Inactivation of Gram-negative pathogens in refrigerated milk by reuterin in combination with nisin or the lactoperoxidase system

Inhibitory activity of reuterin (β-hydroxypropionaldehyde) combined with the antimicrobial peptide nisin or the lactoperoxidase system (LPOS) against food-borne Gram-negative pathogens in milk refrigerated at 4 and 8 °C was investigated. At 4 °C, reuterin (8 AU/ml) and LPOS were bactericidal against Salmonella enterica, Campylobacter jejuni, Aeromonas hydrophila and Yersinia enterocolitica, whereas the only effect recorded for nisin was a slight inhibition of Escherichia coli O157:H7. At 8 °C, reuterin was bactericidal against all the Gram-negative pathogens studied. Same results were attained with LPOS, except for its effect on Y. enterocolitica which was only bacteriostatic. The combination of reuterin with nisin did not enhance the antimicrobial effect of reuterin. A strong synergistic bactericidal activity of reuterin in combination with LPOS on E. coli O157:H7 and S. enterica was observed in milk at 4 °C, and against all the Gram-negative bacteria assayed in milk refrigerated at 8 °C. The application of both antimicrobials would be a useful means to inhibit pathogenic microorganisms, which may be present in milk due to postpasteurization contamination.     

Effects of combinations of lactoperoxidase system and nisin on the behaviour of Listeria monocytogenes ATCC 15313 in skim milk

Individual or combined effects of nisin (100 or 200 IU/ml) and the lactoperoxidase system (LPS) were analysed against 1 x 10(4) cfu/ml Listeria monocytogenes ATCC 15313 cells in skim milk, at 25 degrees C for 15 days. Nisin induced an immediate bactericidal effect and LPS a 48 h bacteriostatic phase which in both cases was followed by re-growth of L. monocytogenes. LPS and nisin added together at t0 showed a synergistic and lasting bactericidal effect which after 8 days and until 15 days resulted in no detectable cells in 1 ml of milk. When LPS was added to cells already in contact with 100 or 200 IU/ml nisin for a period of 4 h, the inhibitory activity was enhanced with no L. monocytogenes detectable after 72 or 48 h, respectively, and until 15 days. When LPS was added after 12 h, the nisin bactericidal phase was followed by re-growth. When nisin, 100 or 200 UI/ml, was added to cells already in contact with LPS over 24 h, L. monocytogenes was not detectable after 196 and 244 h, respectively, without any re-growth. For nisin addition after 72 h, cell counts were 8 log10 cycles lower than in the control milk after 196 h, but population levels were similar to the control within 15 days. The best combination to inhibit L. monocytogenes ATCC 15313 was nisin present at t0 followed by the LPS addition 4 h later, when the maximum inhibitory effect of nisin was reached.     

Inhibitory combinations of nisin, sodium chloride, and pH on Listeria monocytogenes ATCC 15313 in broth by an experimental design approach

The influence of pH (5.0-8.2), NaCl concentrations (0-6% w/v), and incubation time (0-24 h) on the inhibitory activity of nisin (0-100 I.U./ml) against Listeria monocytogenes (10(3) cfu/ml) was studied using the Doehlert experimental design and was confirmed by kinetic experiments. Predicted values were in agreement with experimental values. Experiments were carried out at 22 degrees C in reconstituted TSB-YE1 broth with or without NaCl. Nisin had an immediate pH-dependent bactericidal effect, which increased with decreasing pH values. In modified TSB-YE1 broth without NaCl, the bactericidal efficacy of nisin (50 I.U./ml) was maximum at pH 6.6, with no L. monocytogenes survivors until 120 h at 22 degrees C. Nisin (50 I.U./ml) action decreased in the presence of NaCl, with a minimal inhibitory effect between 2 and 4%. This partially protective effect was cancelled at higher levels of nisin.     

The antimicrobial effect of thyme essential oil, nisin, and their combination against Listeria monocytogenes in minced beef during refrigerated storage

The antimicrobial effect of thyme essential oil (EO) at 0.3%, 0.6%, or 0.9%, nisin at 500 or 1000IU/g, and their combination against Listeria monocytogenes was examined in both tryptic soy broth (TSB) and minced beef meat. Thyme EO at 0.3% possessed a weak antibacterial activity against the pathogen in TSB, whereas at 0.9% showed unacceptable organoleptic properties in minced meat. Thus, only the level of 0.6% of EO was further examined against the pathogen in minced meat. Treatment of minced beef meat with nisin at 500 or 1000IU/g showed antibacterial activity against L. monocytogenes, which was dependent on the concentration level of nisin and the strains used. Treatment of minced beef meat with EO at 0.6% showed stronger inhibitory activity against L. monocytogenes than treatment with nisin at 500 or 1000IU/g. All treatments showed stronger inhibitory activity against the pathogens at 10 degrees C than at 4 degrees C. The combined addition of EO at 0.6% and nisin at 500 or 1000IU/g showed a synergistic activity against the pathogen. Most efficient among treatments was the combination of EO at 0.6% with nisin at 1000IU/g, which decreased the population of L. monocytogenes below the official limit of the European Union recently set at 2logcfu/g, during storage at 4 degrees C.     

Nisin-curvaticin 13 combinations for avoiding the regrowth of bacteriocin resistant cells of Listeria monocytogenes ATCC 15313

Nisin (25-100 IU/ml) and curvaticin 13 (160 AU/ml), a bacteriocin produced by Lactobacillus curvatus SB13, were shown to have a bactericidal effect against Listeria monocytogenes ATCC 15313 in TSB-YE broth (pH 6.5), but it was only transitory. Regrowth was not due to the loss of bacteriocin activity. Cells surviving nisin or curvaticin 13 were more resistant to the respective bacteriocin than the parental strain. Survivors to curvaticin 13 were resistant to the class IIa bacteriocins (camocin CP5, pediocin AcH) but remained sensitive to nisin. The frequencies of spontaneous nisin resistants decreased with increasing bacteriocin concentration and the presence of salts (NaCl, K2HPO4). The behaviour of nisin (1000 IU/ml) or curvaticin 13 (640 AU/ml) resistant variants (Nis1000, Curv645) was investigated in the presence of nisin (100 IU/ml) or curvaticin 13 (320 AU/ml) at 22 and 37 degrees C, and compared with that of the parental strain. The effectiveness of nisin was the same at both temperatures, whereas curvaticin 13 displayed a faster bactericidal action at 37 degrees C. Nis1000 cells were less sensitive to curvaticin 13 than the parental strain, whereas Curv640 cells were more sensitive to nisin than the parental strain. Simultaneous or sequential additions of nisin (50 IU/ml) and curvaticin 13 (160 AU/ml) were performed at 22 degrees C in broth inoculated with the parental strain. All combinations induced a greater inhibitory effect than the use of a single bacteriocin. Simultaneous addition of bacteriocins at t0 led to the absence of viable cells in the broth after 48 h.     

Application of reuterin produced by Lactobacillus reuteri 12002 for meat decontamination and preservation

Lactobacillus reuteri strain 12002 was used for reuterin production in the two-step fermentation process. A batch culture fermentation was used to produce a maximum biomass of L. reuteri. Then cells were harvested, resuspended in a glycerol-water solution, and anaerobically incubated to produce reuterin. The lyophilized supernatants (approximately 4000 activity units (AU) of reuterin per ml) were diluted in distilled water for decontamination and preservation trials. The MIC values of reuterin for Escherichia coli O157:H7 and Listeria monocytogenes were 4 and 8 AU/ml, respectively. In meat decontamination experiments, the surface of cooked pork was inoculated with either L. monocytogenes or E. coli O157:H7 at a level of approximately log10 5 CFU/cm2, incubated for 30 min at 7 degrees C, and decontaminated by exposure to reuterin (500 AU/ml). The bactericidal effect of reuterin was analyzed 15 s and 24 h after exposure at 7 degrees C. After 15 s of exposure to reuterin, viable numbers decreased by 0.45 and 0.3 log10 CFU/cm2 for E. coli O157:H7 and L. monocytogenes, respectively. After 24 h the numbers decreased by 2.7 log10 CFU/cm2 for E. coli O157:H7 and by 0.63 log10 CFU/cm2 for L. monocytogenes. In the same experiment, the combined effect of reuterin and lactic acid was also investigated. Adding lactic acid (5%, vol/vol) to reuterin significantly enhanced (P < or = 0.05) the efficacy of reuterin. No additional effect (P < or = 0.05) was found when ethanol (40%) was added to the mixture of reuterin and lactic acid. To evaluate the preservative effect of reuterin during meat storage, reuterin was added to raw ground pork contaminated with E. coli O157:H7 or L. monocytogenes. Reuterin at a concentration of 100 AU/g resulted in a 5.0-log10 reduction of the viability of E. coli O157:H7 after 1 day of storage at 7 degrees C. Reuterin at a concentration of 250 AU/g reduced the number of the viable cells of L. monocytogenes by log10 3.0 cycles after 1 week of storage at 7 degrees C.     

Effectiveness of Some Natural Antimicrobial Compounds in Controlling Pathogen or Spoilage Bacteria in Lightly Fermented Chinese Cabbage

ABSTRACT: This study was designed to evaluate the bactericidal or bacteriostatic effect of chitosan, an allyl isothiocyanate (AIT) product, and nisin for the artificially inoculated pathogenic bacteria ( Escherichia coli , Salmonella Enteritidis, Staphylococcus aureus , and Listeria monocytogenes ) or natural microflora of fermented Chinese cabbage. Addition of 0.1% chitosan decreased the population of pathogens from 0.7 to 1.7 log colony-forming units (CFU)/g after 4 d of storage at 10 °C. The bactericidal activity of chitosan was found to be stronger than that of nisin (0.05 mg/g). Addition of 0.2% of the AIT product (containing AIT and hop extract) exhibited a bacteriostatic effect. However, a combination of AIT product and chitosan enhanced bactericidal efficacy against L. monocytogenes . The addition of chitosan or AIT product was observed to suppress the populations of mesophilic and coliform bacteria during storage at 10 °C for 4 d. Moreover, the use of chitosan or the AIT product did not change the sensory quality of the lightly fermented vegetable. Therefore, these results suggest that chitosan or the AIT product could be useful to improve the microbial safety and quality of lightly fermented vegetable.     

Inhibition of Listeria monocytogenes using nisin with grape seed extract on turkey frankfurters stored at 4 and 10 degrees C

Recontamination of cooked ready-to-eat (RTE) chicken and beef products with Listeria monocytogenes has been a major safety concern. Natural antimicrobials in combinations can be an alternative approach for controlling L. monocytogenes. Therefore, the objectives of this study were to evaluate the inhibitory activities against L. monocytogenes of nisin (6,400 IU/ ml), grape seed extract (GSE; 1%), and the combination of nisin and GSE both in tryptic soy broth with 0.6% yeast extract (TSBYE) and on the surface of full-fat turkey frankfurters. TSBYE was incubated at 37 degrees C for 72 h and turkey frankfurters at 4 or 10'C for 28 days. Inocula were 6.7 or 5 log CFU per ml or g for TSBYE or frankfurters, respectively. After 72 h in TSBYE, nisin alone did not show any inhibitory activity against L. monocytogenes. The combination of nisin and GSE gave the greatest inhibitory activity in both TSBYE and on turkey frankfurters with reductions of L. monocytogenes populations to undetectable levels after 15 h and 21 days, respectively. This combination of two natural antimicrobials has the potential to control the growth and recontamination of L. monocytogenes on RTE meat products.     

Characterization and antimicrobial activity of bacteriocin 217 produced by natural isolate Lactobacillus paracasei subsp. paracasei BGBUK2-16

The strain Lactobacillus paracasei subsp. paracasei BGBUK2-16. which was isolated from traditionally homemade white-pickled cheese, produces bacteriocin 217 (Bac217; approximately 7 kDa). The onset of Bac217 biosynthesis was observed in the logarithmic phase of growth, and the production plateau was reached after 9 or 12 h of incubation at 37 and 30 degrees C, respectively, when culture entered the early stationary phase. Biochemical characterization showed that Bac217 retained antimicrobial activity within the range of pH 3 to 12 or after treatment at 100 degrees C for 15 min. Bac217 antimicrobial activity also remained unchanged after storage at 4 degrees C for 6 months or -20 degrees C for up to 12 months. However, Bac217 activity was completely lost after treatment with different proteolytic enzymes. BGBUK2-16 contains only one plasmid about 80 kb in size. Plasmid curing indicated that genes coding for Bac217 synthesis and immunity seem to be located on this plasmid. Bac217 exhibited antimicrobial activity against some pathogenic bacteria, such as Staphylococcus aureus and Bacillus cereus. Interestingly, Bac217 showed activity against Salmonella sp. and Pseudomonas aeruginosa ATCC27853. The inhibitory effect of BGBUK2-16 on the growth of S. aureus in mixed culture was observed. S. aureus treatment with Bac217 led to a considerable decrease (CFU/ml) within a short period of time. The mode of Bac217 action on S. aureus was identified as bactericidal. It should be noted that the strain BGBUK2-16 was shown to be resistant to bacteriocin nisin, which is otherwise widely used as a food additive for fermented dairy products.     

Transmission electron microscopy study of Listeria monocytogenes treated with nisin in combination with either grape seed or green tea extract

The objective of this study was to use transmission electron microscopy to investigate the morphological changes that occurred in Listeria monocytogenes cells treated with grape seed extract (GSE), green tea extract (GTE), nisin, and combinations of nisin with either GSE or GTE. The test solutions were prepared with (i) 1% GSE, 1% GTE, 6,400 IU of nisin, and the combination of these dilutions with nisin or with (ii) the pure major phenolic constituents of GSE (0.02% epicatechin plus 0.02% catechin) or GTE (0.02% epicatechin plus 0.02% caffeic acid) and their combinations with 6,400 IU of nisin in tryptic soy broth with 0.6% yeast extract (TSBYE). Test solutions were inoculated with L. monocytogenes at approximately 10(6) CFU/ml and incubated for 3 or 24 h at 37 degrees C. After 3 h of incubation, cells were harvested and evaluated under a transmission electron microscope (JEOL-100 CX) operating at 80 kV (50,000X). Microscopic examination revealed an altered cell membrane and condensed cytoplasm when L. monocytogenes cells were exposed to a combination of nisin with either GSE or GTE or to pure compounds of the major phenolic constituents in combination. After 24 h of incubation at 37 degrees C, the combinations of nisin with GSE and nisin with GTE reduced the L. monocytogenes population to undetectable levels and 3.7 log CFU/ml, respectively. These observations indicate that the combination of nisin with either GSE or GTE had a synergistic effect, and the combinations of nisin with the major phenolic constituents were most likely associated with the L. monocytogenes cell damage during inactivation in TSBYE at 37 degrees C.     

Effect of combining nisin and/or lysozyme with in-package pasteurization on thermal inactivation of Listeria monocytogenes in ready-to-eat turkey bologna

Achieving a targeted lethality with minimum exposure to heat and preservation of product quality during pasteurization is a challenge. The objective of this study was to evaluate the effect of nisin and/or lysozyme in combination with in-package pasteurization of a ready-to-eat low-fat turkey bologna on the inactivation of Listeria monocytogenes. Sterile bologna samples were initially treated with solutions of nisin (2 mg/ml = 5,000 AU/ml = 31.25 AU/cm2), lysozyme (10 mg/ml = 80 AU/ml = 0.5 AU/cm2), and a mixture of nisin and lysozyme (2 mg/ml nisin + 10 mg/ml lysozyme = 31.75 AU/cm2). Bologna surfaces were uniformly inoculated with a Listeria suspension resulting in a population of approximately 0.5 log CFU/cm2. Samples were vacuum packaged and subjected to heat treatment (60, 62.5, or 65 degrees C). Two nonlinear models (Weibull and log logistic) were used to analyze the data. From the model parameters, the time needed to achieve a 4-log reduction was calculated. The nisin-lysozyme combination and nisin treatments were effective in reducing the time required for 4-log reductions at 62.5 and 65 degrees C but not at 60 degrees C. At 62.5 degrees C, nisin-lysozyme-treated samples required 23% less time than did the control sample to achieve a 4-log reduction and 31% less time at 65 degrees C. Lysozyme alone did not enhance antilisterial activity with heat. Results from this study can be useful to the industry for developing an efficient intervention strategy against contamination of ready-to-eat meat products by L. monocytogenes.     

Behavior of Listeria monocytogenes and Staphylococcus aureus in yogurt fermented with a bacteriocin-producing thermophilic starter

Streptococcus salivarius subsp. thermophilus B producing a bacteriocin active against Listeria monocytogenes ATCC 7644 and Staphylococcus aureus SAD 30 was isolated from bakery yeast. The bacteriocin was partially purified by an adsorption/desorption technique, and its spectrum of action was compared to that of a neutralized cell-free supernatant (CFS). Although the CFS inhibited a number of gram-positive and -negative bacteria of health and spoilage significance, the spectrum of action of the partially purified bacteriocin was limited to gram-positive bacteria. L. monocytogenes was the most sensitive to both preparations. The bacteriocin-producing streptococcal strain was used in combination with a Bac- Lactobacillus delbrueckii subsp. bulgaricus CY strain isolated from commercial yogurt to assess the effectiveness of the resulting thermophilic starter in controlling L. monocytogenes and S. aureus in yogurt during fermentation and storage at refrigeration (ca. 7 degrees C) or abuse (ca. 22 degrees C) temperature. Yogurt samples were contaminated with L. monocytogenes or S. aureus to the approximate levels of 10(3) and 10(6) CFU/ml of milk, respectively. The results showed that in situ bacteriocin production was more active against L. monocytogenes than against S. aureus in vitro and in contaminated samples. While L. monocytogenes leveled off below the detectable limit in a 1-ml sample of yogurt within 24 h of processing, S. aureus survived in Bac+ and Bac- samples during 10 days of storage at room temperature (ca. 22 degrees C). Use of a Bac+ starter resulted in a 5-day extension of the shelf life.     

Destruction of Listeria monocytogenes in sturgeon (Acipenser transmontanus) caviar by a combination of nisin with chemical antimicrobials or moderate heat

The objective of this study was to investigate the effect of nisin in combination with heat or antimicrobial chemical treatments (such as lactic acid, chlorous acid, and sodium hypochlorite) on the inhibition of Listeria monocytogenes and total mesophiles in sturgeon (Acipenser transmontanus) caviar. The effects of nisin (250, 500, 750, and 1,000 IU/ml), lactic acid (1, 2, and 3%), chlorous acid (134 and 268 ppm), sodium hypochlorite (150 and 300 ppm), and heat at 60 degrees C for 3 min were evaluated for a five-strain mixture of L. monocytogenes and total mesophiles in sturgeon caviar containing 3.5% salt. Selected combinations of these antimicrobial treatments were also tested. Injured and viable L. monocytogenes cells were recovered using an overlay method. Treating caviar with > or =500 IU/ml nisin initially reduced L. monocytogenes by 2 to 2.5 log units. Chlorous acid (268 ppm) reduced L. monocytogenes from 7.7 log units to undetectable (<0.48 log units) after 4 days of storage at 4 degrees C. However, there were no synergistic effects observed for combinations of nisin (500 or 750 IU/ml) plus either lactic acid or chlorous acid. Lactic acid caused a slight reduction (approximately 1 log unit) in the microbial load during a 6-day period at 4 degrees C. Sodium hypochlorite was ineffective at the levels tested. Mild heating (60 degrees C for 3 min) with nisin synergistically reduced viable counts of L. monocytogenes and total mesophiles. No L. monocytogenes cells (<0.48 log units) were recovered from caviar treated with heat and nisin (750 IU/ml) after a storage period of 28 days at 4 degrees C.     

Effectiveness and stability of plastic films coated with nisin for inhibition of Listeria monocytogenes

Plastic films were coated with a cellulose-based carrier solution containing nisin, a natural antimicrobial peptide with the potential to inhibit growth of food spoilage and pathogenic microorganisms such as Listeria monocytogenes. Five commercial plastic films with different chemical compositions and surface properties were compared in this study: low-density polyethylene, ethylene-vinyl acetate copolymer, and three types of ethylene-methacrylic acid copolymers: Surlyn 1601, Nucrel 0403, and Nucrel 0903. The films were coated with nisin at a concentration of 1000 IU/cm2. Nisin-coated films were stored at room temperature (21 degrees C) and at 4 degrees C and analyzed weekly for 12 weeks. Antimicrobial activity of the different nisin-coated films against a nisin indicator strain, Lactococcus lactis subsp. cremoris ATCC 14365, and against L. monocytogenes ATCC 19115 was assessed using an inhibition zone assay. Nisin incorporated into the films was recovered by a boiling and extraction procedure, and its activity was quantified using an agar well diffusion assay. Film type did not have any significant effect on the antimicrobial activity of the nisin-coated films (P < 0.05); all five film types had comparable inhibition zones on both assays. The films maintained stable activity for the duration of the study, both at room temperature and refrigeration. The results of this study demonstrate that commercially available packaging films can be coated with nisin and the resulting antimicrobial films can be conveniently stored at room temperature with no adverse effect on nisin activity.     

Enhancement of nisin, lysozyme, and monolaurin antimicrobial activities by ethylenediaminetetraacetic acid and lactoferrin

A microtiter plate assay was employed to systematically assess the interaction between ethylenediaminetetraacetic acid (EDTA) or lactoferrin and nisin, lysozyme, or monolaurin against strains of Listeria monocytogenes, Escherichia coli, Salmonella enteritidis, and Pseudomonas fluorescens. Low levels of EDTA acted synergistically with nisin and lysozyme against L. monocytogenes but EDTA and monolaurin interacted additively against this microorganism. EDTA synergistically enhanced the activity of nisin, monolaurin, and lysozyme in tryptic soy broth (TSB) against two enterohemorrhagic E. coli strains. In addition, various combinations of nisin, lysozyme, and monolaurin with EDTA were bactericidal to some gram-negative bacteria whereas none of the antimicrobials alone were bactericidal. Lactoferrin alone (2000 microg ml(-1)) did not inhibit any of the bacterial strains, but did enhance nisin activity against both L. monocytogenes strains. Lactoferrin in combination with monolaurin inhibited growth of E. coli O157:H7 but not E. coli O104:H21. While lactoferrin combined with nisin or monolaurin did not completely inhibit growth of the gram-negative bacteria, there was some growth inhibition. All combinations of EDTA or lactoferrin with antimicrobials were less effective in 2% fat UHT milk than in TSB. S. enteritidis and P. fluorescens strains were consistently more resistant to antimicrobial combinations. Resistance may be due to differences in the outer membrane and/or LPS structure.     

Antibacterial effect of water-soluble tea extracts on foodborne pathogens in laboratory medium and in a food model

The microbial inhibition of foodborne pathogens was determined in brain heart infusion broth with 10% (wt/vol) water-soluble extracts of green, jasmine, black, dungglre, and oolong tea against Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, Listeria monocytogenes, and Staphylococcus aureus. The mixed culture (approximately 6.0 log CFU/ml), which was composed of the four pathogens, was inoculated into brain heart infusion broth with and without tea extracts. After incubation at 35 degrees C for 0, 1, 3, and 5 days, proper dilution of each sample was spiral plated on each selective agar. Viable cell counts were performed after incubation at 35 degrees C for 24 to 36 h. Green, jasmine, and black tea exhibited an approximately 5.0 log suppression of S. aureus compared with the control from days 1 to 5. Green and jasmine tea also suppressed the growth of L. monocytogenes by approximately 3.0 log CFU/ml on day 5. In contrast, no tea extracts inactivated E. coli O157:H7 and Salmonella Enteritidis. Based on the result in liquid medium, green and jasmine teas of 0.1% (vol/wt) were individually evaluated for their antimicrobial activity against L. monocytogenes and S. aureus in a food model (ground beef) stored at 7 degrees C for 0, 1, 3, 5, and 7 days. Viable cell counts of total bacteria, L. monocytogenes, and S. aureus in ground beef were not significantly different among green and jasmine tea and the control.     

Antibacterial activity of xanthorrhizol isolated from Curcuma xanthorrhiza Roxb. against foodborne pathogens

Xanthorrhizol, isolated from the ethanol extract of Curcuma xanthorrhiza Roxb., is a sesquiterpene compound with a molecular weight of 218. The aim of this study was to investigate the antibacterial activity of xanthorrhizol against foodborne pathogens. The antibacterial activity of xanthorrhizol was measured in terms of the MIC and the MBC. MICs and MBCs of xanthorrhizol against Bacillus cereus, Clostridium perfringens, Listeria monocytogenes, Staphylococcus aureus, Salmonella Typhimurium, and Vibrio parahaemolyticus were 8, 16, 8, 8, 16, 8 microg/ml and 16, 32, 16, 16, 16, 16 microg/ml, respectively. The bactericidal study, as determined by the viable cell count method, revealed that xanthorrhizol treatment at 4 x MIC reduced viable cells by at least 6 to 8 log for all six foodborne pathogens in 4 h. Xanthorrhizol maintained its antibacterial activity after thermal treatments (121 degrees C, 15 min) under various pH ranges (pH 3.0, 7.0, and 11.0). These results strongly suggest that xanthorrhizol, conferring strong antibacterial activity with thermal and pH stability, can be effectively used as a natural preservative to prevent the growth of foodborne pathogens.     

Effect of nisin on growth boundaries of Listeria monocytogenes Scott A, at various temperatures, pH and water activities

The effect of nisin on growth boundaries of Listeria monocytogenes Scott A in Tryptone Soy Broth (TSB) under different a(w)s, pH, and temperatures was studied. Growth/no growth turbidity data was modeled using logistic regression. Combinations of various temperatures (5-35 degrees C), pH (4.05-6.70) adjusted with HCl, a(w)s (0.937-0.998) NaCl (0.5-10.5%) and nisin (0-100 IU/ml) were used to monitor the growth/no growth response of L. monocytogenes Scott A for 60 days. The concordance of the logistic regression model was 99.4%, indicating successful data fitting. The minimum pH at which growth was observed was 4.81 at the temperature range of 25-35 degrees C and at a(w) as high as 0.992. Growth was observed at a(w) as low as 0.937, at pH 6.7, at the temperature range of 25-35 degrees C. Increasing nisin concentrations above 25 IU/ml resulted in a more inhibitory environment for L. monocytogenes. Presence of 100 IU/ml resulted in a minimum pH for growth at 5.20, and a minimum a(w) at 0.967 at the temperature range of 25-35 degrees C. It was remarkable that low to medium salt concentrations (2.5-4.5 NaCl% w/v) provided a protective effect against inhibition of L. monocytogenes by nisin. The present study points out the applicability of growth/no growth modeling in order to study any interactions between various factors affecting initiation of growth of micro-organisms, in which its turn helps the understudying of microbe-food ecosystem relations and the development of safer food.     

Synergy between nisin and select lactates against Listeria monocytogenes is due to the metal cations

Listeria monocytogenes, a major foodborne pathogen, has been responsible for many outbreaks and recalls. Organic acids and antimicrobial peptides (bacteriocins) such as nisin are produced by lactic acid bacteria and are commercially used to control pathogens in some foods. This study examined the effects of lactic acid (LA) and its salts in combination with a commercial nisin preparation on the growth of L. monocytogenes Scott A and its nisin-resistant mutant. Because of an increase in its activity at a lower pH, nisin was more active against L. monocytogenes when used in combination with LA. Most of the salts of LA, including potassium lactate, at up to 5% partially inhibited the growth of L. monocytogenes and had no synergy with nisin. Zinc and aluminum lactate, as well as zinc and aluminum chloride (0.1%), worked synergistically with 100 IU of nisin per ml to control the growth of L. monocytogenes Scott A. No synergy was observed when zinc or aluminum lactate was used with nisin against nisin-resistant L. monocytogenes. The nisin-resistant strain was more sensitive to Zn lactate than was wild-type L. monocytogenes Scott A; however, the cellular ATP levels of the nisin-resistant strain were not significantly affected. Changes in the intracellular ATP levels of the wild-type strain support our hypothesis that pretreatment with zinc lactate sensitizes cells to nisin. The similar effects of thesalts of hydrochloric and lactic acids support the hypothesis that metal cations are responsible for synergy with nisin.     

Fate of Listeria monocytogenes inoculated onto the surface of model Turkey frankfurter pieces treated with zein coatings containing nisin, sodium diacetate, and sodium lactate at 4 degrees C

The antimicrobial effects of zein coatings containing nisin, sodium lactate, and sodium diacetate against Listeria monocytogenes on turkey frankfurters at 4 degrees C were determined. Our objectives were to determine whether zein, nisin, lactate, and diacetate alone or in combination could control the growth of L. monocytogenes on full-fat turkey frankfurters at 4 degrees C and to determine whether lactate or diacetate had any synergistic effect on the activity of nisin. Turkey frankfurter pieces surface inoculated with L. monocytogenes strain V7 were treated with zein-ethanol-glycerol (ZEG), zein-propylene-glycol (ZPR), ethanol-glycerol (EG), propylene glycol (PR), nisin (N), sodium lactate (L), or sodium diacetate (D) alone or in combination. Over 28 days, treatment with N or D alone reduced L. monocytogenes counts on frankfurters by 6.6 or 6.3 log CFU/g, respectively. N-D treatment reduced L. monocytogenes by 6 log CFU/g. The zein solvents EG and PR reduced L. monocytogenes by about 5.6 and 5.2 log CFU/g, respectively, similar to the results obtained with ZEG and ZPR, which suggests that zein powder per se had no antimicrobial activity. After 28 days, ZEG-N-D, ZEG-N-D-L, ZPR-N-D, and ZPR-N-D-L yielded no detectable CFU. L alone was ineffective. No synergies were observed. N and D when used singly and the combinations of N-D, ZEG-N-D, ZEG-N-D-L, ZPR-N-D, ZPR-N-D-L, EG, and PR were effective as inhibitors of the growth of recontaminating L. monocytogenes cells on full-fat turkey frankfurters.