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Improving the control of Listeria monocytogenes in cold smoked salmon
Trends in Food Science & Technology 10 (1999) 211±216
Review
Improving the control of Listeria monocytogenes in cold smoked salmon FreÂdeÂrique Dues INRA, Laboratoire de Recherches de Technologie LaitieÁre, 65 rue de Saint-Brieuc, 35042 Rennes cedex, France ( fax: +33-2-99-28-53-50; e-mail: du[email protected]). Smoked salmon is no longer a luxury product and its consumption has spread beyond special occasions. Unfortunately, this food is highly susceptible to contamination with Listeria monocytogenes. The processing steps sensitive to potential L. monocytogenes contamination, the current and future methodologies to ®ght against such ¯ora are presented with a particular emphasis on the ecacy of anti-listerial peptides. # 1999 Elsevier Science Ltd. All rights reserved.
The international market for smoked salmon was 32,173 tons in 1992, compared with the world salmon catch of 1 million tons, with 28 exporting and 52 importing countries [1]. France, with a production of 12,380 t in 1997 (turnover of 2.5 billion francs), realized by 35 operators, is the world's leading country for processing salmon. Between 1980 and 1995, price was reduced two-fold and consumer demand increased up to six-fold. Consumer demand pushed the industry in France, in 1992, towards a quality charter named `Label Rouge'. Cold smoked salmon (CSS) is one of the most perishable food products. Determination of shelf life is the responsibility of the manufacturers. In France, a maximum of three weeks from the packing date is recommended.
In some countries, it could be eight weeks. Quality depends on raw material, processing and storage conditions. Fish processing was initially designed to enhance the shelf life but changes in consumer taste led to a lighter smoking and salting which allows microbial development and growth. Spoilage of CSS is mainly the result of microbial activities leading to changes in the structure of the ¯esh and o odours. Pathogenic bacteria, such as L. monocytogenes, are frequently isolated (10%) [2, 3] and CSS must be considered as a potential source of infection [4]. The ®ght against Listeria is of special safety and economic concern. Common methods are reviewed with a special emphasis on the promising use of bacteriocins produced by lactic acid bacteria (LAB). Emergence of bacteriocin-resistant cells is also considered.
Cold-smoked salmon
Salmon used could be received raw or frozen with head and viscera removed or not. Salting (2±4%) contributes to eliminating part of the constituent water but also ®rms up the ¯esh, in¯uences texture, taste and prevents bleaching of the product. Dierent salting methods exist, according to dierent countries, using dried salt (kench curing with sodium chloride or nitrated salt), brine (70±80% NaCl) or injected brine. Drying is carried out only before cold smoking to decrease water concentration. Three types of process are used: air or contact drying, vacuum drying or freeze-drying. The smoking step gives the speci®c colour and taste to the product. Traditionally, ®llets are exposed to smoke produced by pyrolysis of wood or sawdust (beech or oak). The ¯esh dehydrates and becomes impregnated with volatile compounds like phenols (guaiacol and eugenol responsible for the taste; syringol responsible for the typical odour), acids, aldehydes, alcohol and hydrocarbons [5]. Hot smoking (60 C) is practised in north European countries (Germany and Poland), whereas cold smoking (20±24 C) is practised in France and gives a more tender and soft texture. There are three methods for smoking: the traditional way, by combustion; by using an electric ®eld which accelerates the smoke deposition or by atomization of a commercial smoke-aroma concentrate solution. Fish is vacuum packed in the form of ®llets, cubes or slices. The packaging has to be watertight. No colouring or preservatives are allowed in France.
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The resulting composition of CSS is: 65±78% water, 8±14% lipids, 16±22% proteins; 2±4.6% salt in the water phase and 0.2±1.5 mg of phenols per 100 g of ¯esh. Immediately after packing, bacterial counts were 103± 104 cfu/g. At this time, microbial ¯ora is typical of fresh ®sh ¯ora with predominance of Gram negative (64%) bacteria such as Shewanella putrefaciens and Aeromonas spp. LAB are also present (32%) with a majority of Carnobacterium spp. [6]. During storage, the ratio between these two ¯oras is inverted and among LAB, growth of lactobacilli is signi®cant. Most of the studies are based on quantitative determination at the rejection time. Total viable ¯ora grows from 103±104 to 107±108 cfu/g in three weeks depending on storage conditions [6±8]. Photobacterium phosphoreum has been described as the speci®c spoilage organism leading to H2S and methylmercaptan production [7], but the involvement of LAB in spoilage has been also suggested [6, 8]. Indeed, LAB predominates at the end of storage [5] whereas H2S-producing bacteria remain at a population level of 1.5 log units less. Among these LAB, the contribution of lactobacilli and carnobacteria to spoilage is subject to controversy. Truelstrup±Hansen [5] found that Lactobacillus sake, in particular, produced o-odours like `sour', `cabbage' and `sulphurous'. Carnobacterium piscicola is described as a spoilage organism because it is associated with ®sh intestine or disease [9] and is recognized as a tyramine producer [10], but did not accelerate the spoilage when inoculated on CSS at 106±107 cfu/g [11] and improved the sensory quality during vacuumpacked storage at 4 C [12]. Volatile compound production is closely related to microbiological contamination. Leroi et al. [6] showed an increase from 3.5 to 17 mg-N per 100 g in three weeks at 8 C which can be linked to S. putrefaciens and P. phosphoreum growth. Total volatile basic nitrogen (TVBN) increased from 15.5 to 53 mg-N per 100 g in the same conditions. No norms exist for CSS, but for fresh salmon no more than 35 mg of TVBN are accepted. Biogenic asinine production is also the result of microbial activity. Only histamine is controlled in tuna ®sh (it must be lower than 100 mg/g [13]), because of its frequent implication in foodborne intoxication. But the presence of high quantities of tyramine (80±230 mg/g), cadaverine (16±700 mg/g), putrecine (50±180 mg/g), histamine (4±190 mg/g), spermine (0±60 mg/g) and spermidine (3±20 mg/g) has been reported and must be taken into account. Microbial analysis is the only method that allows the speci®c detection of pathogens. Legislation tolerates: 106 cfu/g of aerobic microorganisms, 1 cfu/g Staphylococcus aureus and the absence of Coliforms, Salmonella and sulphite-reducing microorganisms [14]. Listeria is not subject to any norm, but fewer than 100 cfu/g are recommended.
Origin of L. monocytogenes contamination and consequences
The overall incidence of L. monocytogenes in smoked ®sh is 10%, with levels below 100 cfu/g [15], and vary between 0% in Scottish samples to 50% in American, Chilean and Norwegian samples [16]. Despite this high rate of contamination, clinical listeriosis in humans is rarely associated with ®sh products. This has been explained by the fact that there is no genetic relationship between L. monocytogenes clones isolated from ®sh and those isolated from human listeriosis [17]. However, L. monocytogenes is of concern in cold smoked ®sh, in particular since one outbreak of listeriosis caused by ®sh was reported [4, 18] in Sweden, nine cases of listeriosis incriminating rainbow trout resulted in two deaths in Finland, victims fell ill with gastroenteritis. Moreover, in the USA, 23.7% of foodstu toxi-infections were due to the presence of L. monocytogenes in seafood products during the 1980s. The ®sh origin (wild or breeding), ®shing season, ®shing technique (trawling or angling), handling, storage conditions (refrigerated or frozen) determine initial contamination. In France, in 1995, raw material (Salmo solar L) came from Norway (70%), Scotland (15%), USA±Canada (10%), Ireland and Chile (5%) with a large proportion of breeding origin. The origin of the salmon is critical: Listeria species are detected in 81% of fresh water and in 30% of marine water samples including 62% of L. monocytogenes [19]. This contamination comes from sewage euents, faecal contamination by animals and run-o from agricultural land [20]. Therefore, the initial and most important source of contamination in processing plants, is the surface (skin, mucus, tail, head) and intestinal cavity of the ®sh at the time of decapitation, peeling and ®lleting. Processing equipment, personnel rotation have been described as a secondary source of contamination [21]. Costs of foodborne illness caused by L. monocytogenes, present in dierent foods, are estimated at 300 billion dollars [22] in 1997 in the USA. No outbreaks has been reported due to CSS consumption but rainbow trout has previously been associated with cases of listeriosis [4, 18]. Recalls of seafood have been reported in north America (smoked Nova Scotia salmon in December 1988 in Washington; smoked salmon in August 1989 in Washington; smoked salmon products in September 1989 in Illinois) and in Europe (smoked salmon Fjord King in December 1998 in France). For the industry, the consequences of Listeria contamination are recalls, destruction, cleaning, adverse publicity and the resulting decrease in consumption leading to the closure of factories.
Behaviour of L. monocytogenes during CSS processing
During the transport and the thawing procedure, part of the vascular system is exposed to water from containers
F. Dues / Trends in Food Science & Technology 10 (1999) 211±216
and thawing tanks which are contaminated in 18.6% of cases with L. monocytogenes [21]. During salmon ®lleting, L. monocytogenes are transferred from the ¯esh to cut surfaces, slicing machines and tables contaminating 60% of these apparatuses, which become potential sources of contamination. Salting selects gram negative, halophilic anaerobic rods and a water activity of 0.95 does not inhibit L. monocytogenes development [23]. Brining, by soaking or injection, is a risky method because baths and needles are re-used. No new contamination occurs at the smoking step, but cold processing does not eliminate the initial contamination. Cold-smoked processing results in 11.3± 17.5% of samples contaminated whereas only 1.5±8.4% of the hot-smoked salmon samples were L. monocytogenes positive [2, 24]. Liquid smoke (0.2 mg phenols per 10 g total deposition) had a signi®cant eect on L. monocytogenes growth [25], whereas solid smoke was less ecient [26]. Refrigeration and vacuum storage do not guarantee prevention of L. monocytogenes growth. Listeria monocytogenes is able to grow from 105 to 109 cfu/g at 12 C and from 104 to 109 or 105 cfu/g at 6 and 0 C, respectively, on craw®sh tail suggesting that chilled temperature storage is not sucient to prevent L. monocytogenes growth [27]. These authors also showed that freezing of the ®nished product did not kill L. monocytogenes and did not aect its development later.
Wrestling with L. monocylogenes
Because current legislation forbids the use of any additive, antioxidant or antibacterial chemical substances, research on L. monocytogenes inhibition is currently based on the use of dierent processing parameters. The use of inhibitory LAB, which are naturally present on the ®sh, is a new opportunity. At 10 C, 6% NaCl reduced the L. monocytogenes count for one log after 30 days of storage under O2permeable conditions whereas no signi®cant eect of NaCl was observed under vacuum packaging, compared with the control [28]. At 4 C, the presence of increasing salt concentration (3±6%) reduces L. monocytogenes for 2±5 logs under permeable conditions. Unfortunately, this amount of salt is not acceptable for consumers. SunÄen [26] has determined the minimum inhibitory concentration of liquid and solid smoke wood extracts. The ®rst result was the con®rmation that liquid was more ecient than solid smoke because of its capacity to penetrate the ¯esh. The second one was that the more active smokes showed the highest phenol concentrations. The last and encouraging result was that, even if not all the liquid smoke tested were eective against L. monocytogenes, the ecient ones were active at levels recommended by the manufacturers and then organoleptically acceptable to consumers.
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The room for manoeuvre on salt concentration and smoking methods is very limited because of consumer requirements. Other methods are being considered and tested to be submitted for legislative approval. Irradiation with 2 kGy reduced L. monocytogenes counts from 104 to less than 100 cfu/g, but this is the upper limit after which sensory quality changes are noticed [29]. Monoglycerides (MC12) slow down L. monocytogenes growth on seafood [30]. The MC12 activity was enhanced by the addition of propyl gallate (200 mg/g), an antioxidant [31], but to our knowledge, no study has been carried out on salmon. Organic acids eective against L. monocytogenes are not much used because of their insolubility, their eect on colour and organoleptic properties of the product [32]. Because many bacteriocins, especially those from LAB, which are commonly used in food, are active against L. monocytogenes and inactive against mammalian cells, there is an increasing interest in their use as biopreservatives. FDA regulates the use of bacteriocins in food in the USA. Requirements for safety assessment are listed in Table 1. Nisin is a bacteriocin produced by Lactococcus lactis. It is the only bacteriocin to receive international acceptance to be used as an additive in some foods (in 1969, Nisaplin1, UK). Though if nisin is not yet usable on CSS, Wessels and Huss [33] tested L. monocytogenes inhibition by insitu bacteriocin-producing strains in CSS. Unfortunately, the nisin-producer Lactococcus lactis ATCC 11454, did not grow on CSS slices at 10 C, resulting in a 1.5 log growth of L. monocytogenes. Nilsson et al. [34] reported that nisin (1000 IU/g) action was enhanced by a CO2-atmosphere: two log reduction on CSS stored at Table 1. FCA requirements for bacteriocin use in food industry No. 1 2 3
4 5
6 7
1
Requirements Bacteriocins or their producers must be characterized and their GRAS status must be established Bacteriocin metabolism by bacteria and catabolism in human tracts must be known Puri®ed or semi-puri®ed bacteriocin production must be detailed, reproducible, guarantee a percentage of purity, absence of toxicity and information must be provided about speci®c manipulations (recombinants, mutants. . .) Intended use levels should be determined and explained compared with natural levels in food if applicable The desired eect must be precise (inhibition of pathogens, spoilage organisms, shelf life increase) and the eectiveness of the bacteriocin should be demonstrated and standardized under typical use conditions The food concerned must be described with its speci®city of processing and storage Secondary eects should be studied: Does it mask spoilage? Does it interact negatively with other substances? Is it toxic at high levels? Is it stable during storage? Fields [49]
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5 C compared with vacuum-packaging. Dues et al. [35] showed that Carnobacterium divergens V41 and C. piscicola V1 were two eective strains in a simulated cold smoked salmon system stored at 4 C, inhibiting L. monocytogenes as early as day 4. Carnobacterium piscicola V1 was bactericidal against L. monocytogenes on sterile and commercial vacuum-packed cold-smoked salmon at 4 and 8 C, while C. divergens V41 presented a bacteriostatic eect. Moreover, no product spoilage could be observed with the use of such bacteriocinproducing strains as demonstrated by good sensorial analyses and low biogenic amine production. Crude extracts of piscicocins were bactericidal at 4 and 8 C. Listeria growth was delayed by divercin V41 at 8 C and was inhibited at 4 C [36]. In Denmark, Nilsson et al. [37] had also shown that C. piscicola A9b had a bacteriostatic eect on L. monocytogenes growth without adversely aecting the sensory quality of CSS. Carnobacterium piscicola strains were promising candidates for L. monocytogenes control on CSS at chilled temperatures. Factors limiting bacteriocin eectiveness in foods are due to the bacteriocin characteristics (structure, conditions of maximum activity), bacteriocin producers (ability to grow and produce the bacteriocin at speci®c conditions of temperature, atmosphere. . .) or food (liquid or solid, content of lipids, carbohydrates. . .). When bacteriocin is dropped, it could be inactivated by food components (proteases, lipids, other microorganisms. . .) or its activity could be aected during the processing (temperature, drying. . .). Bacteriocin eectiveness depends on the population of target cells: how many molecules of bacteriocins are required to cause the bacterial death of a cell? Bacteriocin mode of action may be speci®c and target cell susceptibility more or less signi®cant. The resistance to bacteriocins of some cell is of particular interest to avoid a situation similar to antibiotic resistance.
Listeria monocytogenes resistance
Most of the studies on antibiotic resistance in human, on stress adaptation [38], on cleaning antibacterial agent eciency [39] or bacteriocin action [40] report that when L. monocytogenes are exposed to such antimicrobial agents, adapted cells appear. An increase and continued use of these antibacterial compounds may select resistant germs or cause mutations that would impair their eciency. The development of acquired or spontaneous nisin resistance has been reported for S. aureus, L. plantarum, Streptococcus thermophilus, and several species of Bacillus and, recently, for L. monocytogenes under laboratory conditions. Fortunately, spontaneous bacteriocin resistance in food-borne bacteria is rarely reported. This could be explained by the fact that, in food, L. monocytogenes has to exceed not only bacteriocin activity but also additional stresses, such as nutrient availability, NaCl content, pH, storage
temperature and the presence of other microorganisms. Resistance is of special concern for antibiotherapy and foodstu security. Poyar-Salmeron et al. [41] showed that movable genetic elements, such as plasmids and transposons, are responsible for the emergence of drug resistance in L. monocytogenes. In the case of bacteriocins, the mechanisms of resistance are not well known. Most of the studies concern nisin and are descriptive. Nevertheless, whatever the bacteriocin considered, some results are already known, such as mutation frequency, maintained between 10ÿ4 and 10ÿ7 [39, 42] in laboratory media. However, the stability of the resistance may be contradicted since Rekhif et al. [40] did not observe reversion of the resistance after several successive cultures in the absence of bacteriocins (mesentericin 52, curvaticin 13 and plantaricin 19). On the contrary, Dykes and Hastings [43] found a reversion frequency within the range of 10ÿ4± 10ÿ5 for leucocins A, B and E and sakacin A. Another feature is that there is no cross-resistance reported between lantibiotics (nisin) and class II bacteriocins (mesentericin, curvaticin, plantaricin, divercin, piscicocins. . .) suggesting that these bacteriocins should have dierent modes of action. Resistance could be due to changes in the outer membrane composition of target cells, especially lipids [44]. Cell wall composition [45], cell surface hydrophobicity [46] or di- and trivalent cations (Ca2+, Mg2+ and Gd3+), which may interact with the negative charges of phospholipid and cancel the electrostatic interactions between phospholipid and the bacteriocin [47], could be implicated in resistance mechanisms. In parallel to the structural modi®cation of the membrane and the cell wall, Davies et al. [46] investigated the possibility of new protein synthesis. Chloramphenicol did not adversely aect the frequency isolation of NR mutants indicating that de novo protein synthesis was not involved. However, the role of proteases, regulation or membrane proteins had to be taken into account. Rekhif et al. [40] shown that L. monocytogenes resistance was not due to bacteriocin inactivation or destruction. Robichon et al. [48] has identi®ed a new s54 factor expressed by L. monocytogenes and responsible for sensitivity to mesentericin Y105. Genes under s54 control are involved in nitrogen metabolism, in pilus production, and dicarboxylic acid transport. Mesentericin could interact with speci®c membrane proteins involved in these pathways.
Conclusion
The likelihood of L. monocytogenes contamination of smoked salmon is high. The increasing volume of production requires ecient protection. Association of in situ bacteriocin production with traditional preservation methods is promising but further studies are needed to control the emergence of potential resistant strains.
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Acknowledgements
The author thanks Rod Jeerson for proof-reading.
References 1 Codex Committee on Fish and Fishery Products, 22eÂme Session, Bergen, Norway, 6±10 May 1996 2 Jemmi, T. (1993) `Listeria Monocytogenes in Smoked Fish: An Overview' in Arch. Lebensmittelhyg. 44, 10±13 3 Johansson, T., Rantala, L., Palmu, L. and Honkanen-Buzalski, T. (1999) `Occurrence and Typing of Listeria monocytogenes Strains in Retail Vacuum-Packed Fish Products and in a Production Plant' in Int. J. Food Microbiol. 47, 111±119 4 Miettinen, M.K., Siitonen, A., Heiskanen, P., Haajanen, H., Bjorkroth, K.J. and Korkeala, H.J. (1999) `Molecular Epidemiology of an Outbreak of Febrile Gastroenteritis Caused by Listeria monocytogenes in Cold-smoked Rainbow Trout' in J. Clin. Microbiol. 37, 2358±2360 5 Truelstrup-Hansen, L. (1995) `Quality of Chilled, Vacuum Packed Cold-Smoked Salmon' in Ph.D. thesis. Danish Institute for Fisheries Research and the Royal Veterinary and Agricultural University of Copenhagen, Denmark 6 Leroi, F., Joraud, J.J., Chevalier, F. and Cardinal, M. (1998) `A Study of the Microbial Ecology of Cold-Smoked Salmon During Storage at 8 C' in Int. J. Food Microbiol. 39, 111±121 7 Donald, B. and Gibson, G.M. (1992) `Preliminary observations of the ¯ora of fresh vacuum-packed salmon steaks' in Quality Assurance in the Fish Industry, (Huss, H.H. et al., eds), pp. 107± 114, Elsevier, London 8 Gram, L. and Huss, H.H. (1996) `Microbial Spoilage of Fish and Fish Products' in Int. J. Food Microbiol. 33, 121±137 9 Toranzo, A.E., Romalde, J.L., NunÄez, S., Figueras, A. and Barja, J.L. (1993) `An Epizootic in Farmed, Market-Size Rainbow Trout in Spain Caused by a Strain of Carnobacterium piscicola of Unusual Virulence' in Dis. Aquat. Org. 17, 87±99 10 Leisner, J.J., Millan, J.C., Huss, H.H. and Larsen, L.M. (1994) `Production of Histamine and Tyramine by Lactic Acid Bacteria Isolated from Vacuum-Packed Sugar-Salted Fish' in J. Appl. Bacteriol. 76, 417±423 11 Paludan-MuÈller, C., Dalgaard, P., Huss, H.H. and Gram, L. (1998) `Evaluation of the Role of Carnobacterium piscicola in Spoilage of Vacuum- and Modi®ed-Packed Cold-Smoked Salmon Stored at 5 C' in Int. J. Food Microbiol. 39, 155±166 12 Leroi, F., Arbey, N., Joraud, J.J. and Chevalier, F. (1996) `Eect of Inoculation with Lactic Acid Bacteria on Extending the Shelf Life of Vacuum-Packed Cold-Smoked Salmon' in Int. J. Food Sci. Technol. 31, 497±504 13 Dehove, R.A. (1998) `ReÂglementation des Produits' in QualiteÂ. Repression des Fraudes. ED, Lamy. 14 NF V 45-065. (1995) Poisson transformeÂ. Saumon fumeÂ. Norme AFNOR 15 Cortesi, M.L., Sarli, T., Santoro, A., Murru, N. and Pepe, T. (1997) `Distribution and Behavior of Listeria monocytogenes in Three Lots of Naturally-Contaminated Vacuum Packed Smoked Salmon Stored at 2 and 10 C' in Int. J. Food Microbiol. 37, 209±214 16 Farber, J.M. (1991) `Listeria monocytogenes in Fish Products' in J. Food Prot. 54, 922±934 17 Boerlin, P., Boerlin-Petzold, F., Bannerman, E., Bille, J. and Jemmi, T. (1997) `Typing Listeria monocytogenes Isolates from Fish Products and Human Listeriosis Cases' in Appl. Environ. Microbiol. 63, 1338±1343 18 Ericsson, H., Eklow, A., Danielson-Tham, M.L., Loncarvic, S., Mentzing, L.O., Persson, L., Unnerstad, H. and Tham, W. (1997) `An Outbreak of Listeriosis Suspected to Have Been Caused by Rainbow Trout' in J. Clin. Microbiol. 35, 2904±2907
215
19 Colburn, K.G., Kaysner, C.A., Abeyta, C. and Welkell, M.M. (1990) `Listeria Species in California Coast Estuarine Environment' in Appl. Environ. Mcrobiol. 56, 2007±2011 20 Fuchs, R.S. and Reilly, P.J.A. (1992) `The Incidence and Signi®cance of Listeria monocytogenes in Seafood' in Quality Assurance in the Fish Industry (Huss, H. H. et al., eds) pp. 217± 227. 21 Eklund, M.W., Poysky, F.T., Paranjpye, R.N., Lashbrook, L.C., Peterson, M.E. and Pelroy, G.A. (1995) `Incidence and Sources of Listeria monocytogenes in Cold-Smoked Fishery Products and Processing Plants' in J. Food Prot. 58, 502±508 22 Klima, R.A. and Montville, T.J. (1995) `The Regulatory and Industrial Responses to Listeriosis in the USA: A Paradigm for Dealing with Emerging Foodborne Pathogens' in Trends in Food Science and Technology 6, 87±93 23 Cheroute-Vialette, M., Lebert, I., Hebraud, M., Labadie, J.C. and Lebert, A. (1998) `Eects of pH or aw Stress on Growth of Listeria monocytogenes' in Int. J. Food Microbiol. 42, 71±77 24 Heinitz, M.L. and Johnson, J.M. (1998) `The Incidence of Listeria spp., Salmonella spp., and Clostridium botulinum in Smoked Fish and Shell®sh' in J. Food Prot. 61, 318±323 25 Thurette, J., MembreÂ, J.M., Han Chin, L., Tailliez, R. and Catteau, M. (1998) `Behavior of Listeria spp. in Smoked Fish Products Aected by Liquid Smoke, NaCI Concentration, and Temperature' in J. Food Prot. 61, 1475±1479 26 SunÄen, E. (1998) `Minimum Inhibitory Concentration of Smoke Wood Extracts Against Spoilage and Pathogenic Micro-Organisms Associated with Foods' in Lett. Appl. Microbiol. 27, 45±48 27 Dorsa, W.J., Marshall, D.L., Moody, M.W. and Hackney, C.R. (1993) `Low Temperature Growth and Thermal Inactivation of Listeria monocytogenes in Precooked Craw®sh Tail Meat' in J. Food Prot. 56, 106±109 28 Peterson, M.E., Pelroy, G.A., Paranjpy, E.R.N., Poysky, F.T., Almond, J.S. and Eklund, M.W. (1993) `Parameters for Control of Listeria monocytogenes in Smoked Fishery Products: Sodium Chloride and Packaging Method' in J. Food Prot. 56, 938±943 29 Andrews, L.S. and Grodner, R.M. (1997) `Radiosensitivity of Listeria monocytogenes Using Split Dose Application of Gamma Irradiation' in J. Food Prot. 60, 262±266 30 Wang, L.L. and Johnson, E.A. (1997) `Control of Listeria monocytogenes by Monoglycerides in Foods' in J. Food Prot. 60, 131±138 31 Baker, R.C., Poon, W., Kline, D. and Vadehra, D.V. (1982) `Antimicrobial Properties of Lauricidin in Mechanically Deboned Chicken, Minced Fish and Chicken Sausage' in J. Food Safety 4, 177±184 32 Bal'a, M.F. and Marshall, D.L. (1998) `Organic Acid Dipping of Cat®sh Fillets: Eect on Color, Microbial Load and Listeria monocytogenes' in J. Food Prot. 61, 1470±1474 33 Wessels, S. and Huss, H.H. (1996) `Suitability of Lactococcus lactis subsp. lactis ATCC I 1454 as a Protective Culture for Lightly Preserved Fish Products' in Food Microbiol. 13, 323±332 34 Nilsson, L., Huss, H.H. and Gram, L. (1997) `Inhibition of Listeria monocytogenes on Cold-Smoked Salmon by Nisin and Carbon Dioxide Atmosphere' in Int. J. Food Microbiol. 38, 217±227 35 Dues, F., Leroi, F., Boyaval, P. and Dousset, X. (1999) `Inhibition of Listeria monocytogenes by Carnobacterium spp. Strains in a Simulated Cold-Smoked Fish System Stored at 4 C' in Int. J. Food Microbiol. 47, 33±42 36 Dues, R, Corre, C., Leroi, F., Dousset, X. and Boyaval, P. `Inhibition of Listeria monocytogenes by In Situ-Produced and SemiPuri®ed Bacteriocins of Carnobacterium spp. on VacuumPacked Refrigerated Cold-Smoked Salmon' in J. Food Prot., in press 37 Nilsson, L., Gram, L. and Huss, H.H. (1999) `Growth Control of Listeria monocytogenes on Cold-Smoked Salmon using a Competitive Lactic Acid Bacteria Flora' in J. Food Prot. 62, 336±342
216
F. Dues / Trends in Food Science & Technology 10 (1999) 211±216
38 Lou, Y. and Yousef, A.E. (1997) `Adaptation to Sublethal Environmental Stresses Protects Listeria monocytogenes Against Lethal Preservation Factors' in Appl. Environ. Microbiol. 63, 1252±1255 39 LemaõÃtre, J.P., Echchannaoui, H., Michaut, G., Divies, C. and Rousset, A. (1998) `Plasmid-Mediated Resistance to Antimicrobial Agents Among Listeriae' in J. Food Prot. 61, 1459±1464 40 Rekhif, N., Atrih, A. and Lefebvre, G. (1994) `Selection and Properties of Spontaneous Mutants of Listeria monocytogenes ATCC15313 Resistant to Dierent Bacteriocins Produced by Lactic Acid Bacteria Strains' in Curr. Microbiol. 28, 237±241 41 Poyar-Salmeron, C., Trieu-Cuot, P., Cartier, C., McGowan, A., McLauchlin, J. and Courvalin, P. (1992) `Genetic Basis of Tetracycline Resistance in Clinical Isolates of Listeria monocytogenes' in Antimicrobial Agents Chemother. 36, 463±466 42 Martinis, E.C.P., Crandall, A.D., Mazotta, A.S. and Montville, T.J. (1997) `In¯uence of pH, Salt and Temperature on Nisin Resistance in Listeria monocytogenes' in J. Food Prot. 60, 420±423 43 Dykes, G.A. and Hastings, W.N. (1998) `Fitness Costs Associated with Class II a Bacteriocin Resistance in Listeria monocytogenes B73' in Lett. Appl. Microbiol. 26, 5±8
44 Ming, X. and Daeshel, M.A. (1995) `Correlation of Cellular Phospholipid Content with Nisin Resistance of Listeria monocytogenes Scott A' in J. Food Prot. 58, 416±420 45 Crandall, A.T. and Montville, T.J. (1998) `Nisin Resistance in Listeria monocytogenes ATCC 700302 is a Complex Phenotype' in Appl. Environ. Microbiol. 64, 231±237 46 Davies, E.A., Falahee, M.B. and Adams, M.R. (1996) `Involvement of the Cell Envelope of Listeria monocytogenes in the Acquisition of Nisin Resistance' in J. Appl. Bacteriol. 81, 139±146 47 Verheul, A., Russell, N.J., Van T'Hof, R., Rombouts, F.M. and Abee, T. (1997) `Modi®cation of Membrane Phospholipid Composition in Nisin-Resistant Listeria monocytogenes Scott A'' in Appl. Environ. Microbiol. 63, 3451±3457 48 Robichon, D., Gouin, E., DeÂbarbouilleÂ, M., Cossart, P., Cenatiempo, Y. and HeÂchard, Y. (1997) `The rpoN (s54) Gene from Listeria monocytogenes is Involved in Resistance to Mesentericin Y105, an Antibacterial Peptide from Leuconostoc mesenteroides' in J. Bacteriol. 79, 7591±7594 49 Fields, F.O. (1996) `Use of Bacteriocins in Food: Regulatory Considerations' in J. Food Prot., supplement, 72±77
Any Suggestions? Articles published in TIFS are usually specially invited by the Editor, with assistance from our International Advisory Editorial Board. However, we welcome ideas from readers for articles on exciting new and developing areas of food research. A brief synopsis of the proposal should ®rst be sent to the Editor, who can provide detailed guidelines on manuscript preparation. Mini-reviews focus on promising areas of food research that are advancing rapidly, or are in need of re-review in the light of recent advances or changing priorities within the food industry. Thus they are shorter than conventional reviews, focusing on the latest developments and discussing likely future applications and research needs. Features are similar in style to mini-reviews, highlighting speci®c topics of broad appeal to the food science community. The Viewpoint section provides a forum to express personal options, observations or hypotheses, to present new perspectives, and to help advance understanding of controversial issues by provoking debate and comment. Conference Reports highlight and assess important developments presented at relevent conferences worldwide. TIFS also welcomes Letters to the Editor concerned with issues raised by published articles or by recent developments in the food sciences. All Review-style articles are subject to editorial and independent peer review by international experts in the appropriate ®eld.
Review
Improving the control of Listeria monocytogenes in cold smoked salmon FreÂdeÂrique Dues INRA, Laboratoire de Recherches de Technologie LaitieÁre, 65 rue de Saint-Brieuc, 35042 Rennes cedex, France ( fax: +33-2-99-28-53-50; e-mail: du[email protected]). Smoked salmon is no longer a luxury product and its consumption has spread beyond special occasions. Unfortunately, this food is highly susceptible to contamination with Listeria monocytogenes. The processing steps sensitive to potential L. monocytogenes contamination, the current and future methodologies to ®ght against such ¯ora are presented with a particular emphasis on the ecacy of anti-listerial peptides. # 1999 Elsevier Science Ltd. All rights reserved.
The international market for smoked salmon was 32,173 tons in 1992, compared with the world salmon catch of 1 million tons, with 28 exporting and 52 importing countries [1]. France, with a production of 12,380 t in 1997 (turnover of 2.5 billion francs), realized by 35 operators, is the world's leading country for processing salmon. Between 1980 and 1995, price was reduced two-fold and consumer demand increased up to six-fold. Consumer demand pushed the industry in France, in 1992, towards a quality charter named `Label Rouge'. Cold smoked salmon (CSS) is one of the most perishable food products. Determination of shelf life is the responsibility of the manufacturers. In France, a maximum of three weeks from the packing date is recommended.
In some countries, it could be eight weeks. Quality depends on raw material, processing and storage conditions. Fish processing was initially designed to enhance the shelf life but changes in consumer taste led to a lighter smoking and salting which allows microbial development and growth. Spoilage of CSS is mainly the result of microbial activities leading to changes in the structure of the ¯esh and o odours. Pathogenic bacteria, such as L. monocytogenes, are frequently isolated (10%) [2, 3] and CSS must be considered as a potential source of infection [4]. The ®ght against Listeria is of special safety and economic concern. Common methods are reviewed with a special emphasis on the promising use of bacteriocins produced by lactic acid bacteria (LAB). Emergence of bacteriocin-resistant cells is also considered.
Cold-smoked salmon
Salmon used could be received raw or frozen with head and viscera removed or not. Salting (2±4%) contributes to eliminating part of the constituent water but also ®rms up the ¯esh, in¯uences texture, taste and prevents bleaching of the product. Dierent salting methods exist, according to dierent countries, using dried salt (kench curing with sodium chloride or nitrated salt), brine (70±80% NaCl) or injected brine. Drying is carried out only before cold smoking to decrease water concentration. Three types of process are used: air or contact drying, vacuum drying or freeze-drying. The smoking step gives the speci®c colour and taste to the product. Traditionally, ®llets are exposed to smoke produced by pyrolysis of wood or sawdust (beech or oak). The ¯esh dehydrates and becomes impregnated with volatile compounds like phenols (guaiacol and eugenol responsible for the taste; syringol responsible for the typical odour), acids, aldehydes, alcohol and hydrocarbons [5]. Hot smoking (60 C) is practised in north European countries (Germany and Poland), whereas cold smoking (20±24 C) is practised in France and gives a more tender and soft texture. There are three methods for smoking: the traditional way, by combustion; by using an electric ®eld which accelerates the smoke deposition or by atomization of a commercial smoke-aroma concentrate solution. Fish is vacuum packed in the form of ®llets, cubes or slices. The packaging has to be watertight. No colouring or preservatives are allowed in France.
0924-2244/99/$ - see front matter Copyright # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 92 4 - 2 24 4 ( 9 9 ) 0 0 05 1 - 5
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The resulting composition of CSS is: 65±78% water, 8±14% lipids, 16±22% proteins; 2±4.6% salt in the water phase and 0.2±1.5 mg of phenols per 100 g of ¯esh. Immediately after packing, bacterial counts were 103± 104 cfu/g. At this time, microbial ¯ora is typical of fresh ®sh ¯ora with predominance of Gram negative (64%) bacteria such as Shewanella putrefaciens and Aeromonas spp. LAB are also present (32%) with a majority of Carnobacterium spp. [6]. During storage, the ratio between these two ¯oras is inverted and among LAB, growth of lactobacilli is signi®cant. Most of the studies are based on quantitative determination at the rejection time. Total viable ¯ora grows from 103±104 to 107±108 cfu/g in three weeks depending on storage conditions [6±8]. Photobacterium phosphoreum has been described as the speci®c spoilage organism leading to H2S and methylmercaptan production [7], but the involvement of LAB in spoilage has been also suggested [6, 8]. Indeed, LAB predominates at the end of storage [5] whereas H2S-producing bacteria remain at a population level of 1.5 log units less. Among these LAB, the contribution of lactobacilli and carnobacteria to spoilage is subject to controversy. Truelstrup±Hansen [5] found that Lactobacillus sake, in particular, produced o-odours like `sour', `cabbage' and `sulphurous'. Carnobacterium piscicola is described as a spoilage organism because it is associated with ®sh intestine or disease [9] and is recognized as a tyramine producer [10], but did not accelerate the spoilage when inoculated on CSS at 106±107 cfu/g [11] and improved the sensory quality during vacuumpacked storage at 4 C [12]. Volatile compound production is closely related to microbiological contamination. Leroi et al. [6] showed an increase from 3.5 to 17 mg-N per 100 g in three weeks at 8 C which can be linked to S. putrefaciens and P. phosphoreum growth. Total volatile basic nitrogen (TVBN) increased from 15.5 to 53 mg-N per 100 g in the same conditions. No norms exist for CSS, but for fresh salmon no more than 35 mg of TVBN are accepted. Biogenic asinine production is also the result of microbial activity. Only histamine is controlled in tuna ®sh (it must be lower than 100 mg/g [13]), because of its frequent implication in foodborne intoxication. But the presence of high quantities of tyramine (80±230 mg/g), cadaverine (16±700 mg/g), putrecine (50±180 mg/g), histamine (4±190 mg/g), spermine (0±60 mg/g) and spermidine (3±20 mg/g) has been reported and must be taken into account. Microbial analysis is the only method that allows the speci®c detection of pathogens. Legislation tolerates: 106 cfu/g of aerobic microorganisms, 1 cfu/g Staphylococcus aureus and the absence of Coliforms, Salmonella and sulphite-reducing microorganisms [14]. Listeria is not subject to any norm, but fewer than 100 cfu/g are recommended.
Origin of L. monocytogenes contamination and consequences
The overall incidence of L. monocytogenes in smoked ®sh is 10%, with levels below 100 cfu/g [15], and vary between 0% in Scottish samples to 50% in American, Chilean and Norwegian samples [16]. Despite this high rate of contamination, clinical listeriosis in humans is rarely associated with ®sh products. This has been explained by the fact that there is no genetic relationship between L. monocytogenes clones isolated from ®sh and those isolated from human listeriosis [17]. However, L. monocytogenes is of concern in cold smoked ®sh, in particular since one outbreak of listeriosis caused by ®sh was reported [4, 18] in Sweden, nine cases of listeriosis incriminating rainbow trout resulted in two deaths in Finland, victims fell ill with gastroenteritis. Moreover, in the USA, 23.7% of foodstu toxi-infections were due to the presence of L. monocytogenes in seafood products during the 1980s. The ®sh origin (wild or breeding), ®shing season, ®shing technique (trawling or angling), handling, storage conditions (refrigerated or frozen) determine initial contamination. In France, in 1995, raw material (Salmo solar L) came from Norway (70%), Scotland (15%), USA±Canada (10%), Ireland and Chile (5%) with a large proportion of breeding origin. The origin of the salmon is critical: Listeria species are detected in 81% of fresh water and in 30% of marine water samples including 62% of L. monocytogenes [19]. This contamination comes from sewage euents, faecal contamination by animals and run-o from agricultural land [20]. Therefore, the initial and most important source of contamination in processing plants, is the surface (skin, mucus, tail, head) and intestinal cavity of the ®sh at the time of decapitation, peeling and ®lleting. Processing equipment, personnel rotation have been described as a secondary source of contamination [21]. Costs of foodborne illness caused by L. monocytogenes, present in dierent foods, are estimated at 300 billion dollars [22] in 1997 in the USA. No outbreaks has been reported due to CSS consumption but rainbow trout has previously been associated with cases of listeriosis [4, 18]. Recalls of seafood have been reported in north America (smoked Nova Scotia salmon in December 1988 in Washington; smoked salmon in August 1989 in Washington; smoked salmon products in September 1989 in Illinois) and in Europe (smoked salmon Fjord King in December 1998 in France). For the industry, the consequences of Listeria contamination are recalls, destruction, cleaning, adverse publicity and the resulting decrease in consumption leading to the closure of factories.
Behaviour of L. monocytogenes during CSS processing
During the transport and the thawing procedure, part of the vascular system is exposed to water from containers
F. Dues / Trends in Food Science & Technology 10 (1999) 211±216
and thawing tanks which are contaminated in 18.6% of cases with L. monocytogenes [21]. During salmon ®lleting, L. monocytogenes are transferred from the ¯esh to cut surfaces, slicing machines and tables contaminating 60% of these apparatuses, which become potential sources of contamination. Salting selects gram negative, halophilic anaerobic rods and a water activity of 0.95 does not inhibit L. monocytogenes development [23]. Brining, by soaking or injection, is a risky method because baths and needles are re-used. No new contamination occurs at the smoking step, but cold processing does not eliminate the initial contamination. Cold-smoked processing results in 11.3± 17.5% of samples contaminated whereas only 1.5±8.4% of the hot-smoked salmon samples were L. monocytogenes positive [2, 24]. Liquid smoke (0.2 mg phenols per 10 g total deposition) had a signi®cant eect on L. monocytogenes growth [25], whereas solid smoke was less ecient [26]. Refrigeration and vacuum storage do not guarantee prevention of L. monocytogenes growth. Listeria monocytogenes is able to grow from 105 to 109 cfu/g at 12 C and from 104 to 109 or 105 cfu/g at 6 and 0 C, respectively, on craw®sh tail suggesting that chilled temperature storage is not sucient to prevent L. monocytogenes growth [27]. These authors also showed that freezing of the ®nished product did not kill L. monocytogenes and did not aect its development later.
Wrestling with L. monocylogenes
Because current legislation forbids the use of any additive, antioxidant or antibacterial chemical substances, research on L. monocytogenes inhibition is currently based on the use of dierent processing parameters. The use of inhibitory LAB, which are naturally present on the ®sh, is a new opportunity. At 10 C, 6% NaCl reduced the L. monocytogenes count for one log after 30 days of storage under O2permeable conditions whereas no signi®cant eect of NaCl was observed under vacuum packaging, compared with the control [28]. At 4 C, the presence of increasing salt concentration (3±6%) reduces L. monocytogenes for 2±5 logs under permeable conditions. Unfortunately, this amount of salt is not acceptable for consumers. SunÄen [26] has determined the minimum inhibitory concentration of liquid and solid smoke wood extracts. The ®rst result was the con®rmation that liquid was more ecient than solid smoke because of its capacity to penetrate the ¯esh. The second one was that the more active smokes showed the highest phenol concentrations. The last and encouraging result was that, even if not all the liquid smoke tested were eective against L. monocytogenes, the ecient ones were active at levels recommended by the manufacturers and then organoleptically acceptable to consumers.
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The room for manoeuvre on salt concentration and smoking methods is very limited because of consumer requirements. Other methods are being considered and tested to be submitted for legislative approval. Irradiation with 2 kGy reduced L. monocytogenes counts from 104 to less than 100 cfu/g, but this is the upper limit after which sensory quality changes are noticed [29]. Monoglycerides (MC12) slow down L. monocytogenes growth on seafood [30]. The MC12 activity was enhanced by the addition of propyl gallate (200 mg/g), an antioxidant [31], but to our knowledge, no study has been carried out on salmon. Organic acids eective against L. monocytogenes are not much used because of their insolubility, their eect on colour and organoleptic properties of the product [32]. Because many bacteriocins, especially those from LAB, which are commonly used in food, are active against L. monocytogenes and inactive against mammalian cells, there is an increasing interest in their use as biopreservatives. FDA regulates the use of bacteriocins in food in the USA. Requirements for safety assessment are listed in Table 1. Nisin is a bacteriocin produced by Lactococcus lactis. It is the only bacteriocin to receive international acceptance to be used as an additive in some foods (in 1969, Nisaplin1, UK). Though if nisin is not yet usable on CSS, Wessels and Huss [33] tested L. monocytogenes inhibition by insitu bacteriocin-producing strains in CSS. Unfortunately, the nisin-producer Lactococcus lactis ATCC 11454, did not grow on CSS slices at 10 C, resulting in a 1.5 log growth of L. monocytogenes. Nilsson et al. [34] reported that nisin (1000 IU/g) action was enhanced by a CO2-atmosphere: two log reduction on CSS stored at Table 1. FCA requirements for bacteriocin use in food industry No. 1 2 3
4 5
6 7
1
Requirements Bacteriocins or their producers must be characterized and their GRAS status must be established Bacteriocin metabolism by bacteria and catabolism in human tracts must be known Puri®ed or semi-puri®ed bacteriocin production must be detailed, reproducible, guarantee a percentage of purity, absence of toxicity and information must be provided about speci®c manipulations (recombinants, mutants. . .) Intended use levels should be determined and explained compared with natural levels in food if applicable The desired eect must be precise (inhibition of pathogens, spoilage organisms, shelf life increase) and the eectiveness of the bacteriocin should be demonstrated and standardized under typical use conditions The food concerned must be described with its speci®city of processing and storage Secondary eects should be studied: Does it mask spoilage? Does it interact negatively with other substances? Is it toxic at high levels? Is it stable during storage? Fields [49]
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5 C compared with vacuum-packaging. Dues et al. [35] showed that Carnobacterium divergens V41 and C. piscicola V1 were two eective strains in a simulated cold smoked salmon system stored at 4 C, inhibiting L. monocytogenes as early as day 4. Carnobacterium piscicola V1 was bactericidal against L. monocytogenes on sterile and commercial vacuum-packed cold-smoked salmon at 4 and 8 C, while C. divergens V41 presented a bacteriostatic eect. Moreover, no product spoilage could be observed with the use of such bacteriocinproducing strains as demonstrated by good sensorial analyses and low biogenic amine production. Crude extracts of piscicocins were bactericidal at 4 and 8 C. Listeria growth was delayed by divercin V41 at 8 C and was inhibited at 4 C [36]. In Denmark, Nilsson et al. [37] had also shown that C. piscicola A9b had a bacteriostatic eect on L. monocytogenes growth without adversely aecting the sensory quality of CSS. Carnobacterium piscicola strains were promising candidates for L. monocytogenes control on CSS at chilled temperatures. Factors limiting bacteriocin eectiveness in foods are due to the bacteriocin characteristics (structure, conditions of maximum activity), bacteriocin producers (ability to grow and produce the bacteriocin at speci®c conditions of temperature, atmosphere. . .) or food (liquid or solid, content of lipids, carbohydrates. . .). When bacteriocin is dropped, it could be inactivated by food components (proteases, lipids, other microorganisms. . .) or its activity could be aected during the processing (temperature, drying. . .). Bacteriocin eectiveness depends on the population of target cells: how many molecules of bacteriocins are required to cause the bacterial death of a cell? Bacteriocin mode of action may be speci®c and target cell susceptibility more or less signi®cant. The resistance to bacteriocins of some cell is of particular interest to avoid a situation similar to antibiotic resistance.
Listeria monocytogenes resistance
Most of the studies on antibiotic resistance in human, on stress adaptation [38], on cleaning antibacterial agent eciency [39] or bacteriocin action [40] report that when L. monocytogenes are exposed to such antimicrobial agents, adapted cells appear. An increase and continued use of these antibacterial compounds may select resistant germs or cause mutations that would impair their eciency. The development of acquired or spontaneous nisin resistance has been reported for S. aureus, L. plantarum, Streptococcus thermophilus, and several species of Bacillus and, recently, for L. monocytogenes under laboratory conditions. Fortunately, spontaneous bacteriocin resistance in food-borne bacteria is rarely reported. This could be explained by the fact that, in food, L. monocytogenes has to exceed not only bacteriocin activity but also additional stresses, such as nutrient availability, NaCl content, pH, storage
temperature and the presence of other microorganisms. Resistance is of special concern for antibiotherapy and foodstu security. Poyar-Salmeron et al. [41] showed that movable genetic elements, such as plasmids and transposons, are responsible for the emergence of drug resistance in L. monocytogenes. In the case of bacteriocins, the mechanisms of resistance are not well known. Most of the studies concern nisin and are descriptive. Nevertheless, whatever the bacteriocin considered, some results are already known, such as mutation frequency, maintained between 10ÿ4 and 10ÿ7 [39, 42] in laboratory media. However, the stability of the resistance may be contradicted since Rekhif et al. [40] did not observe reversion of the resistance after several successive cultures in the absence of bacteriocins (mesentericin 52, curvaticin 13 and plantaricin 19). On the contrary, Dykes and Hastings [43] found a reversion frequency within the range of 10ÿ4± 10ÿ5 for leucocins A, B and E and sakacin A. Another feature is that there is no cross-resistance reported between lantibiotics (nisin) and class II bacteriocins (mesentericin, curvaticin, plantaricin, divercin, piscicocins. . .) suggesting that these bacteriocins should have dierent modes of action. Resistance could be due to changes in the outer membrane composition of target cells, especially lipids [44]. Cell wall composition [45], cell surface hydrophobicity [46] or di- and trivalent cations (Ca2+, Mg2+ and Gd3+), which may interact with the negative charges of phospholipid and cancel the electrostatic interactions between phospholipid and the bacteriocin [47], could be implicated in resistance mechanisms. In parallel to the structural modi®cation of the membrane and the cell wall, Davies et al. [46] investigated the possibility of new protein synthesis. Chloramphenicol did not adversely aect the frequency isolation of NR mutants indicating that de novo protein synthesis was not involved. However, the role of proteases, regulation or membrane proteins had to be taken into account. Rekhif et al. [40] shown that L. monocytogenes resistance was not due to bacteriocin inactivation or destruction. Robichon et al. [48] has identi®ed a new s54 factor expressed by L. monocytogenes and responsible for sensitivity to mesentericin Y105. Genes under s54 control are involved in nitrogen metabolism, in pilus production, and dicarboxylic acid transport. Mesentericin could interact with speci®c membrane proteins involved in these pathways.
Conclusion
The likelihood of L. monocytogenes contamination of smoked salmon is high. The increasing volume of production requires ecient protection. Association of in situ bacteriocin production with traditional preservation methods is promising but further studies are needed to control the emergence of potential resistant strains.
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Acknowledgements
The author thanks Rod Jeerson for proof-reading.
References 1 Codex Committee on Fish and Fishery Products, 22eÂme Session, Bergen, Norway, 6±10 May 1996 2 Jemmi, T. (1993) `Listeria Monocytogenes in Smoked Fish: An Overview' in Arch. Lebensmittelhyg. 44, 10±13 3 Johansson, T., Rantala, L., Palmu, L. and Honkanen-Buzalski, T. (1999) `Occurrence and Typing of Listeria monocytogenes Strains in Retail Vacuum-Packed Fish Products and in a Production Plant' in Int. J. Food Microbiol. 47, 111±119 4 Miettinen, M.K., Siitonen, A., Heiskanen, P., Haajanen, H., Bjorkroth, K.J. and Korkeala, H.J. (1999) `Molecular Epidemiology of an Outbreak of Febrile Gastroenteritis Caused by Listeria monocytogenes in Cold-smoked Rainbow Trout' in J. Clin. Microbiol. 37, 2358±2360 5 Truelstrup-Hansen, L. (1995) `Quality of Chilled, Vacuum Packed Cold-Smoked Salmon' in Ph.D. thesis. Danish Institute for Fisheries Research and the Royal Veterinary and Agricultural University of Copenhagen, Denmark 6 Leroi, F., Joraud, J.J., Chevalier, F. and Cardinal, M. (1998) `A Study of the Microbial Ecology of Cold-Smoked Salmon During Storage at 8 C' in Int. J. Food Microbiol. 39, 111±121 7 Donald, B. and Gibson, G.M. (1992) `Preliminary observations of the ¯ora of fresh vacuum-packed salmon steaks' in Quality Assurance in the Fish Industry, (Huss, H.H. et al., eds), pp. 107± 114, Elsevier, London 8 Gram, L. and Huss, H.H. (1996) `Microbial Spoilage of Fish and Fish Products' in Int. J. Food Microbiol. 33, 121±137 9 Toranzo, A.E., Romalde, J.L., NunÄez, S., Figueras, A. and Barja, J.L. (1993) `An Epizootic in Farmed, Market-Size Rainbow Trout in Spain Caused by a Strain of Carnobacterium piscicola of Unusual Virulence' in Dis. Aquat. Org. 17, 87±99 10 Leisner, J.J., Millan, J.C., Huss, H.H. and Larsen, L.M. (1994) `Production of Histamine and Tyramine by Lactic Acid Bacteria Isolated from Vacuum-Packed Sugar-Salted Fish' in J. Appl. Bacteriol. 76, 417±423 11 Paludan-MuÈller, C., Dalgaard, P., Huss, H.H. and Gram, L. (1998) `Evaluation of the Role of Carnobacterium piscicola in Spoilage of Vacuum- and Modi®ed-Packed Cold-Smoked Salmon Stored at 5 C' in Int. J. Food Microbiol. 39, 155±166 12 Leroi, F., Arbey, N., Joraud, J.J. and Chevalier, F. (1996) `Eect of Inoculation with Lactic Acid Bacteria on Extending the Shelf Life of Vacuum-Packed Cold-Smoked Salmon' in Int. J. Food Sci. Technol. 31, 497±504 13 Dehove, R.A. (1998) `ReÂglementation des Produits' in QualiteÂ. Repression des Fraudes. ED, Lamy. 14 NF V 45-065. (1995) Poisson transformeÂ. Saumon fumeÂ. Norme AFNOR 15 Cortesi, M.L., Sarli, T., Santoro, A., Murru, N. and Pepe, T. (1997) `Distribution and Behavior of Listeria monocytogenes in Three Lots of Naturally-Contaminated Vacuum Packed Smoked Salmon Stored at 2 and 10 C' in Int. J. Food Microbiol. 37, 209±214 16 Farber, J.M. (1991) `Listeria monocytogenes in Fish Products' in J. Food Prot. 54, 922±934 17 Boerlin, P., Boerlin-Petzold, F., Bannerman, E., Bille, J. and Jemmi, T. (1997) `Typing Listeria monocytogenes Isolates from Fish Products and Human Listeriosis Cases' in Appl. Environ. Microbiol. 63, 1338±1343 18 Ericsson, H., Eklow, A., Danielson-Tham, M.L., Loncarvic, S., Mentzing, L.O., Persson, L., Unnerstad, H. and Tham, W. (1997) `An Outbreak of Listeriosis Suspected to Have Been Caused by Rainbow Trout' in J. Clin. Microbiol. 35, 2904±2907
215
19 Colburn, K.G., Kaysner, C.A., Abeyta, C. and Welkell, M.M. (1990) `Listeria Species in California Coast Estuarine Environment' in Appl. Environ. Mcrobiol. 56, 2007±2011 20 Fuchs, R.S. and Reilly, P.J.A. (1992) `The Incidence and Signi®cance of Listeria monocytogenes in Seafood' in Quality Assurance in the Fish Industry (Huss, H. H. et al., eds) pp. 217± 227. 21 Eklund, M.W., Poysky, F.T., Paranjpye, R.N., Lashbrook, L.C., Peterson, M.E. and Pelroy, G.A. (1995) `Incidence and Sources of Listeria monocytogenes in Cold-Smoked Fishery Products and Processing Plants' in J. Food Prot. 58, 502±508 22 Klima, R.A. and Montville, T.J. (1995) `The Regulatory and Industrial Responses to Listeriosis in the USA: A Paradigm for Dealing with Emerging Foodborne Pathogens' in Trends in Food Science and Technology 6, 87±93 23 Cheroute-Vialette, M., Lebert, I., Hebraud, M., Labadie, J.C. and Lebert, A. (1998) `Eects of pH or aw Stress on Growth of Listeria monocytogenes' in Int. J. Food Microbiol. 42, 71±77 24 Heinitz, M.L. and Johnson, J.M. (1998) `The Incidence of Listeria spp., Salmonella spp., and Clostridium botulinum in Smoked Fish and Shell®sh' in J. Food Prot. 61, 318±323 25 Thurette, J., MembreÂ, J.M., Han Chin, L., Tailliez, R. and Catteau, M. (1998) `Behavior of Listeria spp. in Smoked Fish Products Aected by Liquid Smoke, NaCI Concentration, and Temperature' in J. Food Prot. 61, 1475±1479 26 SunÄen, E. (1998) `Minimum Inhibitory Concentration of Smoke Wood Extracts Against Spoilage and Pathogenic Micro-Organisms Associated with Foods' in Lett. Appl. Microbiol. 27, 45±48 27 Dorsa, W.J., Marshall, D.L., Moody, M.W. and Hackney, C.R. (1993) `Low Temperature Growth and Thermal Inactivation of Listeria monocytogenes in Precooked Craw®sh Tail Meat' in J. Food Prot. 56, 106±109 28 Peterson, M.E., Pelroy, G.A., Paranjpy, E.R.N., Poysky, F.T., Almond, J.S. and Eklund, M.W. (1993) `Parameters for Control of Listeria monocytogenes in Smoked Fishery Products: Sodium Chloride and Packaging Method' in J. Food Prot. 56, 938±943 29 Andrews, L.S. and Grodner, R.M. (1997) `Radiosensitivity of Listeria monocytogenes Using Split Dose Application of Gamma Irradiation' in J. Food Prot. 60, 262±266 30 Wang, L.L. and Johnson, E.A. (1997) `Control of Listeria monocytogenes by Monoglycerides in Foods' in J. Food Prot. 60, 131±138 31 Baker, R.C., Poon, W., Kline, D. and Vadehra, D.V. (1982) `Antimicrobial Properties of Lauricidin in Mechanically Deboned Chicken, Minced Fish and Chicken Sausage' in J. Food Safety 4, 177±184 32 Bal'a, M.F. and Marshall, D.L. (1998) `Organic Acid Dipping of Cat®sh Fillets: Eect on Color, Microbial Load and Listeria monocytogenes' in J. Food Prot. 61, 1470±1474 33 Wessels, S. and Huss, H.H. (1996) `Suitability of Lactococcus lactis subsp. lactis ATCC I 1454 as a Protective Culture for Lightly Preserved Fish Products' in Food Microbiol. 13, 323±332 34 Nilsson, L., Huss, H.H. and Gram, L. (1997) `Inhibition of Listeria monocytogenes on Cold-Smoked Salmon by Nisin and Carbon Dioxide Atmosphere' in Int. J. Food Microbiol. 38, 217±227 35 Dues, F., Leroi, F., Boyaval, P. and Dousset, X. (1999) `Inhibition of Listeria monocytogenes by Carnobacterium spp. Strains in a Simulated Cold-Smoked Fish System Stored at 4 C' in Int. J. Food Microbiol. 47, 33±42 36 Dues, R, Corre, C., Leroi, F., Dousset, X. and Boyaval, P. `Inhibition of Listeria monocytogenes by In Situ-Produced and SemiPuri®ed Bacteriocins of Carnobacterium spp. on VacuumPacked Refrigerated Cold-Smoked Salmon' in J. Food Prot., in press 37 Nilsson, L., Gram, L. and Huss, H.H. (1999) `Growth Control of Listeria monocytogenes on Cold-Smoked Salmon using a Competitive Lactic Acid Bacteria Flora' in J. Food Prot. 62, 336±342
216
F. Dues / Trends in Food Science & Technology 10 (1999) 211±216
38 Lou, Y. and Yousef, A.E. (1997) `Adaptation to Sublethal Environmental Stresses Protects Listeria monocytogenes Against Lethal Preservation Factors' in Appl. Environ. Microbiol. 63, 1252±1255 39 LemaõÃtre, J.P., Echchannaoui, H., Michaut, G., Divies, C. and Rousset, A. (1998) `Plasmid-Mediated Resistance to Antimicrobial Agents Among Listeriae' in J. Food Prot. 61, 1459±1464 40 Rekhif, N., Atrih, A. and Lefebvre, G. (1994) `Selection and Properties of Spontaneous Mutants of Listeria monocytogenes ATCC15313 Resistant to Dierent Bacteriocins Produced by Lactic Acid Bacteria Strains' in Curr. Microbiol. 28, 237±241 41 Poyar-Salmeron, C., Trieu-Cuot, P., Cartier, C., McGowan, A., McLauchlin, J. and Courvalin, P. (1992) `Genetic Basis of Tetracycline Resistance in Clinical Isolates of Listeria monocytogenes' in Antimicrobial Agents Chemother. 36, 463±466 42 Martinis, E.C.P., Crandall, A.D., Mazotta, A.S. and Montville, T.J. (1997) `In¯uence of pH, Salt and Temperature on Nisin Resistance in Listeria monocytogenes' in J. Food Prot. 60, 420±423 43 Dykes, G.A. and Hastings, W.N. (1998) `Fitness Costs Associated with Class II a Bacteriocin Resistance in Listeria monocytogenes B73' in Lett. Appl. Microbiol. 26, 5±8
44 Ming, X. and Daeshel, M.A. (1995) `Correlation of Cellular Phospholipid Content with Nisin Resistance of Listeria monocytogenes Scott A' in J. Food Prot. 58, 416±420 45 Crandall, A.T. and Montville, T.J. (1998) `Nisin Resistance in Listeria monocytogenes ATCC 700302 is a Complex Phenotype' in Appl. Environ. Microbiol. 64, 231±237 46 Davies, E.A., Falahee, M.B. and Adams, M.R. (1996) `Involvement of the Cell Envelope of Listeria monocytogenes in the Acquisition of Nisin Resistance' in J. Appl. Bacteriol. 81, 139±146 47 Verheul, A., Russell, N.J., Van T'Hof, R., Rombouts, F.M. and Abee, T. (1997) `Modi®cation of Membrane Phospholipid Composition in Nisin-Resistant Listeria monocytogenes Scott A'' in Appl. Environ. Microbiol. 63, 3451±3457 48 Robichon, D., Gouin, E., DeÂbarbouilleÂ, M., Cossart, P., Cenatiempo, Y. and HeÂchard, Y. (1997) `The rpoN (s54) Gene from Listeria monocytogenes is Involved in Resistance to Mesentericin Y105, an Antibacterial Peptide from Leuconostoc mesenteroides' in J. Bacteriol. 79, 7591±7594 49 Fields, F.O. (1996) `Use of Bacteriocins in Food: Regulatory Considerations' in J. Food Prot., supplement, 72±77
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