Enterococcus faecium LM-2, a multi-bacteriocinogenic strain naturally occurring in “Byaslag”, a traditional cheese of Inner Mongolia in China

A bacteriocin-producing strain LM-2 was isolated from “Byaslag”, a traditional cheese in Inner Mongolia of China, and identified as Enterococcus faecium. The strain produced the maximum activity of enterocin (5120 AU ml^?1) in MRS medium in the mid-stationary phase of growth (i.e. after 30 h at 37 °C). Purified enterocin LM-2 was obtained by using a two-step procedure consisting of ammonium sulfate precipitation and cation exchange chromatography. Tricine–SDS–PAGE of the purified enteriocin LM-2 contained two active protein bands between 3.5 and 6.4 kDa. It showed a broad spectrum of activity against most of the tested strains in the genera Listeria, Staphylococcus, Bacillus, Clostridium, Salmonella and Pseudomonas. All Listeria strains tested, including L. monocytogenes, were highly sensitive to this bacteriocin. The inhibitory activity of enterocin LM-2 against L. monocytogenes was bactericidal without concomitant cell growth, and the highest level of adsorption (87.5%) to the surface of L. monocytogenes 54002 cells was founded at pH 6.0. This strong inhibitory activity against L. monocytogenes, coupled with its broad pH-resistance and heat-stability indicated its potential use as a food preservative to enhance the safety of fermented foods. The presence of enterocin P and L50-like structural genes in E. faecium LM-2 was established by PCR and direct sequencing, which indicated strain LM-2 may be a new multi-bacteriocin producer strain.     

Antimicrobial effect of cranberry juice and extracts

The antimicrobial effect of cranberry juice and of three cranberry extracts (water-soluble (E1) and apolar phenolic compounds (E2), and anthocyanins (E3)) was investigated against seven bacterial strains (Enterococcus faecium resistant to vancomycin (ERV), Escherichia coli O157:H7 EDL 933, Escherichia coli ATCC 25922, Listeria monocytogenes HPB 2812, Pseudomonas aeruginosa ATCC 15442, Salmonella Typhimurium SL1344, and Staphylococcus aureus ATCC 29213). Each cranberry sample was analyzed to determine the minimum inhibitory concentration (MIC) and the maximal tolerated concentration (MTC) at neutral pH. The results, reported in μg phenol/mL, indicated that all the bacterial strains, both Gram-positive and Gram-negative, were selectively inhibited by the cranberry phenolic compounds. The extract rich in water-soluble phenolic compounds caused the most important growth inhibitions. The bacteria ERV, and to a lesser degree, P. aeruginosa, S. aureus and E. coli ATCC 25922, were the most sensitive to the antimicrobial activity of extract E1. The growth of P. aeruginosa and E. coli ATCC was also affected by the presence of the anthocyanin-rich cranberry extract E3, although the observed antibacterial effect was not as important as with extract E1. In general, L. monocytogenes, E. coli O157:H7 and S. Typhimurium were the most resistant to the antibacterial activity of the cranberry extracts. Within 30 min of exposure with pure neutralized cranberry juice, L. monocytogenes and ERV were completely inactivated.     

Influence of Listeria innocua on the growth of Listeria monocytogenes

The growth of Listeria monocytogenes and Listeria innocua strains was monitored during this study: (i) in TSB–YE media and (ii) in a food matrix (pasteurized milk) according to the ISO 11290-1 methodology. Different inocula concentrations and mixtures were tested. The response was shown to be strain dependent. In TSB–YE the inhibition of a L. monocytogenes strain was observed in just one of the three mixtures (L. monocytogenes_1340 with L. innocua_11288) showing a reduction of 1.37 log cfu/ml after 42.5 h and 1.85 log after 66.5 h of incubation. In pasteurized milk the inhibition of L. monocytogenes by L. innocua was always observed when L. innocua was present in higher concentrations than L. monocytogenes. The reverse was also observed but only in one mixture (cocktail of six L. monocytogenes with L. innocua_2030c) when the initial concentration of L. monocytogenes was 100 times higher than L. innocua suggesting the phenomenon of quorum sensing. Furthermore, inhibitory activity was not caused by bacteriocins, and no correlation between the growth rate and inhibition was demonstrated.     

Occurrence of antilisterial structural bacteriocins genes in meat borne lactic acid bacteria

The ability to inhibit the growth of Listeria cells and the presence of bacteriocin encoding genes was examined in 115 LAB strains isolated from Argentinean vacuum-packaged beef and different traditional fermented sausages. Lactobacillus (L) sakei, Lactobacillus (L) curvatus and Enterococcus (E) faecium showed a great inhibition of all Listeria strains evaluated while Pediococcus (P) acidilactici and Lactobacillus (L) plantarum demonstrated a limited or absent antilisterial activity. Both L. curvatus and L. sakei carried the sppA, sppQ and sapA structural genes, encoding for sakacin P, sakacin Q and curvacin A bacteriocins, respectively. Whilst L. curvatus exhibited a higher occurrence of these genes, L. sakei strains were more effective at inhibiting Listeria (L) strains, Listeria monocytogenes UC8159 and Listeria innocua 7 being the most sensitive to these bacteriocins. Among analyzed E. faecium strains, the wide distribution of entA, entB and entP genes accounted for the high antilisterial activity particularly observed against L. monocytogenes FBUNT. The structural gene plantEF was mostly present in Lactobacillus plantarum strains and no pedA gene was found in P. acidilactici evaluated strains. The antilisterial potential of L. sakei and E. faecium offers great possibilities for the meat industry as biopreservative cultures, although more studies are needed in order to conclude about this issue.     

Incidence and genetic variability of Listeria species from three milk processing plants

The presence of Listeria in three milk processing environments as a potential source of milk contamination was assessed. Swab samples (n = 210) taken from milk processing plants were examined. Sample sites included the milk processing equipment, besides areas handling raw and pasteurized milk. The USDA Listeria-selective enrichment procedure was used to process the samples. Forty one (19.52%) Listeria isolates were recovered. The isolates were further subjected to biochemical and genotypic characterization. Out of 41 isolates, 16 (7.62%) were confirmed as Listeria monocytogenes, 2 (0.95%) as L. ivanovii, 19 (9.05%) as L. innocua. 1 (0.48%) as L. seeligeri and 3 (1.43%) as L. grayi. All the L. monocytogenes isolates were positive for the hlyA gene. PCR based serotyping revealed all L. monocytogenes to be of 1/2a, 1/2c, 3a and 3c serovar group. AscI and ApaI restriction analysis yielded four PFGE clusters for 16 L. monocytogenes isolates obtained from raw milk collector, milk silos, buttermilk mixer, cheese and other milk product processor. No predominant PFGE cluster was observed among these L. monocytogenes isolates. The main sources of L. monocytogenes were found to be raw milk collector and milk silos. In the present study L. monocytogenes was isolated from milk and milk products processing plants which could cross-contaminate the processed products and may possess a potential threat to public health.     

Characterization of bacPPK34 a bacteriocin produced by Pediococcus pentosaceus strain K34 isolated from “Alheira”

Different lactic acid bacteria were isolated during different stages in the production of “Alheiras”, a traditionally fermented sausage produced in the north of Portugal, between 2005 and 2007, in a total of 484 isolates. One of 484 isolates (K34) produced a bacteriocin, designated as bacPPK34, and was identified as a strain of Pediococcus pentosaceus by 16S rRNA sequencing. The highest bacteriocin production was noted at late log/early stationary phase after 15–18 h of growth in MRS broth at 37 °C (3200 AU/ml) against Enterococcus faecalis ATCC 29212 and 12800 AU/ml against Listeria monocytogenes (L1, L2, L3). bacPPK34 was between 2.5 kDa and 6.2 kDa in size, as determined by tricine-SDS-PAGE. Complete inactivation or significant reduction in antimicrobial activity was observed after treatment of cell-free supernatants with proteinase K, pepsin and trypsin. No change in activity was recorded when treated with catalase. The bacteriocin was resistant to treatments with lipase and detergents Triton X-100, Tween 20, SDS, NaCl, urea and EDTA. Furthermore, the bacteriocin remained active after 2 h at pH 2–12 and temperature treatments at 60, 80, 100 °C, 1 month of storage at ?20 and 4 °C and 20 min at 121 °C. Addition of bacPPK34 to a mid-log culture of L. monocytogenes and E. faecalis ATCC 29212 inhibited growth. The bacteriocin did not adhere to the surface of the producer cells.     

Inhibition of Listeria monocytogenes by pomegranate (Punica granatum) peel extract in meat paté at different temperatures

Natural antimicrobials are being more and more considered as alternative approach for controlling growth of microorganisms in food. The objective of this study was to evaluate the pomegranate extract’s (PE) potential to be used as a natural preservative in ready to eat meats. Listeria monocytogenes was the main target. In a preliminary assessment with the disk diffusion method PE showed inhibitory effect against all five tested species, in the following order of increasing sensitivity: L. monocytogenes, Bacillus subtilis, Bacillus cereus, Escherichia coli and Staphylococcus aureus.No viable cells of L. monocytogenes were detected after incubation in BHI broth in presence of 7.5% v/v of the liquid PE (or 24.7 mg dry PE/ml). This concentration was considered as the Minimal Bactericidal Concentration (MBC) of the tested PE. Two pure components commonly found in PE, namely gallic and ellagic acids were also tested in BHI broth, however they did not show considerable inhibition of L. monocytogenes. PE in a concentration equal to the measured MBC was tested against L. monocytogenes in meat paté at different temperatures. At 4 °C during 46 days the extract inhibited the growth in meat paté by 4.1 log CFU/g compared to the control, which had reached log 9.2 CFU/g already on the 18th day. Inhibition was less pronounced at higher temperatures. The results indicate that the PE has a potential to be used as a natural preservative in meat products.     

Pomegranate peel (Punica granatum L) extract and Chinese gall (Galla chinensis) extract inhibit Vibrio parahaemolyticus and Listeria monocytogenes on cooked shrimp and raw tuna

Vibrio parahaemolyticus and Listeria monocytogenes are bacterial pathogens associated with raw or ready-to-eat seafood products. Many compounds extracted from plant material have shown promise for inhibiting bacterial pathogens when applied to some foods. In this study, aqueous methanol extracts from pomegranate peel (Punica granatum L.) and Chinese gallnut (Galla chinensis) were tested against V. parahaemolyticus and L. monocytogenes on cooked shrimp and raw homogenized tuna. The extracts were applied to the shrimp by soaking for 2 min (5 mg/ml). The extracts (1.7 mg ml) were added to homogenized tuna and stirred. The antimicrobial assay on V. parahaemolyticus was conducted at 12 °C, and the assay on tuna was conducted at both 4° and 12 °C. Both Chinese gall and pomegranate peel extracts significantly inhibited the growth of V. parahaemolyticus in both shrimp and tuna. Only Chinese gall extract significantly inhibited growth of L. monocytogenes. Overall V. parahaemolyticus was more sensitive to both plant extracts compared with L. monocytogenes. Both plant extracts had stronger antimicrobial activity on shrimp compared with the tuna. Neither extract completely inhibited the growth of V. parahaemolyticus or L. monocytogenes.     

Development of a multiplex real-time PCR method for simultaneous detection of Vibrio parahaemolyticus,Listeria monocytogenes and Salmonella spp. in raw shrimp

Vibrio parahaemolyticus, Listeria monocytogenes and Salmonella spp. are important pathogens contaminating seafood in China. In this study, we developed an efficient multiplex real-time PCR for the simultaneous detection of V. parahaemolyticus, L. monocytogenes and Salmonella spp. in raw shrimp without a prior enrichment step. In a test using 28 target and non-target strains only the targets were detected and two calibration curves, for pure cultures and artificially contaminated samples, were used to evaluate the efficiencies of this method. Amplification efficiencies of this multiplex real-time PCR were excellent in pure cultures and artificially contaminated shrimps. The limits of detection in artificially contaminated shrimps were 112 CFU/g for V. parahaemolyticus, 158 CFU/g for L. monocytogenes and 103 CFU/g for Salmonella spp. We validated this multiplex real-time PCR method on 48 commercial samples and the results were comparable to standard culture methods. This efficient multiplex real-time PCR, where each test takes only 50 min after DNA extraction, is a useful tool for high-throughput surveillance of V. parahaemolyticus, L. monocytogenes and Salmonella spp. in seafood products.     

Sardinian goat’s milk as source of bacteriocinogenic potential protective cultures

Goat breeding in Sardinia constitutes an important source of income for farming and shepherding activities. In this study 170 LAB strains were isolated from Sardinian goat’s milk and tested for bacteriocins production against several food-borne pathogenic microorganisms. Four isolates (SD1, SD2, SD3 and SD4) were selected for their effective inhibition on Listeria monocytogenes. The strains were classified as members of Enterococcus genus, according to their biochemical and physiological characteristics, and then genetically identified as Enterococcus faecium. In MRS broth at 37 °C, bacteriocins SD1 and SD2 were produced at much higher levels (51200 AU/ml) compared to bacteriocin SD3 (3200 AU/ml) and bacteriocin SD4 (800 AU/ml). Their peptides were inactivated by proteolytic enzymes, but not when treated with α-amylase, catalase and lipase. The four bacteriocins remained stable at pH from 2.0 to 12.0, after exposure to 100 °C for 120 min and were not affected by the presence of surfactants and salts (N-Laourylsarcosine, NaCl, SDS, Triton X-100, Tween 20, Tween 80 and urea). Their molecular size was determined to be approximately 5 kDa by tricine-SDS-PAGE.Since the strains exhibited a strong antimicrobial activity against 21 L. monocytogenes strains and 6 Salmonella spp. isolates, they should be considered as potential bio-preservatives cultures for fermented food productions. Moreover, due to their technological features, the four strains could be taken in account for using as adjunct NSLAB (non-starter lactic acid bacteria) rather than as starter culture.     

Lactobacillus sakei 1 and its bacteriocin influence adhesion of Listeria monocytogenes on stainless steel surface

Listeria monocytogenes is of particular concern for the food industry due to its psychrotolerant and ubiquitous nature. In this work, the ability of L. monocytogenes culturable cells to adhere to stainless steel coupons was studied in co-culture with the bacteriocin-producing food isolate Lactobacillus sakei 1 as well as in the presence of the cell-free neutralized supernatant of L. sakei 1 (CFSN-S1) containing sakacin 1. Results were compared with counts obtained using a non bacteriocin-producing strain (L. sakei ATCC 15521) and its bacteriocin free supernatant (CFSN-SA). Culturable adherent L. monocytogenes and lactobacilli cells were enumerated respectively on PALCAM and MRS agars at 3-h intervals for up to 12 h and after 24 and 48 h of incubation. Bacteriocin activity was evaluated by critical dilution method. After 6 h of incubation, the number of adhered L. monocytogenes cells in pure culture increased from 3.8 to 5.3 log CFU/cm² (48h). Co-culture with L. sakei 1 decreased the number of adhered L. monocytogenes cells (P < 0.001) during all sampling times with counts lower than 3.0 log CFU/cm². The CFNS-S1 also led to a significant and similar reduction in culturable adhered L. monocytogenes counts for up to 24 h of incubation, however after 48 h of incubation, re-growth of L. monocytogenes number of adhered cells was observed, likely due to lack of competition for nutrients. L. sakei ATCC 15521 or its supernatant (CFNS-SA) did not reduce the number of adhered L. monocytogenes cells on stainless steel surface and from 6 h of incubation, listerial counts were between 4.3 and 4.5 log CFU/cm². These results indicate that L. sakei 1 and its bacteriocin sakacin 1 may be useful to inhibit early stages of L. monocytogenes adherence to abiotic surface.     

The effect of the pediocin PA-1 produced by Pediococcus acidilactici against Listeria monocytogenes and Clostridium perfringens in Spanish dry-fermented sausages and frankfurters

The inhibitory effects of the pediocin PA-1 in frankfurters and the Pediococcus acidilactici MCH14 pediocin-producing strain itself in Spanish dry-fermented sausages were evaluated. The experiments were carried out by in situ assays using Listeria monocytogenes and Clostridium perfringens as target bacteria strains. The inhibition of L. monocytogenes by the P. acidilactici MCH14 pediocin-producing strain was effective in Spanish dry-fermented sausages, being the counts of this pathogen reduced by 2 log cycles compared to the control. In frankfurters, the counts of L. monocytogenes, using 5000 bacteriocin units/ml (BU/ml) of the pediocin PA-1 produced by P. acidilactici MCH14, decreased by 2 and 0.6 log cycles after storage at 4 °C for 60 days and at 15 °C for 30 days, respectively, with respect to the control. Similar results were obtained for C. perfringens, with 5000 BU/ml the counts of this strain were reduced by 2 and 0.8 log cycles after storage at 10 °C for 60 days and at 15 °C for 30 days, respectively, with respect to the control. Both the pediocin PA-1 and the P. acidilactici MCH14 pediocin-producing strain could be a useful alternative for protection against these food-borne pathogens in meat products.     

Influence of Listeria innocua on the attachment of Listeria monocytogenes to stainless steel and aluminum surfaces

Listeria monocytogenes is an important foodborne pathogen that may be transmitted from the food-processing environment to food; however, the ecology and interaction of this organism with microbial residents on surfaces within the food industry is not well understood. The current study was undertaken to investigate the influence of Listeria innocua on the growth and attachment of L. monocytogenes to stainless steel or aluminum surfaces at 23 °C. When grown in broth as a mixed culture, L. innocua reached a higher cell count at 24 h than did L. monocytogenes. Attachment was evaluated by placing an aliquot containing 10³ CFU/ml of L. innocua and 10³ CFU/ml of L. monocytogenes on the coupons and by quantifying attached cells after 24 and 72 h. Attachment of L. monocytogenes was decreased by the presence of L. innocua. When compared to L. monocytogenes alone, there was a significant reduction of attachment of L. monocytogenes at 24 and 72 h on stainless steel and 72 h on aluminum surface when L. innocua was added at the same time. L. innocua exhibited an effect on the attachment of L. monocytogenes, increasing our knowledge of the behavior of L. monocytogenes in the presence of another Listeria species.     

Antilisterial activity and stability of nanovesicle-encapsulated antimicrobial peptide P34 in milk

Bacillus sp. P34, a strain isolated from aquatic environments of Brazilian Amazon basin, produces a bacteriocin-like substance (BLS) which was encapsulated in nanovesicles prepared from partially purified soy lecithin. The efficiency of free and encapsulated BLS P34 to control the development of L. monocytogenes and maintenance of antimicrobial activity was assessed over time in milk. The antimicrobial activity of free and encapsulated BLS P34 decreased approximately 50% after 4 days of storage (<4 °C) in skim and whole milk. After this period there was not significant loss of activity up to 21 days. The viable counts of Listeria monocytogenes in skim and whole milk containing 3200 AU/ml of free or encapsulated BLS P34 were always lower than those observed in controls without bacteriocin at both 30 °C and 7 °C. At 1600 AU/ml concentration, free and encapsulated BLS P34 were inhibitory to L. monocytogenes in skim milk, when compared with the control at 7 days. Nanovesicle-encapsulated and free BLS P34 shows potential use as biopreservative for application in milk-derived products.     

Growth inhibition of Listeria monocytogenes by bacteriocin-producing Staphylococcus equorum SE3 in cheese models

The anti-listerial activity of Staphylococcus equorum SE3 isolated from cheese brine was tested in two different model cheese systems to ascertain its potential for use as a protective culture for smear cheese ripening. Co-cultivation of Listeria monocytogenes L129 and the antilisterial S. equorum SE3 was performed on “milk agar” or “modified milk agar” (model cheese surface systems). S. equorum inoculated at concentrations of 10⁶ cfu/cm² completely inhibited growth of L. monocytogenes inoculated at 10–500 cfu/cm² on modified milk agar within 24 h of incubation, in the negative controls L129 grew to >10⁷ cfu/cm². At a higher inoculation level, growth inhibition was still more than 7 log units after 24 h. L. monocytogenes strains of different serotypes were also inhibited. Co-cultivation of S. equorum SE3 with other smear bacteria or yeasts, however, showed no growth inhibition of these important ripening microorganisms. The antilisterial effect was not diminished on the modified milk agar when co-cultivation was performed with the added smear cheese microbiota. However, on milk agar with no adjuncts (“green cheese model”), only a slight (<1 log unit) growth inhibition of L. monocytogenes was observed. Addition of peptides or amino acids to milk agar could restore growth inhibition of listeriae at different levels.     

Screening and characterisation of bacteriophage P100 insensitive Listeria monocytogenes isolates in Austrian dairy plants

Listeria monocytogenes is a major health threat in food production premises. The use of bacteriophages, such as listeriaphage P100, to minimize food pathogens is a promising alternative to eliminate bacteria, as is application of Listex™ P100. However, the use of phages may result in resistant L. monocytogenes strains. In this study 486 L. monocytogenes isolates obtained from 59 dairies over 15 years were screened for the presence of P100 insensitive L. monocytogenes. The overall number of P100 insensitive L. monocytogenes isolates was 13 (2.7%) for all years and all investigated plants. Insensitivities were detected in five of 59 dairies. Detected insensitive isolates did not appear randomly, but were in connection with phage treatments.The influence of dilution on the efficacy of P100 was tested by different host pathogen ratios. Additionally, artificially induced insensitive isolates were selected. Adsorption tests suggested that the detected insensitive L. monocytogenes isolates had receptor modifications.     

Antimicrobial effects of fractions from cranberry products on the growth of seven pathogenic bacteria

The antimicrobial effect of thirty HPLC fractions of different polarity obtained from two cranberry juices and three extracts (anthocyanins, water-soluble and apolar phenolic compounds) isolated from frozen cranberries and pomace was investigated against seven bacterial strains (Enterococcus faecium resistant to vancomycin (ERV), Escherichia coli O157:H7 EDL 933, Escherichia coli ATCC 25922, Listeria monocytogenes HPB 2812, Pseudomonas aeruginosa ATCC 15442; Salmonella Typhimurium SL1344 and Staphylococcus aureus ATCC 29213) The minimum inhibitory concentration (MIC) and the maximal tolerated concentration (MTC) of each fraction were determined for each pathogen using a 96-well microtiter plate method. The results, reported in μg phenol/mL, indicated that all the bacterial strains, both Gram-positive and Gram-negative, were selectively inhibited by the cranberry phenolic compounds. All pathogens were very sensitive to at least seven fractions with MTCs below 2 μg phenol/mL and five fractions with MICs below 10 μg phenol/mL. In addition, four fractions rich in apolar phenolic compounds were very efficient against all bacteria with MICs below 10 μg phenol/mL, and twenty five fractions completely inhibited microbial growth with MICs below100 μg phenol/well. L. monocytogenes exhibited the highest sensitivity with twelve very active fractions (MTCs and MICs below 1 and 10 μg phenol/mL, respectively) while E. coli O157H7 was the least sensitive to twenty seven fractions (with the highest MICs). Also, it appears that the technological process to manufacture cranberry juice can reduce the antimicrobial activity of phenolic fractions.     

Listeria monocytogenes in ready-to-eat vacuum and modified atmosphere packaged meat and fish products of Estonian origin at retail level

The prevalence, counts and genetic diversity of Listeria monocytogenes in ready-to-eat (RTE) vacuum and modified atmosphere packaged meat and fish products was studied in Estonia. Within two consecutive years 370 RTE food samples were collected at retail level from which 11% were found to be positive for L. monocytogenes. Contamination was higher among RTE fish products (17%) than in RTE meat products (6%). Generally, the counts of L. monocytogenes in positive products remained under ten colony forming units (CFU) per gram of product. Only 1.6% of the RTE meat and fish products contained L. monocytogenes in range of 10–100 CFU/g and 0.3% more than 100 CFU/g at the end of shelf-life. The food category containing highest L. monocytogenes prevalence was RTE lightly salted fish products with the prevalence of 32%. Only one (0.3%) RTE food sample exceeded the 100 CFU/g food safety criterion set out in the EU Regulation 2073/2005. Pulsed-field gel electrophoresis (PFGE) characterization of the isolates showed an overall similarity higher than 70%, and nine clusters based on 100% similarity were revealed. PFGE genotyping revealed that the few predominant pulsotypes were associated with particular food plants.     

Effect of the competitive growth of Lactobacillus sakei MN on the growth kinetics of Listeria monocytogenes Scott A in model meat gravy

Listeria monocytogenes is a foodborne pathogen capable of growth under refrigeration temperatures. The use of bacteriocin-producing lactic acid bacteria to inhibit Gram-positive pathogen growth may be an important tool to enhance the safety of refrigerated foods. The influence of three different populations of the bacteriocin-producing strain Lactobacillus sakei MN on the growth kinetic parameters of three different populations of L. monocytogenes Scott A co-cultured in model meat gravy at 4, 10, 16, and 22 °C was studied. The Baranyi growth model was used to estimate the kinetic parameters of L. monocytogenes and L. sakei for each strain cultured alone or in co-culture. The highest L. monocytogenes populations were achieved by pure cultures, decreasing in co-culture with the different inocula of L. sakei, at all temperatures. A modified logistic model was applied which includes a factor β that adjusts the effect of L. sakei on L. monocytogenes depending on the environmental conditions. The co-cultures of low (∼1log) L. monocytogenes inocula showed a decrease in β values when temperature increased, indicating that inter-species competition changes with temperature; the 2log- and 4log-inocula of L. monocytogenes co-cultures also showed this behavior but only with the higher initial population of L. sakei.     

Effect of temperature and salt on thermal inactivation of Listeria monocytogenes in salmon roe

Listeria monocytogenes is a potentially fatal foodborne pathogen that can be found in ready-to-eat seafood products, such as fresh salmon roe. Once contaminated, salmon roe must be decontaminated prior to human consumption. This study was conducted to determine the thermal inactivation kinetics of L. monocytogenes in raw salmon roe as affected by bacterial strain, temperature, and salt concentration. Three different strains of L. monocytogenes, including serotype 4b (F2365), 1/2b (F4260), and 1/2a (V7), were individually inoculated to salmon roe supplemented with salt (0–4.5%), and heated under different temperatures (57.5–65.0 °C) to evaluate the survival of the bacterium during heating and determine the D-values. Results showed that the thermal resistance (log D) of L. monocytogenes was significantly affected by bacterial strain, temperature, and salt and by their interactive effects, with strain F2365 being the most heat-resistant among all three strains tested. Salt added to salmon roe significantly increased the thermal resistance of the bacteria. For L. monocytogenes F2365, the z value of the bacterium in salmon roe was 5.99 °C, and its heat resistance increased with the level of salt in a linear manner. The results of kinetic analysis and the models obtained in this study may be used by the seafood industry to develop proper thermal processes to eliminate L. monocytogenes in raw salmon roe and to ensure microbial safety and prevent foodborne illness.