementary osmolytes? Environmental Chemistry 3: 105e110. Kogej T, Ramos J, Plemenitas A, Gunde-Cimerman N, 2005. The halophilic fungus Hortaea werneckii and the halotolerant fungus Aureobasidium pullulans maintain low intracellular cation concentrations in hypersaline environments. Applied and Environmental Microbiology 71: 6600e6605. Kogej T, Stein M, Volkmann M, Gorbushina AA, Galinski EA, Gunde-Cimerman N, 2007. Osmotic adaptation of the halophilic fungus Hortaea werneckii: role of osmolytes and melanization. Microbiology 153: 4261e4273. Kumamoto CA, 2008. Niche-specific gene expression during C. albicans infection. Current Opinion in Microbiology 11: 325e330. Langsrud S, Sundheim G, Holck AL, 2004. Cross-resistance to antibiotics of Escherichia coli adapted to benzalkonium chloride or exposed to stress-inducers. Journal of Applied Microbiology 96: 201e208. Lennon JT, Jones SE, 2011. Microbial seed banks: the ecological and evolutionary implications of dormancy. Nature Reviews Microbiology 9: 119e130. Levy SB, 2001. Antibacterial household products: cause for concern. Emerging Infectious Diseases 7: 512e515. Li DM, de Hoog GS, Saunte DML, van den Ende AHGG, Chen XR, 2008. Coniosporium epidermidis sp. nov., a new species from human skin. Studies in Mycology 61: 131e136. Li DM, Li RY, de Hoog GS, Sudhadham M, Wang DL. Fatal Exophiala infections in China, with a report of seven cases. Mycoses, in press, doi:10.1111/j.1439-0507.2010.01859.x. Lian X, de Hoog GS, 2010. Indoor wet cells harbour melanized agents of cutaneous infection. Medical Mycology 48: 622e628. Macia MD, Borrell N, Segura M, Gomez C, Perez JL, Oliver A, 2006. Efficacy and potential for resistance selection of antipseudomonal treatments in a mouse model of lung infection by

Evolution of Fungal Pathogens in Domestic Environments?

hypermutable Pseudomonas aeruginosa. Antimicrobial Agents and Chemotherapy 50: 975e983. Mangalappalli-Illathu AK, Korber DR, 2006. Adaptive resistance and differential protein expression of Salmonella enterica serovar Enteritidis biofilms exposed to benzalkonium chloride. Antimicrobial Agents and Chemotherapy 50: 3588e3596. Matos T, de Hoog GS, de Boer AG, de Crom I, Haase G, 2002. High prevalence of the neurotrope Exophiala dermatitidis and related oligotrophic black yeasts in sauna facilities. Mycoses 45: 373e377. Mc Cay PH, Ocampo-Sosa AA, Fleming GT, 2010. Effect of subinhibitory concentrations of benzalkonium chloride on the competitiveness of Pseudomonas aeruginosa grown in continuous culture. Microbiology 156: 30e38. Moranova Z, Kawamoto S, Raclavsky V, 2009. Hypoxia sensing in Cryptococcus neoformans: biofilm-like adaptation for dormancy? Biomedical Papers-Olomouc 153: 189e193. Nappi AJ, Ottaviani E, 2000. Cytotoxicity and cytotoxic molecules in invertebrates. Bioessays 22: 469e480. Nishimura K, Miyaji M, Taguchi H, Tanaka R, 1987. Fungi in bathwater and sludge of bathroom drainpipes. Frequent isolation of Exophiala species. Mycopathologia 97: 17e23. Nishiuchi Y, Tamaru A, Kitada S, Taguri T, Matsumoto S, Tateishi Y, Yoshimura M, Ozeki Y, Matsumura N, Ogura H, Maekura R, 2009. Mycobacterium avium complex organisms predominantly colonize in the bathtub inlets of patients’ bathrooms. Japanese Journal of Infectious Diseases 62: 182e186. Nisiotou AA, Chorianopoulos N, Nychas GJE, Panagou EZ, 2010. Yeast heterogeneity during spontaneous fermentation of black Conservolea olives in different brine solutions. Journal of Applied Microbiology 108: 396e405. Ojima M, Toshima Y, Koya E, Ara K, Tokuda H, Kawai S, Kasuga F, Ueda N, 2002. Hygiene measures considering actual distributions of microorganisms in Japanese households. Journal of Applied Microbiology 93: 800e809. Onofri S, 1999. Antarctic microfungi. In: Seckbach J (ed), Enigmatic Microorganisms and Life in Extreme Environments. Kluwer Academic, Dordrecht; London, pp. 323e336. Patnaik R, 2008. Engineering complex phenotypes in industrial strains. Biotechnology Progress 24: 38e47.
U, 2006. Role of oxidative stress in the extremely salt-tolPetrovic erant yeast Hortaea werneckii. FEMS Yeast Research 6: 816e822. Porteous MB, Reddimg SW, Thompsom EH, Grooters AM, de Hoog GS, Sutton DA, 2003. Isolation of an unusual fungus in treated dental unit waterlines. Journal of the American Dental Association 134: 853e858. Prenafeta-Boldu FX, Summerbell R, de Hoog GS, 2006. Fungi growing on aromatic hydrocarbons: biotechnology’s unexpected encounter with biohazard? FEMS Microbiology Reviews 30: 109e130. Ranta HM, 1990. Effect of simulated acid rain on quantity of epiphytic microfungi on Scots pine (Pinus sylvestris L.) needles. Environmental Pollution 67: 349e359. Rauch ME, Graef HW, Rozenzhak SM, Jones SE, Bleckmann CA, Kruger RL, Naik RR, Stone MO, 2006. Characterization of microbial contamination in United States air force aviation fuel tanks. Journal of Industrial Microbiology & Biotechnology 33: 29e36. Robert VA, Casadevall A, 2009. Vertebrate endothermy restricts most fungi as potential pathogens. Journal of Infectious Diseases 200: 1623e1626. Rosenberg SM, Hastings PJ, 2004. Genomes: worming into genetic instability. Nature 430: 625e626. Ruibal C, Gueidan C, Selbmann L, Gorbushina AA, Crous PW, Groenewald JZ, Muggia L, Grube M, Isola D, Schoch CL, Staley JT, Lutzoni F, de Hoog GS, 2009. Phylogeny of rock-inhabiting fungi related to Dothideomycetes. Studies in Mycology 64: 123e133.

1017

Russell AD, 2002. Introduction of biocides into clinical practice and the impact on antibiotic-resistant bacteria. Journal of Applied Microbiology 92: 121se135s. Russo G, Libkind D, Sampaio JP, van Broock MR, 2008. Yeast diversity in the acidic Rio Agrio-Lake Caviahue volcanic environment (Patagonia, Argentina). FEMS Microbiology Ecology 65: 415e424. SaldanhaDaGama A, MalfeitoFerreira M, Loureiro V, 1997. Characterization of yeasts associated with Portuguese pork-based products. International Journal of Food Microbiology 37: 201e207. Samani P, Bell G, 2010. Adaptation of experimental yeast populations to stressful conditions in relation to population size. Journal of Evolutionary Biology 23: 791e796. Samson RA, Hoekstra ES, Frisvad JC, 2004. Introduction to Food- and Airborne Fungi, 7th edn. Centraalbureau voor Schimmelcultures, Utrecht, The Netherlands. Scheffer RP, 1991. Role of toxins in evolution and ecology of plant pathogenic fungi. Experientia 47: 804e811. Schnitzler N, Peltroche-Llacsahuanga H, Bestier N, Zundorf J, Lutticken R, Haase G, 1999. Effect of melanin and carotenoids of Exophiala (Wangiella) dermatitidis on phagocytosis, oxidative burst, and killing by human neutrophils. Infection and Immunity 67: 94e101. Scott E, Duty S, Callahan M, 2008. A pilot study to isolate Staphylococcus aureus and methicillin-resistant S. aureus from environmental surfaces in the home. American Journal of Infection Control 36: 458e460. Selbmann L, de Hoog GS, Mazzaglia A, Friedmann EI, Onofri S, 2005. Fungi at the edge of life: cryptoendolithic black fungi from Antarctic desert. Studies in Mycology 51: 1e32. Shah AA, Hasan F, Hameed A, Ahmed S, 2008. Biological degradation of plastics: a comprehensive review. Biotechnology Advances 26: 246e265. Shiomi N, Yasuda T, Inoue Y, Kusumoto N, Iwasaki S, Katsuda T, Katoh S, 2004. Characteristics of neutralization of acids by newly isolated fungal cells. Journal of Bioscience and Bioengineering 97: 54e58. Shirakawa MA, Beech IB, Tapper R, Cincotto MA, Gambale W, 2003. The development of a method to evaluate bioreceptivity of indoor mortar plastering to fungal growth. International Biodeterioration & Biodegradation 51: 83e92. Slepecky RA, Starmer WT, 2009. Phenotypic plasticity in fungi: a review with observations on Aureobasidium pullulans. Mycologia 101: 823e832. Sorensen SJ, Bailey M, Hansen LH, Kroer N, Wuertz S, 2005. Studying plasmid horizontal transfer in situ: a critical review. Nature Reviews Microbiology 3: 700e710. Sterflinger K, 1998. Temperature and NaCl-tolerance of rock-inhabiting meristematic fungi. Antonie van Leeuwenhoek 74: 271e281. Straus DC, 2009. Molds, mycotoxins, and sick building syndrome. Toxicology and Industrial Health 25: 617e635. Su HJ, Rotnitzky A, Burge HA, Spengler JD, 1992. Examination of fungi in domestic interiors by using factor-analysis - correlations and associations with home factors. Applied and Environmental Microbiology 58: 181e186. Summerbell RC, Staib F, Dales R, Nolard N, Kane J, Zwanenburg H, Burnett R, Krajden S, Fung D, Leong D, 1992. Ecology of fungi in human dwellings. Journal of Medical and Veterinary Mycology 30: 279e285. Szomolay B, Klapper I, Dockery J, Stewart PS, 2005. Adaptive responses to antimicrobial agents in biofilms. Environmental Microbiology 7: 1186e1191. Thevissen K, Warnecke DC, Francois EJA, Leipelt M, Heinz E, Ott C, Zahringer U, Thomma BPHJ, Ferkel KKA, Cammue BPA, 2004. Defensins from insects and plants interact with fungal glucosylceramides. Journal of Biological Chemistry 279: 3900e3905.

1018

Thrasher JD, Crawley S, 2009. The biocontaminants and complexity of damp indoor spaces: more than what meets the eyes. Toxicology and Industrial Health 25: 583e615. Tringe SG, Zhang T, Liu XG, Yu YT, Lee WH, Yap J, Yao F, Suan ST, Ing SK, Haynes M, Rohwer F, Wei CL, Tan P, Bristow J, Rubin EM, Ruan YJ, 2008. The airborne metagenome in an indoor urban environment. PLoS ONE 3. Tuon FF, Costa SF, 2008. Rhodotorula infection. A systematic review of 128 cases from literature. Revista Iberoamericana de Micologıa 25: 135e140. Turk M, Abramovic Z, Plemenitas A, Gunde-Cimerman N, 2007. Salt stress and plasma-membrane fluidity in selected extremophilic yeasts and yeast-like fungi. FEMS Yeast Research 7: 550e557. Vadkertiova R, Slavikova E, 1995. Killer activity of yeasts isolated from the water environment. Canadian Journal of Microbiology 41: 759e766. Villegas LB, Amoroso MJ, De Figueroa LIC, 2005. Copper tolerant yeasts isolated from polluted area of Argentina. Journal of Basic Microbiology 45: 381e391. Wallstrom S, Karlsson S, 2004. Biofilms on silicone rubber insulators; microbial composition and diagnostics of removal by use of ESEM/EDS - composition of biofilms infecting silicone rubber insulators. Polymer Degradation and Stability 85: 841e846. Wang WL, Chi ZM, Chi Z, Li J, Wang XH, 2009. Siderophore production by the marine-derived Aureobasidium pullulans and its antimicrobial activity. Bioresource Technology 100: 2639e2641. West-Eberhard MJ, 2005. Developmental plasticity and the origin of species differences. Proceedings of the National Academy of Sciences of the United States of America 102: 6543e6549.


ar et al. C. Gostinc

Xin G, Glawe D, Doty SL, 2009. Characterization of three endophytic, indole-3-acetic acid-producing yeasts occurring in Populus trees. Mycological Research 113: 973e980. Zalar P, Frisvad JC, Gunde-Cimerman N, Varga J, Samson RA, 2008a. Four new species of Emericella from the Mediterranean region of Europe. Mycologia 100: 779e795.
ar C, de Hoog GS, Ur

V, Sudhadham M, GundeZalar P, Gostinc sic Cimerman N, 2008b. Redefinition of Aureobasidium pullulans and its varieties. Studies in Mycology 61: 21e38. Zalar P, de Hoog GS, Schroers HJ, Crous PW, Groenewald JZ, Gunde-Cimerman N, 2007. Phylogeny and ecology of the ubiquitous saprobe Cladosporium sphaerospermum, with descriptions of seven new species from hypersaline environments. Studies in Mycology 58: 157e183. Zalar P, Novak M, de Hoog GS, Gunde-Cimerman N, 2011. Dishwashers: man-made enrichment machinery for human opportunistic fungi? Fungal Biology 115: 997e1007. Zaragoza O, Rodrigues ML, De Jesus M, Frases S, Dadachova E, Casadevall A, 2009. The capsule of the fungal pathogen Cryptococcus neoformans. Advances in Applied Microbiology 68: 133e216. Zeng JS, Sutton DA, Fothergill AW, Rinaldi MG, Harrak MJ, de Hoog GS, 2007. Spectrum of clinically relevant Exophiala species in the United States. Journal of Clinical Microbiology 45: 3713e3720. Zhang ZH, Qian WF, Zhang JZ, 2009. Positive selection for elevated gene expression noise in yeast. Molecular Systems Biology 5: 299. Zheng CL, Zhou JT, Wang J, Wang J, Qu BC, 2008. Isolation and characterization of a nitrobenzene degrading yeast strain from activated sludge. Journal of Hazardous Materials 160: 194e199.