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.     

Reduction of Escherichia coli population following treatment with bacteriocins from lactic acid bacteria and chelators

The inhibitory activity of lactocin 705/AL705 (2133 arbitrary units per ml (AU ml(-1))), two bacteriocins produced by Lactobacillus curvatus CRL705 and nisin (1066AU ml(-1)) produced by Lactococcus lactis CRL1109 in combination with chelating agents against Escherichia coli strains in TSB medium at 21 and 6 degrees C was investigated. Treatment with EDTA (500 and 1000 mm) and Na lactate (800 mm) alone produced a variable effect depending on the strain, Na lactate being inhibitory against E. coli NCTC12900 at both assayed temperatures while EDTA (1000 mm) led to its inactivation only at 6 degrees C. Direct and deferred strategies using EDTA and Na lactate showed that the direct addition of bacteriocins and chelators was not as effective as compared to deferred treatments. When the deferred treatment effectiveness was evaluated at 6 degrees C, the use of EDTA (500 and 1000 mm) and Na lactate (800 mm) in combination with lactocin 705/AL705 demonstrated to be the most inhibitory strategy against both E. coli strains. Nevertheless, treatments with chelators and bacteriocins was highly dependent upon strain sensitivity. Permeabilization of the outer membrane of E. coli strains with EDTA and Na lactate combined with lactocin 705/AL705 showed to be valuable in controlling this foodborne bacteria at low temperatures.     

Antagonistic action of Lactobacillus lactis toward Salmonella spp. and Escherichia coli O157:H7 during growth and refrigerated storage

Cells of Lactobacillus lactis were added to trypticase soy broth that contained cells of Escherichia coli O157:H7 or cells of Salmonella spp. in order to determine if L. lactis inhibited the pathogens. The inhibition of all pathogens was examined during growth at 37 degrees C for 24 h. Inhibition of Salmonella spp. was also examined at refrigeration temperatures (6 degrees C) for 5 days. One strain each of E. coli O157:H7, Salmonella Typhimurium, and Salmonella Enteritidis was examined. E. coli was enumerated on violet red bile agar, and Salmonella spp. were enumerated on brilliant green agar. In all experiments at 37 degrees C, the L. lactis completely inhibited all pathogens, producing numbers that were not detectable after 24 h of incubation. There were significant (P > 0.05) increases in numbers of the pathogens in the control samples containing no L. lactis. There were significant (P < 0.05) declines in the pH of both control and L. lactis inoculated samples. There was a significantly (P < 0.05) larger decline in the pH of samples inoculated with L.lactis. Interaction studies with pH-neutralized broth indicated that acid production by L lactis was primarily responsible for the inhibition. Numbers of Salmonella spp. incubated at 6 degrees C did not decline significantly (P > 0.05) for control or inoculated samples, which suggests that this strain of L. lactis does not inhibit Salmonella spp. at refrigeration temperatures. Additionally, there were no significant (P > 0.05) changes in pH or in numbers of L. lactis during refrigerated storage.     

Antimicrobial potential of immobilized Lactococcus lactis subsp. lactis ATCC 11454 against selected bacteria

Immobilization of living cells of lactic acid bacteria could be an alternative or complementary method of immobilizing organic acids and bacteriocins and inhibit undesirable bacteria in foods. This study evaluated the inhibition potential of immobilized Lactococcus lactis subsp. lactis ATCC 11454 on selected bacteria by a modified method of the agar spot test. L. lactis was immobilized in calcium alginate (1 to 2%)-whey protein concentrate (0 and 1%) beads. The antimicrobial potential of immobilized L. lactis was evaluated in microbiological media against pathogenic bacteria (Escherichia coli, Salmonella, and Staphylococcus aureus) or Pseudomonas putida, a natural meat contaminant, and against seven gram-positive bacteria used as indicator strains. Results obtained in this study indicated that immobilized L. lactis inhibited the growth of S. aureus, Enterococcus faecalis, Enterococcus faecium, Lactobacillus curvatus, Lactobacillus sakei, Kocuria varians, and Pediococcus acidilactici. Only 4 h of incubation at 35 degrees C resulted in a clear inhibition zone around the beads that increased with time. With the addition of 10 mM of a chelating agent (EDTA) to the media, results showed growth inhibition of E. coli; however, P. putida and Salmonella Typhi were unaffected by this treatment. These results indicate that immobilized lactic acid bacteria strains can be successfully used to produce nisin and inhibit bacterial growth in semisolid synthetic media.     

Optimization of Medium Composition for Nisin Fermentation with Response Surface Methodology

ABSTRACT:  Nisin is an effective food biopreservative widely used in food industry. However, 1 problem of concern is limited production rate and final nisin concentration. A nisin-producing strain, L. lactis Lac2, a mutant strain with high yield of nisin, was obtained in our laboratory recently. In the present study, a fractional factorial design was applied to investigate the main factors that affect the yield of L. lactis Lac2. Central composite experimental design and response surface methodology were adopted to derive a statistical model for optimizing the composition of the medium. The results showed that the optimum medium for nisin production of L. lactis Lac2 was composed of 2.68% sucrose (w/v), 0.5% tryptone (w/v), 1% yeast extract (w/v), 0.3% Tween-80 (w/v), 0.02% MgSO₄·7H₂O (w/v), 0.81% NaCl (w/v), 1.91% K₂HPO₄ (w/v), 0.05% ascorbic acid (w/v), and 2% agar (w/v) (if necessary) at pH 6.5. When cultured in the optimum medium, the nisin yield is an average of 3381.81 IU/mL, which nearly doubled the yield when incubated in the initial medium. Also, the concentration of tryptone was decreased while that of the sucrose was increased when compared with CM broth, which means a reduction of the fermentation cost.     

Resistance of Escherichia coli and Salmonella against nisin and curvacin A.

We have determined the effects of the following factors on the resistance of Gram-negative bacteria against nisin and curvacin A: (i) chemotype of the lipopolysaccharide (LPS), (ii) addition of agents permeabilizing the outer membrane, (iii) the fatty acid supply of the growth medium, and (iv) the adaptation to acid and salt stress. Bacteriocin activity was determined against growing and resting cells as well as protoplasts. All smooth strains of Escherichia coli and Salmonella enterica serovar Typhimurium were highly resistant towards the bacteriocins, whereas mutants that possess the core of the LPS, but not the O antigen, as well as deep rough LPS mutants were sensitive. Antibiotics with outer membrane permeabilizing activity, polymyxin B and polymyxin B nonapeptide, increased the sensitivity of smooth E. coli towards nisin, but not that of deep rough mutants. Incorporation of 1 g l(-1) of either oleic acid or linoleic acid to the growth media greatly increased the susceptibility of E. coli LTH1600 and LTH4346 towards bacteriocins. Both strains of E. coli were sensitive to nisin and curvacin A at a pH of less than 5.5 and more than 3% (w/v) NaCl. Adaptation to sublethal pH or higher NaCl concentrations (pH 4.54 and 5.35 or 4.5% (w/v) NaCl) provided only limited protection against the bacteriocidal activity of nisin and curvacin A. Adaptation to 4.5% (w/v) NaCl did not result in cross protection to bacteriocin activity at pH 4.4, but rendered the cells more sensitive towards bacteriocins.     

Reduction of Listeria monocytogenes and Escherichia coli O157:H7 numbers on vacuum-packaged fresh beef treated with nisin or nisin combined with EDTA.

Nisin or nisin combined with EDTA was used to treat fresh beef. Beef cubes (2.5 by 2.5 by 2.5 cm) that were inoculated with approximately 7 log CFU/ml of Listeria monocytogenes Scott A or Escherichia coli O157:H7 505 B were dipped in the following solutions: (i) H2O, (ii) HCl, (iii) nisin, (iv) EDTA, or (v) nisin combined with EDTA, respectively, for 10 min each, with an exception of one set of control beef samples without treatment. Beef samples were then drip-dried for 15 min, vacuum packaged, and stored at 4 degrees C for up to 30 days. The pH on beef after different treatments was not a key factor in preventing bacterial growth. Treatment with nisin or with nisin combined with EDTA reduced the population of L. monocytogenes by 2.01 and 0.99 log CFU/cm2 as compared to the control, respectively, under the conditions of vacuum package and storage at 4 degrees C for up to 30 days. However, the effect of nisin and nisin combined with EDTA against E. coli O157:H7 505 B was marginal at 1.02 log CFU/cm2 and 0.8 log CFU/cm2 reductions, respectively.     

Efficacy of nisin in combination with protective cultures against Listeria monocytogenes Scott A in tofu.

Nisin can be used as a biopreservative to control growth of Listeria monocytogenes in various minimally processed foods. Tofu is an example of a non-fermented soybean product, which may allow growth of Listeria at refrigeration temperatures and in which nisin may be applied to prevent multiplication of Listeria. The efficacy of nisin against Listeria may be compromised by the emergence of spontaneous nisin-resistant mutants. Exposure of L. monocytogenes Scott A to nisin in a culture medium or in a food product results in an initial reduction of Listeria population which is followed by regrowth of survivors to nisin during further incubation. In vitro studies using Standard I Nutrient broth showed that Enterococcus faecium BFE 900-6a and Lactobacillus sakei Lb 706-1a used as protective cultures in combination with nisin were able to suppress proliferation of Listeria cells not killed by nisin at 10 degrees C. Growth and bacteriocin production of these two strains and a third protective culture, Lactococcus lactis BFE 902 was also observed in soymilk and tofu at 10 degrees C. Inoculation studies with tofu prepared with nisin and protective cultures showed that lower amounts of nisin are required for an effective inhibition of L. monocytogenes Scott A when either E. faecium BFE 900-6a or Lc. lactis BFE 902 are used in addition. The combination of nisin with these bacteriocinogenic lactic acid bacteria (LAB) resulted in a complete suppression of listerial growth in homemade tofu stored at 10 degrees C for 1 week. Lb. sakei Lb 706-1a was less effective and did not prevent a slight increase of L. monocytogenes Scott A numbers during storage.     

Effect of Nisin on the Outgrowth of Clostridium botulinum Spores

Nisin, an antibiotic produced by certain strains of Streptococcus lactis, is effective in preventing the outgrowth of Clostridium botulinum spores. Type A C. botulinum spores were the most resistant to the inhibitory action of nisin requiring 1000-2000 I.U. of nisin/ml for a 50% inhibition of outgrowth on TPYG agar plates. Type E spores were more sensitive requiring only 50-100 I.U./ml for 50% inhibition of outgrowth on TPYG agar plates. Type B spores displayed an intermediate level of sensitivity requiring 500-1000 I.U. of nisin/ml for 50% inhibition of outgrowth on TPYG agar plates. Similar levels of nisin were necessary to prevent spore outgrowth in TPYG broth and BHI broth over a 7-day incubation period. With prolonged incubation periods of up to 65 days in TPYG broth, spore outgrowth was observed sporadically at higher nisin levels with the type A and B spores which may indicate some decomposition of nisin with storage. Nisin levels of 5000 I.U./ml for the type A spores and 2000 I.U./ml for the type B spores and the Minnesota E spores were insufficient to prevent spore outgrowth by C. botulinurn in cooked meat medium. For the Beluga E spores, a nisin level of 2000 I.U./ml was necessary to prevent spore outgrowth in cooked meat medium. The need for higher levels of nisin in cooked medium to prevent spore outgrowth may be due to the binding of the nisin by meat particles.     

Isolation, identification, and selection of lactic acid bacteria from alfalfa sprouts for competitive inhibition of foodborne pathogens

Several studies have investigated the control of pathogens on alfalfa sprouts, and some treatments have been shown to be effective in reducing pathogen populations. However, control methods investigated thus far only provide pathogen control at a given point in the sprouting process and can affect germination. Competitive inhibition of pathogens with lactic acid bacteria might provide pathogen control throughout the sprouting process and up to consumption. The purpose of this study was to isolate and identify lactic acid bacteria from alfalfa sprouts to inhibit the growth of foodborne pathogens. Fifty-eight lactic acid bacteria isolates were obtained from alfalfa seeds and sprouts. These isolates were evaluated for inhibitory action against Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes by agar spot tests. All pathogens were inhibited by 32 (55%) of the isolates, S. enterica by 56 (97%), E. coli O157:H7 by 49 (84%), and L. monocytogenes by 41 (71%). The isolates were identified by the Analytical Profile Index evaluation of carbohydrate utilization. Isolates obtained from a sample of alfalfa seeds and identified as Lactococcus lactis subsp. lactis showed zones of inhibition of 4.0 mm or greater for all pathogens. One of these isolates, Lactococcus lactis subsp. lactis (L7), and an isolate previously obtained, Pediococcus acidilactici (D3), were evaluated for competitive inhibition of S. enterica, E. coli O157:H7, and L. monocytogenes in deMan Rogosa Sharpe agar and broth. Pathogen populations were significantly reduced by day 5. The selected isolates will be further evaluated in future studies for inhibitory action toward S. enterica, E. coli O157:H7, and L. monocytogenes during sprouting.     

Biodegradable polylactic acid polymer with nisin for use in antimicrobial food packaging

ABSTRACT:  Biodegradable polylactic acid (PLA) polymer was evaluated for its application as a material for antimicrobial food packaging. PLA films were incorporated with nisin to for control of foodborne pathogens. Antimicrobial activity of PLA/nisin films against Listeria monocytogenes , Escherichia coli O157:H7, and Salmonella Enteritidis were evaluated in culture media and liquid foods (orange juice and liquid egg white). Scanned electron micrograph and confocal laser microscopy revealed that nisin particles were evenly distributed in PLA polymer matrix on the surface and inside of the PLA/nisin films. PLA/nisin significantly inhibited growth of L . monocytogenes in culture medium and liquid egg white. The greatest inhibition occurred at 24 h when the cell counts of L. monocytogenes in the PLA/nisin samples were 4.5 log CFU/mL less than the controls. PLA/nisin reduced the cell population of E. coli O157:H7 in orange juice from 7.5 to 3.5 log at 72 h whereas the control remained at about 6 log CFU/mL. PLA/nisin treatment resulted in a 2 log reduction of S. Enteritidis in liquid egg white at 24 °C. After 21 d at 4 °C the S. Enteritidis population from PLA/nisin treated liquid egg white (3.5 log CFU/mL) was significantly less than the control (6.8 log CFU/mL). E. coli O157:H7 in orange juice was more sensitive to PLA/nisin treatments than in culture medium. The results of this research demonstrated the retention of nisin activity when incorporated into the PLA polymer and its antimicrobial effectiveness against foodborne pathogens. The combination of a biopolymer and natural bacteriocin has potential for use in antimicrobial food packaging.     

Combined effect of water activity and pH on the inhibition of Escherichia coli by nisin

The Doehlert design and surface response methodology were used to study the influence of pH and water activity (aw) on Escherichia coli inhibition by nisin. Combining stress factors at levels where they are not inhibitory by themselves, a reduction of E. coli survival fraction can be achieved with lower nisin doses than in a single nisin treatment. For all the pH values assayed, a synergistic effect of aw and nisin concentration was detected, and the isoresponse lines showed the existence of an area of maximum inhibition. Factors that reduced viable cell counts by 4 to 5 log cycles were 1,000 to 1,400 IU of nisin per ml at pH 5.5 to 6.5 and a water activity of 0.97 and 0.98. The addition of different ionic and nonionic solutes to control aw suggested that the effect of aw in the inhibitory action of nisin on E. coli cells was not solute-specific. The use of the Doehlert experimental design was effective to determine the optimal combination of stress factors, as well as to point out the most important variables that affected E. coli inhibition.     

Nisin的生产、提纯和检测

Nisin是一种由乳酸乳球菌产生的羊毛硫氨酸类细菌素 ,在许多国家被许可作为生物防腐剂。Nisin的产量受许多因素的制约 ,如产生菌性能、培养基组成 (碳源、氮源、磷源和阳离子 )、发酵条件 (pH、温度、搅拌、通风 )、发酵类型 (分批发酵、连续发酵、自由细胞、固定化细胞 )等 ;大规模回收和纯化Nisin主要采用一些基于吸附-解析或者相分配原理的方法 ;最常用的定量检测Nisin的方法主要有生物分析法和免疫检测法 ,采用各种特定nisin抗体的免疫检测方法具有迅速、灵敏、准确等特性并能实现Nisin的在线检测。     

Further Studies on the Antibotulinal Effectiveness of Nisin in Acidic Media

The antibotulinal effectiveness of nisin in TPYG broth was increased somewhat by lowering the pH to pH 5.5. The ability of nisin to inhibit the outgrowth of strain 56A spores was markedly increase at pH 5.5 by comparison to its effectiveness at higher pHs observed in previous studies. The increased effectiveness of nisin at pH 5.5 was less notable for the strain 69A, 113B, and 213B spores. The nisin sensitivity of the type E spores was essentially unchanged from that observed in earlier studies at higher pHs. At pH 6, nisin levels of 5000 I.U./ml were insufficient to prevent spore outgrowth by C. botulinum in cooked meat medium. Comparatively, much lower levels of nisin were effective in preventing botulinal outgrowth in TPYG broth at pH 6. The decreased effectiveness of nisin in cooked meat medium may be due to the binding of nisin to meat particles, and this binding is apparently not affected by lowering the pH to pH 6.0.     

乳酸菌对肠出血性大肠杆菌O157:H7抑制作用的研究

为探讨乳酸菌对肠出血性大肠杆菌O157：H7 ATCC43895(E．Coli O157：H7)的抑制作用，在培养基上进行了研究。将E．Coli O157：H7与干酪乳杆菌干酪亚种、植物乳杆菌、发酵乳杆菌、乳酸乳球菌和瑞士乳杆菌同时接种在培养基中，E．Coli O157：H7的活性不受影响；将E．Coli O157：H7接种到培养了24h的乳酸茵培养液中，E．Coli O157：H7活性显下降。以乳酸调整的低pH值对E．Coli O157：H7有一定的杀灭作用。本研究表明：乳酸菌的代谢产物乳酸对E．Coli O157：H7有杀灭作用。     

Growth patterns of Escherichia coli O157:H7 in ground beef treated with nisin,chelators, organic acids and their combinations immobilized in calcium alginate gels

The effects of antimicrobial substances including nisin, acetic acid, lactic acid, potassium sorbate and chelators (disodium ethylenediamine tetraacetic acid [EDTA] and sodium hexametaphosphate [HMP]), alone or in combination and, with or without immobilization in calcium alginate gels, on the growth of Escherichia coli O157:H7 in ground beef were investigated. Results showed that acetic acid and potassium sorbate could inhibit the growth of E. coli O157:H7 effectively at 10°C and at 30°C. Both EDTA and HMP did not halt the growth of E. coli O157:H7. In an antimicrobial system immobilized with calcium alginate, most of the antimicrobials could not inhibit the growth of E. coli O157:H7 in ground beef at 10°C and at 30°C, with the exception of acetic acid and lactic acid. Immobilization did not enhance the effectiveness of acetic acid against E. coli O157:H7 in ground beef at 10°C and at 30°C (P>0.05) but it did enhance the effectiveness of lactic acid at 10°C. In a system combining different antimicrobials, treatment with nisin /EDTA or nisin/potassium sorbate at 10°C revealed a significantly lower population change of E. coli O157:H7 compared to samples treated with nisin, EDTA or potassium sorbate alone. The use of calcium alginate immobilization further enhanced the effectiveness of the combination system of nisin/EDTA, nisin/acetic acid and nisin/potassium sorbate on the growth of E. coli O157:H7 in ground beef at 10°C but it was not effective at 30°C.     

Incidence of nisin Z production in Lactococcus lactis subsp. lactis TFF 221 isolated from Thai fermented foods

Lactic acid bacteria isolated from various Thai fermented foods were screened for the presence of nisin gene by using PCR with primers specific to nisin A structural gene. Only one strain, Lactococcus lactis subsp. lactis TFF 221, isolated from kung jom, a traditional shrimp paste, was found to carry a nisin gene. The TFF 221 nisin had antimicrobial activity against not only closely related lactic acid bacteria but also some foodborne pathogens. It was heat stable and inactivated by alpha-chymotrypsin and proteinase K. Some characteristics of TFF 221 nisin were found to be very similar to those of nisin A produced by Lactococcus lactis subsp. lactis NCDO 2111. Both of them had the same antimicrobial spectrum and MICs against all indicator bacteria. However, when assayed with indicator organisms, in all cases the TFF 221 nisin produced larger zones of inhibition in agar diffusion assays than the nisin A did. Sequencing of the TFF 221 nisin gene showed that it was the natural nisin variant, nisin Z, as indicated by the substitution of asparagine residue instead of histidine at position 27. The nisin determinant in strain TFF 221 was found to be located on a conjugative transposon residing in the chromosome. The ability of the nisin produced by L. lactis subsp. lactis TFF 221 to inhibit a wide range of foodborne pathogens may be useful in improving the food safety of the fermented product, especially in the Thai environment, which suffers from perennial problems of poor food hygiene.     

Purification and characterization of X-prolyl-dipeptidyl-aminopeptidase from Lactobacillus lactis and from Streptococcus thermophilus

X-Prolyl-dipeptidyl-aminopeptidase recently was found in several lactic acid bacteria. This article describes the purification of the enzymes from Lactobacillus lactis and Streptococcus thermophilus and compares their characteristics. Enzymes from both strains are serine-peptidases. They both have a molecular weight of about 165,000 daltons, an isoelectric point near 4.5, and are constituted of two subunits. The pH optimum of the enzyme isolated from L. lactis is 7.0, whereas the enzyme from S. thermophilus possesses a broad pH optimum between 6.5 and 8.2 with glycyl-L-prolyl-aminomethylcoumarin as substrate. Below pH 5, both enzymes are unstable; however, that from S. thermophilus is more rapidly denatured. The enzyme from S. thermophilus is more sensitive to heat than the corresponding enzyme from L. lactis. Enzymes from the both strains have different specificities towards various substrates and are differently effected by metals, chelators, and other inhibitors. The importance of this enzyme for the metabolism of lactic acid bacteria is discussed.     

Enhanced inhibition of Escherichia coli O157:H7 by lysozyme and chelators

This study examined the effects of three chelating agents (EDTA, disodium pyrophosphate [DSPP], and pentasodium tripolyphosphate [PSTPP]) on the inhibition of the growth of Escherichia coli O157:H7 by lysozyme. The objective of this study was to identify replacement chelators that exhibit synergistic properties similar to those of EDTA. The inhibitory effects of EDTA at 300 to 1,500 microg/ml and of DSPP and PSTPP at 3,000 to 15,000 microg/ml in combination with lysozyme at 200 to 600 microg/ml for up to 48 h at pHs of 6.0, 7.0, and 8.0 on four strains of E. coli O157:H7 was studied with the use of a microbroth dilution assay. The addition of EDTA enhanced lysozyme's inhibitory effect on strains of E. coli O157:H7. EDTA at > or = 300 microg/ml combined with lysozyme at 200 to 600 microg/ml was sufficient to inhibit the growth of the strains at pHs of 6.0 and 8.0. At pH 7.0, lysozyme at 200 to 600 microg/ml and EDTA concentrations of > or = 1,000 microg/ml were effective in inhibiting three of the four strains. DSPP at pH 6.0 was inhibitory at > or = 10,000 microg/ml when combined with lysozyme at 200 to 300 microg/ml. In contrast, PSTPP increased the inhibitory activity of lysozyme more effectively at pH 8.0. Lysozyme at 200 to 600 microg/ml was effective against two strains of E. coli O157:H7 when used in conjunction with PSTPP at > or = 5,000 microg/ml. The remaining strains were inhibited by PSTPP at > or = 10,000 microg/ml. Our results indicate that inhibition occurred with each lysozyme-chelator combination, but the concentrations of phosphates required to increase the antimicrobial spectrum of lysozyme against E. coli O157:H7 were higher than the EDTA concentrations required to achieve the same effect.     

Inhibition of Listeria innocua in hummus by a combination of nisin and citric acid

The effect of nisin or citric acid or combinations of these two inhibitors on the inactivation of a cocktail of three Listeria innocua strains was investigated in a model brain heart infusion (BHI) broth and hummus (chickpea dip). In BHI broth, citric acid had a limited ability to inhibit L. innocua growth. Nisin initially reduced L. innocua concentrations by about 3 log cycles; however, L. innocua reached concentrations similar to those of the control after 5 days at 22 degrees C. In combination, the effects of 500 IU/ml nisin and 0.2% citric acid were synergistic and resulted in complete elimination of L. innocua in the BHI broth. The inhibition of L. innocua by nisin (500 or 1,000 IU/g), citric acid (0.1, 0.2, or 0.3%), or their combinations also was evaluated in hummus. Citric acid alone did not affect L. innocua growth or the aerobic bacterial plate count. A combination of 1,000 IU/g nisin and 0.3% citric acid was somewhat effective (approximately 1.5-log reduction) in controlling the concentration of L. innocua and the aerobic plate count for up to 6 days. This combination also may be useful, in addition to proper hygienic practices, for minimizing the growth of the pathogen Listeria monocytogenes in hummus.