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Patterns of partnership: surveillance and mimicry in host-microbiota mutualisms
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ScienceDirect Patterns of partnership: surveillance and mimicry in host-microbiota mutualisms Travis J Wiles1 and Karen Guillemin1,2 The repertoire of microbial cues monitored by animal and plant tissues encompasses not just molecules but also microbial activities. These include typical pathogen strategies of injuring membranes, degrading cellular material, and scavenging resources. These activities, however, are not exclusive to pathogens. Instead, they characterize the competitive strategies of microbes living in multispecies communities, like those typically found colonizing host tissues. Similar activities are also deployed by host tissues to keep microbes in check. We propose that host surveillance and mimicry of MicrobialAssociated Competitive Activities (MACAs), derived from an evolutionary history of living in mixed microbial communities, has shaped contemporary animal and plant tissue programs of defense, repair, metabolism, and development. Addresses 1 Institute of Molecular Biology, University of Oregon, Eugene, OR, USA 2 Humans and the Microbiome Program, CIFAR, Toronto, ON, Canada Corresponding author: Guillemin, Karen ([email protected])
Current Opinion in Microbiology 2019, 48:87–94 This review comes from a themed issue on Stanley Falkow alumni Edited by Denise Monack and Igor Brodsky
https://doi.org/10.1016/j.mib.2020.01.012 1369-5274/ã 2020 Elsevier Ltd. All rights reserved.
Stanley Falkow made many seminal contributions to the field of bacterial pathogenesis, not least of which was to continually remind us to think from the point of view of the microbes [1]. As researchers studying microbe–host interactions, it is all too easy to fall into the trap of formulating hypotheses based on unconscious, preconceived biases about the primacy of the host in these relationships. Stan’s words remind us of the limitations of our myopic human perspectives. One human-centric proclivity is to bin microbes into discrete categories. We classify them as pathogens, pathobionts, commensals, or mutualists, imposing our value systems on them. These assignments can be useful for characterizing the epidemiology of infectious agents and the potential for pathological outcomes of interactions www.sciencedirect.com
with certain microbes [2]. But by many measures, the microbes that we narrowly label as pathogens and mutualists actually share common strategies of engaging with us [3]. Stan famously said that he never met a microbe he didn’t like [4], and he even proposed that pathogens may be good for our health [5]. He emphasized the pointlessness of such rigid microbial categorization: “It really doesn’t matter how we define a pathogen! To underestimate the evolutionary potential of microorganisms and their ability to survive, even in the face of enormous pressure to eradicate them, would be a mistake” [4]. Here again he was urging us to think like a microbe, for which such human-centric categorizations are irrelevant in the face of broader ecological challenges to compete and survive. Our insistence on imposing these categories is a natural extension of our human desire to make sense of our complicated relationships with microbes, which range from acute life-threatening infections to life-long physiological and developmental dependencies. A human-centric view assumes that microbes make products specifically designed to manipulate us: pathogens inject nefarious toxins to make us ill while mutualists proffer soothing salves to promote our health. With the recent explosion of information about the complex microbial communities that inhabit our bodies, it becomes increasingly untenable to imagine that microbial-based diseases can be explained as binary interactions between individual microbes and ourselves. Indeed, Stan recognized that many of these diseases are the consequence of changing microbial communities responding to altered human ecology [6]. In this review, we attempt to rethink, from a microbial perspective, the origins and nature of pathogenic and salubrious microbial products that host cells detect and respond to during intimate encounters. When we take our human selves out of the center of the picture, we begin to see that the microbial molecules that exert their influences on us are not exclusively designed for manipulating or communicating with us. Rather, we hypothesize that the microbial molecules that elicit responses in us are the signatures of cellular competitions for resources and space that originated from a much more ancient and expansive history of microbes living and competing with other microbes.
Molecular signatures of microbes: from PAMPs to MAMPs To understand the microbial molecules that elicit responses in hosts like ourselves, we need to understand the receptors Current Opinion in Microbiology 2020, 54:87–94
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of these molecules. Taking into account seminal discoveries showing that Toll-like Receptor 4 (TLR4) recognizes the Gram-negative cell wall component lipopolysaccharide (LPS) [7], Charles Janeway and colleagues formulated the concept of Pattern Recognition Receptors (PRRs) that detect conserved molecules they termed Pathogen-Associated Molecular Patterns (PAMPs) [8]. Other examples of PAMPs include viral-associated nucleic acids, fungal-associated sugars, and additional bacterial cell wall components such as peptidoglycan. Sensing and responding to these microbial signatures makes sense from the perspective of defending against infectious threats. However, although these molecules are characteristic of specific classes of microbes, they are not exclusive to disease-causing organisms. Margaret McFall-Ngai and colleagues provided a compelling example, in the symbiosis between the Hawaiian bobtail squid and the luminescent bacterium Vibrio fischeri, of the peptidoglycan monomer known as tracheal cytotoxin (TCT) serving to stimulate host tissue responses essential for the establishment of the partnership [9]. At the tissue level, TCT elicits changes in the squid light organ epithelium reminiscent of tracheal epithelial damage induced by the pathogen Bordetella pertussis, for which TCT was named, but at the organismal level, this tissue regression is beneficial for the symbiosis. In accordance with this finding, they proposed renaming ‘PAMP’ as MicrobialAssociated Molecular Pattern or ‘MAMP’ to remain agnostic about the impacts of perception of these kinds of molecules by host tissues. MAMP is an appropriate term to use when considering that the last common eukaryotic ancestor hunted bacteria as food [10]. The PRR repertoire of this ancestor would have included proteins with shared domains of animal and plant PRRs, such as Toll/interleukin-1 receptor/resistance protein (TIR) and leucine rich repeats (LRR) domains [11]. Extant single-celled eukaryotes serve as useful models of ancient bacterial–eukaryotic interactions and display a diversity of relationships. For example, the social amoeba Dictyostelium preys on bacteria, experiences bacterial infections, and in its multicellular slug form, harbors an endogenous microbiota that increases its capacity for food bacteria carriage [12,13,14]. Dictyostelium uses a TIR domain protein to sense LPS [11] and responds to Gram-positive bacteria by deploying distinct suites of genes, such as alyL encoding the peptidoglycan cleaving enzyme, lysozyme, which it requires to grow specifically on Gram-positive bacteria [15]. Another bacterivore and the closest singlecelled ancestor of animals, choanoflagellates form multicellular structures in response to specific bacterial lipids [16]. These examples highlight how MAMP sensing has been integrated into fundamental aspects of eukaryotic growth and development before the evolution of multicellular hosts. Current Opinion in Microbiology 2020, 54:87–94
In animals, we now can point to numerous examples of MAMPs and PRRs playing important roles in normal development, such as peptidoglycan-mediated maturation of the immune and nervous systems [17,18]. In the developing zebrafish, innate immune sensing via Myd88, a common adaptor for the TLRs and interleukin-1 receptor, functions in establishing rates of intestinal epithelial cell proliferation [19], increasing mucus secreting versus absorptive cell types [20], and promoting enterocyte maturation [21]. These examples demonstrate how animals benefit from monitoring their microbial status to make developmental decisions (e.g., by proceeding with light organ maturation only after symbiont colonization) and to fine-tune tissue homeostasis (e.g., by increasing epithelial turnover and mucosal fortification and lubrication in the presence of microbes). Intriguingly, some PRRs have been coopted into developmental roles independent of microbial sensing, such as the founding Toll receptor that regulates Drosophila dorsal–ventral patterning. Not only do hosts monitor microbes for more than defense against infection, but they have intricate systems for spatially regulating MAMP sensing. For example, in the murine gastrointestinal tract, different TLRs are expressed in different proximal–distal regions and cell types and at different developmental stages [22]. In the zebrafish intestine, transcription of the brush border enzyme, intestinal alkaline phosphatase, is induced by the MAMP LPS, and the upregulated enzyme in turn serves to dephosphorylate LPS in the intestinal lumen, rendering it a less potent stimulator of innate immune signaling [21]. This negative feedback loop of LPSstimulated detoxification of LPS is spatially segregated to the gut lumen, the site of innocuous resident bacteria. Thus, hosts exert spatial control over both the location of MAMP sensing and MAMP availability.
More than collections of molecules: signatures of microbial activities and the MACA hypothesis The importance of context in sensing microbes is a point that Russel Vance, Ralph Isberg, and Dan Portnoy emphasized in ‘Patterns of Pathogenesis’ [23]. In this review, they expanded the concept of host sensing of microbes from conserved molecules to conserved microbial activities “that live pathogens use to invade, manipulate, replicate within, or spread among their hosts”. They explored how different cellular sensors of microbial products provide contextual information on microbial activities. For example, detection of extracellular bacterial flagellin by cell-surface localized TLR5 elicits a different host response than sensing of intracellular flagellin, a hallmark of invading bacteria, by the cytoplasmic nucleotide-binding domain leucine-rich repeat (NLR) family member, NAIP5. www.sciencedirect.com
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Emerging mechanistic insights into the NLR family of innate immune sensors reveal how they are poised to sense perturbations of cellular processes frequently targeted by pathogens [24,25]. Once cells are primed by cues such as MAMPs, the NLRs can induce the assembly of multiprotein inflammasome complexes upon sensing certain microbial activities. The plant immunity field provides multiple examples of NLR family ‘guard’ proteins that surveil the activity of bacterial effector proteins, and similar examples are emerging from animal cells [26]. For example, by sensing its own partial degradation, the NLRP1B inflammasome detects abnormal levels of proteolysis caused by bacterial proteases or effectors that activate the proteosome [27,28]. Similarly, NLRP3 senses efflux of potassium ions as a hallmark of membrane disruption caused by diverse insults such as bacterial pore-forming toxins or secretion systems [29,30].
nutrient-rich contents of cells, is called into question by various lines of investigation [35,36]. Contrary to expectations, strain cytotoxicity of Staphylococcus aureus clinical isolates correlates with asymptomatic carriage and inversely correlates with tissue invasion [37,38]. At a cellular level, Staphylococcus pore-forming toxins can promote biofilm formation, possibly by lysing neighboring bacterial cells to release material such as DNA for biofilm matrices [35]. Another class of cytolytic molecules secreted by Staphylococcus spp., surfactant-like phenolsoluble modulins, may function primarily as antimicrobials against other members of the nasal epithelium rather than to destroy host tissue [35]. In this expanded view of microbial surveillance, host cells are more often innocent bystanders bracing themselves against the collateral damage of intense microbial warfare than they are suffering as targets of pathogenic attackers.
The idea of immune detection of microbial activities provides a powerful framework for understanding host– microbe interactions, but as with the PAMP concept, it falls short with the assumption that these activities are exclusive to pathogens. For example, although bacterial type 3 secretion systems have been studied extensively in pathogens, these systems are found broadly distributed across the genomes of non-disease-associated bacteria. In the Bradyrhizobium–legume symbiosis, type 3 secretion systems play crucial roles in the establishment of the partnership, for example by suppressing plant immune responses [31]. A recent study of Bradyrhizobium type 3 secretion effector proteins identified ErnA, that is targeted to plant cell nuclei and is sufficient to induce formation of nitrogenfixing nodules in plant roots [32]. Looking to single-celled eukaryotes for instances of bacterial activity sensing, choanoflagellates provide a dramatic example of responding to a secreted Vibrio chondroitinase by initiating meiosis and mating, a response that is not elicited by an inactive version of the enzyme [33].
To capture this expanded picture of host surveillance of microbial consortia, we introduce the concept of Microbial-Associated Competitive Activities or ‘MACAs’. MACAs represent the expanse of activities displayed by microbial cells to compete with other cells, both microbial and host. We specifically define MACAs as competitive activities of microbes that target fundamental cellular vulnerabilities common to prokaryotic and eukaryotic cells (Figure 1). Examples of MACAs include membrane disrupting molecules like lytic antimicrobial peptides, degradative enzymes that target essential macromolecules like peptidoglycan glycoside hydrolases [39], and nutrient scavenging molecules like siderophores that sequester scarce resources [40]. We propose that eukaryotic cells sense MACAs as part of ancient strategies for coexisting with microbial communities. Over evolutionary time, MACA sensing would have become incorporated into basic cellular processes that govern defense, repair, growth, and metabolism. In contemporary multicellular eukaryotic hosts, cell type-specific responses to MACAs could further coordinate tissue and organismal structure and function.
These examples motivate yet another expansion of our view of host microbial surveillance beyond activities exclusive to disease-causing microbes to encompass the vastly larger universe of microbes that interact with animals and plants without causing overt harm. Monitoring the activities of complex microbial consortia provides hosts with useful information not just to modulate defensive responses but also to make decisions about metabolism, growth, and development. At the molecular level, microbial monitoring involves detecting specific activities of microbes growing and competing in complex communities. Indeed, competing microbes possess diverse arsenals of contact-dependent and secreted weapons [34] that resemble prototypic pathogen weaponry known to be detected by PRRs. Bacterial Type 4 and 6 secretion systems, for example, are often deployed during interbacterial conflict. Even the host-directed nature of cytolytic toxins, presumed to allow pathogens access to www.sciencedirect.com
Signal integration: DAMPs as MACA mimics Eukaryotic cells and tissues coexisting with warring microbes would need strategies for counterattacks as well as surveillance. Closer inspection of our own repertoires of host defense molecules reveals that they are strikingly similar to MACAs, including membrane targeted antimicrobial peptides like cathelicidins, cellular material degrading enzymes like lysozymes, and nutrient scavenging molecules like transferrin. These defense strategies were found in the last common eukaryotic ancestor [11] and may share evolutionary histories with prokaryotic MACAs or may have evolved convergently to similar solutions for targeting universal cellular vulnerabilities. We refer to these activities produced by macroscopic organisms as ‘MACA mimics’. Current Opinion in Microbiology 2020, 54:87–94
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Figure 1
(a)
(b)
(c)
Current Opinion in Microbiology
MACA sensing and mimicry. (a) Primordial unicellular eukaryotes coexisted in a world of competing microbes and possess systems for sensing Microbial-Associated Competitive Activities (MACAs). (b) Contemporary multicellular animals and plants have evolved ‘MACA mimics’ to control resident microbial communities and potential pathogenic invaders at tissue surfaces. (c) Over evolutionary time, multicellular hosts have coopted MACA and MACA mimic sensory pathways to coordinate immunity and defense, healing and regeneration, and growth and development.
If our immune surveillance systems are tuned to detecting not just the molecules but also the activities of microbes, then they should also be stimulated by hostderived MACA mimics. The universality of the cellular targets of MACA and MACA mimics ensures broad protection but poses the danger of self-harm. For example, many host antimicrobial proteins that target the membranes of microbes also cause collateral damage to host cell membranes [41]. Indeed, the NLRP3 inflammasome senses attacks on membrane integrity from both Current Opinion in Microbiology 2020, 54:87–94
prokaryotic and eukaryotic molecules, including antibiotics such as nigericin from Streptomyces, bacterial pore-forming toxins such as alpha toxin from Staphylococcus, and host derived antimicrobial proteins such as cathelicidin [42]. This notion of self-inflicted damage by MACA mimics can be useful for understanding how plant and animal PPRs are activated by endogenously produced host factors, which are collectively referred to as Damage www.sciencedirect.com
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Associated Molecular Patterns or ‘DAMPs’ [43]. The list of host factors that function as DAMPs is diverse and eclectic—including, potassium ions, ATP, nucleic acid fragments, extracellular matrix fragments, antimicrobial peptides, lysozyme, calprotectin—but can be viewed broadly as the activities or products of MACA mimics. We propose that the expression patterns of MACA mimics have been evolutionarily honed to modulate host immune response pathways to maximize host fitness in response to microbes. Many genes encoding MACA mimics, such as antimicrobial proteins, are direct transcriptional targets of PRRs in response to microbial cues or tissue injury. Many of these antimicrobial proteins have also been shown to function as DAMPs and can have complex immunomodulatory activities [44]. For example, a recent study showed that upregulation of the antimicrobial peptide Cathelicidin during Pseudomonas aeruginosa infection of airway epithelia activates the NLRP3 inflammasome and leads to a protective inflammatory response against this pathogen [45]. Hinting at more complex ways in which MACA mimics have evolved to regulate tissue growth and repair, several host antimicrobial proteins were originally discovered in other contexts: the Reg3 proteins were named for their abundance in regenerating pancreatic islets [46], and the angiogenins were originally found to stimulate capillary growth in tumors [47]. Adding another layer of complexity to immune modulation by MACA mimicry, animal immune systems deploy a number of effector proteins that target host cell membranes but share a common mechanism of action with bacterial membrane perturbing proteins. A key downstream effector of inflammasome activation is the pore-forming protein Gasdermin D that, upon caspasemediated cleavage, oligomerizes in host cell membranes, allowing the release of Interleukin-1 and causing either a state of hyperactivation or a specialized form of cell death called pyroptosis [48,49]. Gasdermin family members bind a number of lipids, including the bacterialmembrane enriched cardiolipin, and will indiscriminately lyse both eukaryotic and prokaryotic cells [50], suggesting a possible additional antimicrobial function or evolutionary origin for this protein family. Other pore-forming effectors of the immune system include members of the Complement System and the killer lymphocyte cytotoxic granule protein Perforin. These proteins share a common membrane attack complex/perforin (MACPF) domain that is structurally and evolutionarily related to the cholesterol-dependent cytolysin (CDC) domain of many bacterial toxins [51]. Another MACPF protein, Torso-like, functions in Drosophila embryonic patterning through a mechanism that can be recapitulated by mechanical introduction of membrane holes [52]. Not only do host immune effectors elicit similar programs of NLRP3 activation as bacterial-derived pore-forming www.sciencedirect.com
toxins and antibiotics, but they also induce similar programs of membrane repair involving both endocytic clearance of damaged membranes and exocytic delivery of new membranes [53,54]. The MACA mimicry framework helps to uncover additional host proteins with antimicrobial activities and complex immunomodulatory functions. For example, Calprotectin inhibits bacterial growth by sequestering transition metals and also functions as a DAMP by activating TLR4 [55]. Phylogenetic reconstruction of the two S100 proteins of the Calprotectin heterodimer demonstrates that the TLR4 activation evolved more recently than the metal sequestration [56]. Calprotectin is released from neutrophil granulocytes in high abundance, making it a robust biomarker of inflammation, and suggesting that its localized deployment is useful for amplifying and refining immune signaling. Another biomarker of inflammation is the siderophore-sequestering Lipocalin 2 protein, whose complex immunomodulatory properties are only partially explained by its manipulation of resident microbiota through iron limitation [57]. In addition to binding siderophores, members of the Lipocalin family bind a number of small hydrophobic molecules with immunomodulatory properties such as steroid hormones and retinol [58]. In the zebrafish intestine, we found that Aeromonas spp. secrete an anti-inflammatory protein, AimA, with structural similarity to Lipocalins [59]. AimA does not appear to bind siderophores or function in iron acquisition for Aeromonas, but we speculate that it may sequester another limiting nutrient while simultaneously stimulating MACA-responsive pathways in the host to exert its anti-inflammatory effect.
MACAs as a framework for understanding patterns of partnership Our proposal that host immune systems continually monitor MACAs provides a new perspective for understanding our mutualism with our resident microbiota. With our human-centric biases, it is tempting to speculate that our resident microbes intentionally support us by promoting our health in exchange for their own longevity or influencing our appetites to encourage ingestion of favored nutrients. However, it is difficult to imagine how certain resident microbes could evolve specific manipulative strategies from which they alone would benefit, without other microbes freeloading on their efforts [60]. A more reasonable assumption is that resident microbes devote their energies to competing with each other, using and innovating on ancient strategies, such as membrane perturbation and nutrient scavenging, which have been coopted as information-rich cues sensed by contemporary host cells. Comparisons between germ-free and conventionally reared animals have taught us that the impacts of resident microbes are subtle but specific [61]. When viewed as part Current Opinion in Microbiology 2020, 54:87–94
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of a continuum of host immune activation, host responses to resident microbes resembles a muted stimulation of tissue damage and repair programs. Colonization of the germ-free vertebrate intestine, for example, stimulates intestinal epithelial cell proliferation and immune cell influx, both responses seen to a greater extent with enteric pathogens [62]. Gnotobiotic studies that test the sufficiency of microbial products to reverse germ-free traits often uncover familiar molecules that are the detritus of microbial existence, such as the examples of peptidoglycan fragments cited above or the fermentation byproduct butyrate that promotes regulatory T cell differentiation in the murine colon [63]. We continue to learn the extent to which host cells monitor such microbial metabolites, not only with canonical PRRs, but also with repertoires of G protein-coupled receptors and orphan hormone receptors [64]. Even in the example of a specific secreted bacterial protein, BefA, that reverses the paucity of pancreatic beta cells in germ-free zebrafish, the producing Aeromonas bacteria do not appear to benefit from manipulating their hosts’ pancreas function [65]. Instead we speculate that BefA is a MACA, providing a competitive advantage against other resident bacteria while non-specifically stimulating beta cell proliferation programs through crosstalk with MACA sensing pathways in the host. In the MACA-informed view of host–microbiota mutualism, resident microbes are engaged primarily in the struggles of competing with other microbes across the ecological landscape of host tissues. A small number of these microbes will elaborate competitive strategies familiar to us as the hallmarks of pathogenesis [66], often coopting the weapons of microbial warfare, such as secretion systems and poreforming molecules, to target host cells and invade host tissues. However, many more will exert limited impacts on their hosts, but their presence will be sensed in subtle ways as the consequence of their competitive activities. Host cellular and tissue responses to these microbial cues will be tailored and amplified by host-derived MACA mimics that engage in similar competitive activities as their microbial counterparts, such as membrane attack and nutrient sequestration, and stimulate similar host response programs of defense and repair. We may perceive such responses as either healthful or pathological, but from the microbes’ perspective, they are simply the opportunities and challenges afforded by living on host tissues among many other microbes.
Conflict of interest Nothing declared.
Acknowledgements We thank members of the Guillemin lab for helpful discussions. Research reported in this publication was supported by the National Institute of General Medical Sciences of the National Institutes of Health under award number 1P01GM125576 (to K.G.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Current Opinion in Microbiology 2020, 54:87–94
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40. Kramer J, Ozkaya O, Kummerli R: Bacterial siderophores in community and host interactions. Nat Rev Microbiol 2019 http:// dx.doi.org/10.1038/s41579-019-0284-4. [Epub ahead of print]. 41. Mukherjee S, Hooper LV: Antimicrobial defense of the intestine. Immunity 2015, 42:28-39. 42. Swanson KV, Deng M, Ting JP: The NLRP3 inflammasome: molecular activation and regulation to therapeutics. Nat Rev Immunol 2019, 19:477-489. 43. Gong T, Liu L, Jiang W, Zhou R: DAMP-sensing receptors in sterile inflammation and inflammatory diseases. Nat Rev Immunol 2020, 20:95-112. 44. Hancock RE, Haney EF, Gill EE: The immunology of host defence peptides: beyond antimicrobial activity. Nat Rev Immunol 2016, 16:321-334. 45. McHugh BJ, Wang R, Li HN, Beaumont PE, Kells R, Stevens H, Young L, Rossi AG, Gray RD, Dorin JR et al.: Cathelicidin is a "fire alarm", generating protective NLRP3-dependent airway epithelial cell inflammatory responses during infection with Pseudomonas aeruginosa. PLoS Pathog 2019, 15:e1007694 This study shows how a Cathelicidin, a MACA mimic, amplifies protective immune responses to Pseudomonas aeruginosa infection by activating NLRP3. 46. Shin JH, Seeley RJ: Reg3 Proteins as gut hormones? Endocrinology 2019, 160:1506-1514. 47. Hooper LV, Stappenbeck TS, Hong CV, Gordon JI: Angiogenins: a new class of microbicidal proteins involved in innate immunity. Nat Immunol 2003, 4:269-273. 48. Evavold CL, Ruan J, Tan Y, Xia S, Wu H, Kagan JC: The pore forming protein gasdermin D regulates interleukin-1 secretion from living macrophages. Immunity 2018, 48:35-44.e6 This study reports how Gasdermin D activation in macrophages can result in a state of hyperactivation, in which the cells persist and secrete Interleukin-1 through Gasdermin D membrane pores. 49. Lieberman J, Wu H, Kagan JC: Gasdermin D activity in inflammation and host defense. Sci Immunol 2019, 4. 50. Ding J, Wang K, Liu W, She Y, Sun Q, Shi J, Sun H, Wang DC, Shao F: Pore-forming activity and structural autoinhibition of the gasdermin family. Nature 2016, 535:111-116. 51. Ni T, Gilbert RJC: Repurposing a pore: highly conserved perforin-like proteins with alternative mechanisms. Philos Trans R Soc Lond B Biol Sci 2017, 372.
34. Granato ET, Meiller-Legrand TA, Foster KR: The evolution and ecology of bacterial warfare. Curr Biol 2019, 29:R521-R537.
52. Mineo A, Fuentes E, Furriols M, Casanova J: Holes in the plasma membrane mimic torso-like perforin in torso tyrosine kinase receptor activation in the drosophila embryo. Genetics 2018, 210:257-262 This study demonstrates that loss of torso-like, a developmental patterning gene in Drosophila that encodes a MACPF pore-forming protein, can be rescued by mechanical introduction of membrane holes in the early embryo.
35. Rudkin JK, McLoughlin RM, Preston A, Massey RC: Bacterial toxins: offensive, defensive, or something else altogether? PLoS Pathog 2017, 13:e1006452.
53. Reiss K, Bhakdi S: Pore-forming bacterial toxins and antimicrobial peptides as modulators of ADAM function. Med Microbiol Immunol 2012, 201:419-426.
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54. Ruhl S, Shkarina K, Demarco B, Heilig R, Santos JC, Broz P: ESCRT-dependent membrane repair negatively regulates pyroptosis downstream of GSDMD activation. Science 2018, 362:956-960 This study demonstrates that ESCRT-mediated exocytosis functions to repair membrane damage and stave off pyroptosis induced by Gastermin D activation. 55. Zackular JP, Chazin WJ, Skaar EP: Nutritional immunity: S100 proteins at the host-pathogen interface. J Biol Chem 2015, 290:18991-18998. 56. Harman JL, Loes AN, Warren GD, Heaphy MC, Lampi KJ, Harms MJ: Evolution of multifunctionality through a pleiotropic substitution in the innate immune protein S100A9. bioRxiv 2019. 57. Xiao X, Yeoh BS, Vijay-Kumar M: Lipocalin 2: an emerging player in iron homeostasis and inflammation. Annu Rev Nutr 2017, 37:103-130. 58. Charkoftaki G, Wang Y, McAndrews M, Bruford EA, Thompson DC, Vasiliou V, Nebert DW: Update on the human and mouse lipocalin (LCN) gene family, including evidence the mouse Mup cluster is result of an "evolutionary bloom". Hum Genomics 2019, 13:11. 59. Rolig AS, Sweeney EG, Kaye LE, DeSantis MD, Perkins A, Banse AV, Hamilton MK, Guillemin K: A bacterial immunomodulatory protein with lipocalin-like domains facilitates host-bacteria mutualism in larval zebrafish. eLife 2018, 7
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This work reports the discovery of a secreted bacterial protein that is a structural homologue of lipocalins and reduces intestinal inflammation in the zebrafish intestine. 60. Johnson KV, Foster KR: Why does the microbiome affect behaviour? Nat Rev Microbiol 2018, 16:647-655. 61. Cheesman SE, Guillemin K: We know you are in there: conversing with the indigenous gut microbiota. Res Microbiol 2007, 158:2-9. 62. Jones TA, Guillemin K: Racing to stay put: how resident microbiota stimulate intestinal epithelial cell proliferation. Curr Pathobiol Rep 2018, 6:23-28. 63. Furusawa Y, Obata Y, Fukuda S, Endo TA, Nakato G, Takahashi D, Nakanishi Y, Uetake C, Kato K, Kato T et al.: Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells. Nature 2013, 504:446-450. 64. Fischbach MA, Segre JA: Signaling in host-associated microbial communities. Cell 2016, 164:1288-1300. 65. Hill JH, Franzosa EA, Huttenhower C, Guillemin K: A conserved bacterial protein induces pancreatic beta cell expansion during zebrafish development. eLife 2016, 5 This study reports the discovery of a secreted bacterial protein that stimulates pancreatic beta cell proliferation in the developing zebrafish. 66. Finlay BB, Falkow S: Common themes in microbial pathogenicity. Microbiol Rev 1989, 53:210-230.
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ScienceDirect Patterns of partnership: surveillance and mimicry in host-microbiota mutualisms Travis J Wiles1 and Karen Guillemin1,2 The repertoire of microbial cues monitored by animal and plant tissues encompasses not just molecules but also microbial activities. These include typical pathogen strategies of injuring membranes, degrading cellular material, and scavenging resources. These activities, however, are not exclusive to pathogens. Instead, they characterize the competitive strategies of microbes living in multispecies communities, like those typically found colonizing host tissues. Similar activities are also deployed by host tissues to keep microbes in check. We propose that host surveillance and mimicry of MicrobialAssociated Competitive Activities (MACAs), derived from an evolutionary history of living in mixed microbial communities, has shaped contemporary animal and plant tissue programs of defense, repair, metabolism, and development. Addresses 1 Institute of Molecular Biology, University of Oregon, Eugene, OR, USA 2 Humans and the Microbiome Program, CIFAR, Toronto, ON, Canada Corresponding author: Guillemin, Karen ([email protected])
Current Opinion in Microbiology 2019, 48:87–94 This review comes from a themed issue on Stanley Falkow alumni Edited by Denise Monack and Igor Brodsky
https://doi.org/10.1016/j.mib.2020.01.012 1369-5274/ã 2020 Elsevier Ltd. All rights reserved.
Stanley Falkow made many seminal contributions to the field of bacterial pathogenesis, not least of which was to continually remind us to think from the point of view of the microbes [1]. As researchers studying microbe–host interactions, it is all too easy to fall into the trap of formulating hypotheses based on unconscious, preconceived biases about the primacy of the host in these relationships. Stan’s words remind us of the limitations of our myopic human perspectives. One human-centric proclivity is to bin microbes into discrete categories. We classify them as pathogens, pathobionts, commensals, or mutualists, imposing our value systems on them. These assignments can be useful for characterizing the epidemiology of infectious agents and the potential for pathological outcomes of interactions www.sciencedirect.com
with certain microbes [2]. But by many measures, the microbes that we narrowly label as pathogens and mutualists actually share common strategies of engaging with us [3]. Stan famously said that he never met a microbe he didn’t like [4], and he even proposed that pathogens may be good for our health [5]. He emphasized the pointlessness of such rigid microbial categorization: “It really doesn’t matter how we define a pathogen! To underestimate the evolutionary potential of microorganisms and their ability to survive, even in the face of enormous pressure to eradicate them, would be a mistake” [4]. Here again he was urging us to think like a microbe, for which such human-centric categorizations are irrelevant in the face of broader ecological challenges to compete and survive. Our insistence on imposing these categories is a natural extension of our human desire to make sense of our complicated relationships with microbes, which range from acute life-threatening infections to life-long physiological and developmental dependencies. A human-centric view assumes that microbes make products specifically designed to manipulate us: pathogens inject nefarious toxins to make us ill while mutualists proffer soothing salves to promote our health. With the recent explosion of information about the complex microbial communities that inhabit our bodies, it becomes increasingly untenable to imagine that microbial-based diseases can be explained as binary interactions between individual microbes and ourselves. Indeed, Stan recognized that many of these diseases are the consequence of changing microbial communities responding to altered human ecology [6]. In this review, we attempt to rethink, from a microbial perspective, the origins and nature of pathogenic and salubrious microbial products that host cells detect and respond to during intimate encounters. When we take our human selves out of the center of the picture, we begin to see that the microbial molecules that exert their influences on us are not exclusively designed for manipulating or communicating with us. Rather, we hypothesize that the microbial molecules that elicit responses in us are the signatures of cellular competitions for resources and space that originated from a much more ancient and expansive history of microbes living and competing with other microbes.
Molecular signatures of microbes: from PAMPs to MAMPs To understand the microbial molecules that elicit responses in hosts like ourselves, we need to understand the receptors Current Opinion in Microbiology 2020, 54:87–94
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of these molecules. Taking into account seminal discoveries showing that Toll-like Receptor 4 (TLR4) recognizes the Gram-negative cell wall component lipopolysaccharide (LPS) [7], Charles Janeway and colleagues formulated the concept of Pattern Recognition Receptors (PRRs) that detect conserved molecules they termed Pathogen-Associated Molecular Patterns (PAMPs) [8]. Other examples of PAMPs include viral-associated nucleic acids, fungal-associated sugars, and additional bacterial cell wall components such as peptidoglycan. Sensing and responding to these microbial signatures makes sense from the perspective of defending against infectious threats. However, although these molecules are characteristic of specific classes of microbes, they are not exclusive to disease-causing organisms. Margaret McFall-Ngai and colleagues provided a compelling example, in the symbiosis between the Hawaiian bobtail squid and the luminescent bacterium Vibrio fischeri, of the peptidoglycan monomer known as tracheal cytotoxin (TCT) serving to stimulate host tissue responses essential for the establishment of the partnership [9]. At the tissue level, TCT elicits changes in the squid light organ epithelium reminiscent of tracheal epithelial damage induced by the pathogen Bordetella pertussis, for which TCT was named, but at the organismal level, this tissue regression is beneficial for the symbiosis. In accordance with this finding, they proposed renaming ‘PAMP’ as MicrobialAssociated Molecular Pattern or ‘MAMP’ to remain agnostic about the impacts of perception of these kinds of molecules by host tissues. MAMP is an appropriate term to use when considering that the last common eukaryotic ancestor hunted bacteria as food [10]. The PRR repertoire of this ancestor would have included proteins with shared domains of animal and plant PRRs, such as Toll/interleukin-1 receptor/resistance protein (TIR) and leucine rich repeats (LRR) domains [11]. Extant single-celled eukaryotes serve as useful models of ancient bacterial–eukaryotic interactions and display a diversity of relationships. For example, the social amoeba Dictyostelium preys on bacteria, experiences bacterial infections, and in its multicellular slug form, harbors an endogenous microbiota that increases its capacity for food bacteria carriage [12,13,14]. Dictyostelium uses a TIR domain protein to sense LPS [11] and responds to Gram-positive bacteria by deploying distinct suites of genes, such as alyL encoding the peptidoglycan cleaving enzyme, lysozyme, which it requires to grow specifically on Gram-positive bacteria [15]. Another bacterivore and the closest singlecelled ancestor of animals, choanoflagellates form multicellular structures in response to specific bacterial lipids [16]. These examples highlight how MAMP sensing has been integrated into fundamental aspects of eukaryotic growth and development before the evolution of multicellular hosts. Current Opinion in Microbiology 2020, 54:87–94
In animals, we now can point to numerous examples of MAMPs and PRRs playing important roles in normal development, such as peptidoglycan-mediated maturation of the immune and nervous systems [17,18]. In the developing zebrafish, innate immune sensing via Myd88, a common adaptor for the TLRs and interleukin-1 receptor, functions in establishing rates of intestinal epithelial cell proliferation [19], increasing mucus secreting versus absorptive cell types [20], and promoting enterocyte maturation [21]. These examples demonstrate how animals benefit from monitoring their microbial status to make developmental decisions (e.g., by proceeding with light organ maturation only after symbiont colonization) and to fine-tune tissue homeostasis (e.g., by increasing epithelial turnover and mucosal fortification and lubrication in the presence of microbes). Intriguingly, some PRRs have been coopted into developmental roles independent of microbial sensing, such as the founding Toll receptor that regulates Drosophila dorsal–ventral patterning. Not only do hosts monitor microbes for more than defense against infection, but they have intricate systems for spatially regulating MAMP sensing. For example, in the murine gastrointestinal tract, different TLRs are expressed in different proximal–distal regions and cell types and at different developmental stages [22]. In the zebrafish intestine, transcription of the brush border enzyme, intestinal alkaline phosphatase, is induced by the MAMP LPS, and the upregulated enzyme in turn serves to dephosphorylate LPS in the intestinal lumen, rendering it a less potent stimulator of innate immune signaling [21]. This negative feedback loop of LPSstimulated detoxification of LPS is spatially segregated to the gut lumen, the site of innocuous resident bacteria. Thus, hosts exert spatial control over both the location of MAMP sensing and MAMP availability.
More than collections of molecules: signatures of microbial activities and the MACA hypothesis The importance of context in sensing microbes is a point that Russel Vance, Ralph Isberg, and Dan Portnoy emphasized in ‘Patterns of Pathogenesis’ [23]. In this review, they expanded the concept of host sensing of microbes from conserved molecules to conserved microbial activities “that live pathogens use to invade, manipulate, replicate within, or spread among their hosts”. They explored how different cellular sensors of microbial products provide contextual information on microbial activities. For example, detection of extracellular bacterial flagellin by cell-surface localized TLR5 elicits a different host response than sensing of intracellular flagellin, a hallmark of invading bacteria, by the cytoplasmic nucleotide-binding domain leucine-rich repeat (NLR) family member, NAIP5. www.sciencedirect.com
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Emerging mechanistic insights into the NLR family of innate immune sensors reveal how they are poised to sense perturbations of cellular processes frequently targeted by pathogens [24,25]. Once cells are primed by cues such as MAMPs, the NLRs can induce the assembly of multiprotein inflammasome complexes upon sensing certain microbial activities. The plant immunity field provides multiple examples of NLR family ‘guard’ proteins that surveil the activity of bacterial effector proteins, and similar examples are emerging from animal cells [26]. For example, by sensing its own partial degradation, the NLRP1B inflammasome detects abnormal levels of proteolysis caused by bacterial proteases or effectors that activate the proteosome [27,28]. Similarly, NLRP3 senses efflux of potassium ions as a hallmark of membrane disruption caused by diverse insults such as bacterial pore-forming toxins or secretion systems [29,30].
nutrient-rich contents of cells, is called into question by various lines of investigation [35,36]. Contrary to expectations, strain cytotoxicity of Staphylococcus aureus clinical isolates correlates with asymptomatic carriage and inversely correlates with tissue invasion [37,38]. At a cellular level, Staphylococcus pore-forming toxins can promote biofilm formation, possibly by lysing neighboring bacterial cells to release material such as DNA for biofilm matrices [35]. Another class of cytolytic molecules secreted by Staphylococcus spp., surfactant-like phenolsoluble modulins, may function primarily as antimicrobials against other members of the nasal epithelium rather than to destroy host tissue [35]. In this expanded view of microbial surveillance, host cells are more often innocent bystanders bracing themselves against the collateral damage of intense microbial warfare than they are suffering as targets of pathogenic attackers.
The idea of immune detection of microbial activities provides a powerful framework for understanding host– microbe interactions, but as with the PAMP concept, it falls short with the assumption that these activities are exclusive to pathogens. For example, although bacterial type 3 secretion systems have been studied extensively in pathogens, these systems are found broadly distributed across the genomes of non-disease-associated bacteria. In the Bradyrhizobium–legume symbiosis, type 3 secretion systems play crucial roles in the establishment of the partnership, for example by suppressing plant immune responses [31]. A recent study of Bradyrhizobium type 3 secretion effector proteins identified ErnA, that is targeted to plant cell nuclei and is sufficient to induce formation of nitrogenfixing nodules in plant roots [32]. Looking to single-celled eukaryotes for instances of bacterial activity sensing, choanoflagellates provide a dramatic example of responding to a secreted Vibrio chondroitinase by initiating meiosis and mating, a response that is not elicited by an inactive version of the enzyme [33].
To capture this expanded picture of host surveillance of microbial consortia, we introduce the concept of Microbial-Associated Competitive Activities or ‘MACAs’. MACAs represent the expanse of activities displayed by microbial cells to compete with other cells, both microbial and host. We specifically define MACAs as competitive activities of microbes that target fundamental cellular vulnerabilities common to prokaryotic and eukaryotic cells (Figure 1). Examples of MACAs include membrane disrupting molecules like lytic antimicrobial peptides, degradative enzymes that target essential macromolecules like peptidoglycan glycoside hydrolases [39], and nutrient scavenging molecules like siderophores that sequester scarce resources [40]. We propose that eukaryotic cells sense MACAs as part of ancient strategies for coexisting with microbial communities. Over evolutionary time, MACA sensing would have become incorporated into basic cellular processes that govern defense, repair, growth, and metabolism. In contemporary multicellular eukaryotic hosts, cell type-specific responses to MACAs could further coordinate tissue and organismal structure and function.
These examples motivate yet another expansion of our view of host microbial surveillance beyond activities exclusive to disease-causing microbes to encompass the vastly larger universe of microbes that interact with animals and plants without causing overt harm. Monitoring the activities of complex microbial consortia provides hosts with useful information not just to modulate defensive responses but also to make decisions about metabolism, growth, and development. At the molecular level, microbial monitoring involves detecting specific activities of microbes growing and competing in complex communities. Indeed, competing microbes possess diverse arsenals of contact-dependent and secreted weapons [34] that resemble prototypic pathogen weaponry known to be detected by PRRs. Bacterial Type 4 and 6 secretion systems, for example, are often deployed during interbacterial conflict. Even the host-directed nature of cytolytic toxins, presumed to allow pathogens access to www.sciencedirect.com
Signal integration: DAMPs as MACA mimics Eukaryotic cells and tissues coexisting with warring microbes would need strategies for counterattacks as well as surveillance. Closer inspection of our own repertoires of host defense molecules reveals that they are strikingly similar to MACAs, including membrane targeted antimicrobial peptides like cathelicidins, cellular material degrading enzymes like lysozymes, and nutrient scavenging molecules like transferrin. These defense strategies were found in the last common eukaryotic ancestor [11] and may share evolutionary histories with prokaryotic MACAs or may have evolved convergently to similar solutions for targeting universal cellular vulnerabilities. We refer to these activities produced by macroscopic organisms as ‘MACA mimics’. Current Opinion in Microbiology 2020, 54:87–94
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Figure 1
(a)
(b)
(c)
Current Opinion in Microbiology
MACA sensing and mimicry. (a) Primordial unicellular eukaryotes coexisted in a world of competing microbes and possess systems for sensing Microbial-Associated Competitive Activities (MACAs). (b) Contemporary multicellular animals and plants have evolved ‘MACA mimics’ to control resident microbial communities and potential pathogenic invaders at tissue surfaces. (c) Over evolutionary time, multicellular hosts have coopted MACA and MACA mimic sensory pathways to coordinate immunity and defense, healing and regeneration, and growth and development.
If our immune surveillance systems are tuned to detecting not just the molecules but also the activities of microbes, then they should also be stimulated by hostderived MACA mimics. The universality of the cellular targets of MACA and MACA mimics ensures broad protection but poses the danger of self-harm. For example, many host antimicrobial proteins that target the membranes of microbes also cause collateral damage to host cell membranes [41]. Indeed, the NLRP3 inflammasome senses attacks on membrane integrity from both Current Opinion in Microbiology 2020, 54:87–94
prokaryotic and eukaryotic molecules, including antibiotics such as nigericin from Streptomyces, bacterial pore-forming toxins such as alpha toxin from Staphylococcus, and host derived antimicrobial proteins such as cathelicidin [42]. This notion of self-inflicted damage by MACA mimics can be useful for understanding how plant and animal PPRs are activated by endogenously produced host factors, which are collectively referred to as Damage www.sciencedirect.com
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Associated Molecular Patterns or ‘DAMPs’ [43]. The list of host factors that function as DAMPs is diverse and eclectic—including, potassium ions, ATP, nucleic acid fragments, extracellular matrix fragments, antimicrobial peptides, lysozyme, calprotectin—but can be viewed broadly as the activities or products of MACA mimics. We propose that the expression patterns of MACA mimics have been evolutionarily honed to modulate host immune response pathways to maximize host fitness in response to microbes. Many genes encoding MACA mimics, such as antimicrobial proteins, are direct transcriptional targets of PRRs in response to microbial cues or tissue injury. Many of these antimicrobial proteins have also been shown to function as DAMPs and can have complex immunomodulatory activities [44]. For example, a recent study showed that upregulation of the antimicrobial peptide Cathelicidin during Pseudomonas aeruginosa infection of airway epithelia activates the NLRP3 inflammasome and leads to a protective inflammatory response against this pathogen [45]. Hinting at more complex ways in which MACA mimics have evolved to regulate tissue growth and repair, several host antimicrobial proteins were originally discovered in other contexts: the Reg3 proteins were named for their abundance in regenerating pancreatic islets [46], and the angiogenins were originally found to stimulate capillary growth in tumors [47]. Adding another layer of complexity to immune modulation by MACA mimicry, animal immune systems deploy a number of effector proteins that target host cell membranes but share a common mechanism of action with bacterial membrane perturbing proteins. A key downstream effector of inflammasome activation is the pore-forming protein Gasdermin D that, upon caspasemediated cleavage, oligomerizes in host cell membranes, allowing the release of Interleukin-1 and causing either a state of hyperactivation or a specialized form of cell death called pyroptosis [48,49]. Gasdermin family members bind a number of lipids, including the bacterialmembrane enriched cardiolipin, and will indiscriminately lyse both eukaryotic and prokaryotic cells [50], suggesting a possible additional antimicrobial function or evolutionary origin for this protein family. Other pore-forming effectors of the immune system include members of the Complement System and the killer lymphocyte cytotoxic granule protein Perforin. These proteins share a common membrane attack complex/perforin (MACPF) domain that is structurally and evolutionarily related to the cholesterol-dependent cytolysin (CDC) domain of many bacterial toxins [51]. Another MACPF protein, Torso-like, functions in Drosophila embryonic patterning through a mechanism that can be recapitulated by mechanical introduction of membrane holes [52]. Not only do host immune effectors elicit similar programs of NLRP3 activation as bacterial-derived pore-forming www.sciencedirect.com
toxins and antibiotics, but they also induce similar programs of membrane repair involving both endocytic clearance of damaged membranes and exocytic delivery of new membranes [53,54]. The MACA mimicry framework helps to uncover additional host proteins with antimicrobial activities and complex immunomodulatory functions. For example, Calprotectin inhibits bacterial growth by sequestering transition metals and also functions as a DAMP by activating TLR4 [55]. Phylogenetic reconstruction of the two S100 proteins of the Calprotectin heterodimer demonstrates that the TLR4 activation evolved more recently than the metal sequestration [56]. Calprotectin is released from neutrophil granulocytes in high abundance, making it a robust biomarker of inflammation, and suggesting that its localized deployment is useful for amplifying and refining immune signaling. Another biomarker of inflammation is the siderophore-sequestering Lipocalin 2 protein, whose complex immunomodulatory properties are only partially explained by its manipulation of resident microbiota through iron limitation [57]. In addition to binding siderophores, members of the Lipocalin family bind a number of small hydrophobic molecules with immunomodulatory properties such as steroid hormones and retinol [58]. In the zebrafish intestine, we found that Aeromonas spp. secrete an anti-inflammatory protein, AimA, with structural similarity to Lipocalins [59]. AimA does not appear to bind siderophores or function in iron acquisition for Aeromonas, but we speculate that it may sequester another limiting nutrient while simultaneously stimulating MACA-responsive pathways in the host to exert its anti-inflammatory effect.
MACAs as a framework for understanding patterns of partnership Our proposal that host immune systems continually monitor MACAs provides a new perspective for understanding our mutualism with our resident microbiota. With our human-centric biases, it is tempting to speculate that our resident microbes intentionally support us by promoting our health in exchange for their own longevity or influencing our appetites to encourage ingestion of favored nutrients. However, it is difficult to imagine how certain resident microbes could evolve specific manipulative strategies from which they alone would benefit, without other microbes freeloading on their efforts [60]. A more reasonable assumption is that resident microbes devote their energies to competing with each other, using and innovating on ancient strategies, such as membrane perturbation and nutrient scavenging, which have been coopted as information-rich cues sensed by contemporary host cells. Comparisons between germ-free and conventionally reared animals have taught us that the impacts of resident microbes are subtle but specific [61]. When viewed as part Current Opinion in Microbiology 2020, 54:87–94
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of a continuum of host immune activation, host responses to resident microbes resembles a muted stimulation of tissue damage and repair programs. Colonization of the germ-free vertebrate intestine, for example, stimulates intestinal epithelial cell proliferation and immune cell influx, both responses seen to a greater extent with enteric pathogens [62]. Gnotobiotic studies that test the sufficiency of microbial products to reverse germ-free traits often uncover familiar molecules that are the detritus of microbial existence, such as the examples of peptidoglycan fragments cited above or the fermentation byproduct butyrate that promotes regulatory T cell differentiation in the murine colon [63]. We continue to learn the extent to which host cells monitor such microbial metabolites, not only with canonical PRRs, but also with repertoires of G protein-coupled receptors and orphan hormone receptors [64]. Even in the example of a specific secreted bacterial protein, BefA, that reverses the paucity of pancreatic beta cells in germ-free zebrafish, the producing Aeromonas bacteria do not appear to benefit from manipulating their hosts’ pancreas function [65]. Instead we speculate that BefA is a MACA, providing a competitive advantage against other resident bacteria while non-specifically stimulating beta cell proliferation programs through crosstalk with MACA sensing pathways in the host. In the MACA-informed view of host–microbiota mutualism, resident microbes are engaged primarily in the struggles of competing with other microbes across the ecological landscape of host tissues. A small number of these microbes will elaborate competitive strategies familiar to us as the hallmarks of pathogenesis [66], often coopting the weapons of microbial warfare, such as secretion systems and poreforming molecules, to target host cells and invade host tissues. However, many more will exert limited impacts on their hosts, but their presence will be sensed in subtle ways as the consequence of their competitive activities. Host cellular and tissue responses to these microbial cues will be tailored and amplified by host-derived MACA mimics that engage in similar competitive activities as their microbial counterparts, such as membrane attack and nutrient sequestration, and stimulate similar host response programs of defense and repair. We may perceive such responses as either healthful or pathological, but from the microbes’ perspective, they are simply the opportunities and challenges afforded by living on host tissues among many other microbes.
Conflict of interest Nothing declared.
Acknowledgements We thank members of the Guillemin lab for helpful discussions. Research reported in this publication was supported by the National Institute of General Medical Sciences of the National Institutes of Health under award number 1P01GM125576 (to K.G.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Current Opinion in Microbiology 2020, 54:87–94
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