Antibacterial activity and mechanism of bifidocin A against Listeria monocytogenes

Bifidocin A, produced by Bifidobacterium animalis BB04, is a novel bacteriocin with antimicrobial activity against a wide range of gram-positive and gram-negative foodborne bacteria. The objective of this study was to investigate the antibacterial activity and mechanism of action of bifidocin A against Listeria monocytogenes, one of the most susceptible bacteria to this bacteriocin. The minimum inhibitory concentration (MIC) of bifidocin A for L. monocytogenes 35152 was 0.029 mg/mL. Time-kill assays showed that bifidocin A effectively inhibited the growth of L. monocytogenes in a time-and concentration-dependent manner. The mechanism of action of bifidocin A was studied by analyzing its effects at a MIC on the cell morphology, intracellular organization, membrane permeability, membrane integrity, and membrane proton motive force (PMF) of L. monocytogenes. Scanning and transmission electron microscopy analyses showed that bifidocin A induced alterations in the morphology and intracellular organization of L. monocytogenes cells. Confocal laser scanning microscopy images showed that L. monocytogenes cells treated with bifidocin A took up propidium iodide. Bifidocin A treatment also induced the leakage of K⁺ and inorganic phosphate, the hydrolysis and release of ATP, and a collapse of the transmembrane electrical potential and pH gradient in L. monocytogenes cells. These results suggested that bifidocin A exerted its anti-Listeria monocytogenes effect through the dissipation of the cytoplasmic membrane PMF, increased membrane permeability, cell membrane pore formation, destruction of membrane integrity, and ultimately complete disintegration of the cells.     

Modelling the recovery of Listeria monocytogenes in high pressure processed simulated cured meat

Current EC regulations require that ready-to-eat products should not exceed the limit of 100 CFU/g for Listeria monocytogenes throughout their shelf-life. On that basis a quantitative analysis for high hydrostatic pressure to produce safe (regarding L. monocytogenes levels) cured meat products with low salt concentration has been developed. An extended Doehlert design for a range of pressures (450–800 MPa), sodium chloride (0–34.9 g/L) and sodium nitrite (mg/L) concentrations, as well as the resulting a_w (0.955–0.987) levels, was generated. Based on the logistic regression analysis, it appears that the recovery of L. monocytogenes is influenced by the applied pressure, the storage time and the synergistic effect of pressure and a_w on inhibiting microbial recovery. This means that the sodium chloride and sodium nitrite concentrations tested indirectly affected the recovery of Listeria and consequently the shelf-life of the product by regulating the a_w levels. The lower the water activity, the less the inactivation and recovery induced by pressure immediately and during storage, respectively.     

Antimicrobial activity of rhamnolipids against Listeria monocytogenes and their synergistic interaction with nisin

The rhamnolipids (RL) are biodegradable biosurfactants which have low toxicity and surface active properties that can be useful for food processing industries. The objective of this study was to evaluate the antimicrobial potential of rhamnolipids against Listeria monocytogenes. Susceptibility tests were performed by the minimal inhibitory concentration (MIC) using the micro-broth dilution technique. The MIC values varied from 78.1 μg/mL to 2500 μg/mL with the 2500 μg/mL being the predominant value. Among the 32 tested cultures, 90.6% were susceptible to RL. Results showed that the rhamnolipid activity was primarily bacteriostatic. The interaction of rhamnolipid with nisin was also investigated. The combined effect of nisin and RL was evaluated against two wild-type isolates of L. monocytogenes, L12 sensitive to RL (MIC 156.2 μg/mL) and L17 less sensitive to RL (2500 μg/mL). The FIC indexes for the isolates were 0.18 and 0.078 for L12 and L17 respectively, indicating a strong synergistic effect. The survival curve of isolates L12 and L17 showed that the combination between nisin and RL was bactericidal at lower concentration than for the individual antimicrobials. For the L12 isolate 78.1 μg/mL of RL and 160 IU/mL of nisin eliminated the population after 30 min of incubation. The combination of 156.2 μg/mL of RL and 320 IU/mL of nisin reduced completely L17 population after 2 h of incubation. Rhamnolipids showed antimicrobial activity against L. monocytogenes and presented a synergistic effect when combined with nisin.     

Characterisation of the resistance and the growth variability of Listeria monocytogenes after high hydrostatic pressure treatments

The use of non-thermal methods for food preservation is due to consumer demands for microbiological safe products, without changes in the sensory and nutritional qualities of the product. High hydrostatic pressure (HHP) has emerged as an alternative to traditional thermal processing methods for foods.Listeria monocytogenes CECT 5672 was treated under HHPs (350, 400 and 450 MPa) for 3, 16 and 23 min. The effects of pH (5, 6 and 7) and sodium chloride concentration (0, 0.5 and 1.0) of the recovery medium were studied on L. monocytogenes. The kinetic parameters were estimated by the method described by Métris, George, and Baranyi (2006). From results obtained, histograms of the lag phase were generated and distributions were fitted. The duration of the lag phase of HHP damaged cells increased with the application of additional stresses. Histograms showed a shift to longer lag phases and an increase in variability with high stress levels in the recovery medium.Using a primary model together with Monte Carlo simulation, predictions of time to growth (100 cfu/g) of L. monocytogenes were established and they were compared with deterministic predictions. It was evidenced that deterministic predictions do not give a good indication of the probability of a certain level of growth.     

Inactivation of Listeria monocytogenes and Listeria innocua in yogurt drink applying combination of high pressure processing and mint essential oils

Influences of high pressure processing (HPP) on some physical properties and on inactivation of Listeria monocytogenes and Listeria innocua in a yogurt drink (ayran) were quantified with or without addition of mint essential oil. Pressure treatment alone or combined with mint essential oil did not cause significant changes in pH, water activity, color and serum protein separation (p > 0.05). HPP of ayran samples at 600 MPa for treatment time of 300 s reduced L. monocytogenes and L. innocua by more than 5-log units (p ≤ 0.05) at ambient temperature. Addition of mint essential oil further enhanced inactivation of both bacteria by more than 1 log cfu mL^?1. Combination of mint essential oil with HPP provided a reduction in pressure treatment severity by 100–300 MPa or by 210 s to achieve the same amount of inactivation relative to HPP alone. Both Weibull distribution and log-logistic models were fitted to survival data. High pressure-processing combined with mint essential oil appeared to be a promising technique for preserving microbiologically-safe ayran with no significant impacts to product quality.     

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.     

Effect of high pressure processing in combination with Weissella viridescens as a protective culture against Listeria monocytogenes in ready-to-eat salads of different pH

This study explored the effect of HPP (400 MPa/1 min) and a Weissella viridescens protective culture, alone or in conjunction, against Listeria monocytogenes in ready-to-eat (RTE) salads with different pH values (4.32 and 5.59) during storage at 4 and 12 °C. HPP was able to reduce the counts of the pathogen after treatment achieving approximately a 4.0 and 1.5 log CFU/g reduction in the low and higher pH RTE salad, respectively. However, L. monocytogenes was able to recover and grow during subsequent storage. W. viridescens grew in both RTE salads at both storage temperatures, with HPP resulting in only a small immediate reduction of W. viridescens ranging from 0.50 to 1.2 log CFU/g depending on the pH of the RTE salad. For the lower pH RTE salad, the protective culture was able to gradually reduce the L. monocytogenes counts during storage whereas for the higher pH RTE salad in some cases it delayed growth significantly or exerted a bacteriostatic effect. exerted a bacteriostatic effect. The results revealed that the increased storage temperature led to an increase in the inactivation/inhibition of L. monocytogenes in the presence of W. viridescens. The combination of HPP and W. viridescens is a promising strategy to control L. monocytogenes and can increase safety even when a break in the chill chain occurs.     

Efficacy of a combination of nisin and p-Anisaldehyde against Listeria monocytogenes

Nisin is used as a food-safe antimicrobial agent and has been widely applied to daily food products to prevent bacterial growth. However, the practical application of nisin is limited. Although nisin inhibits the growth of Listeria monocytogenes, exposure to this agent is also a risk factor for the development of nisin resistance. p-Anisaldehyde (AS) is used in the pharmaceutical industry to manufacture antimicrobial drugs, but few studies have been reported describing the antibacterial activity of AS in the food field. Combining nisin with AS may enhance antimicrobial activity. The synergy observed in vitro was confirmed by checkerboard microdilution method, with the fractional inhibitory concentration index (FICI) values ranging from 0.125 to 0.281. When AS was combined with nisin, a strong synergistic effect was observed using the time-kill and agar-diffusion assays. SEM and LIVE/DEAD BacLight experiment results also suggest that the bactericidal mechanism of the drug combination involves cell wall lysis and membrane damage. We can use this synergy to achieve a stable antimicrobial effect to overcome antimicrobial drug resistance.     

Antibacterial activity of Oregano,Rosmarinus and Thymus essential oils against Staphylococcus aureus and Listeria monocytogenes in beef meatballs

Antimicrobial activity of five essential oils (EOs) was investigated up to 72 h against foodborne pathogens (Staphylococcus aureus, Listeria monocytogenes, Salmonella enteritidis, Campylobacter jejuni) through disk diffusion and determination of Minimum Inhibitory Concentrations. The most active EOs were Thymus vulgaris and Origanum vulgare, followed by Cinnamomum zeylanicum, Rosmarinus officinalis, and Salvia officinalis. The antimicrobial activity of O. vulgare, Rosmarinus officinalis and T. vulgaris was investigated against five enterotoxin producers of S. aureus and five L. monocytogenes strains, for different amounts of time (up to 14 days), at 4 °C, in meatballs. Concentrations of 2% and 1% restricted the growth of both the pathogens but, as a result of panel tests, altered the meat flavor. The cooked meatballs containing 0.5% of EO were acceptable in terms of taste, and the oils were able to suppress concentrations of <10² CFU/g of the pathogens, revealing the potential use of R. officinalis, T. vulgaris and O. vulgare as food preservatives at this concentration.     

Effects of lactic acid and hot water treatments on Salmonella Typhimurium and Listeria monocytogenes on beef

The present study is designed to determine alone and combined effects of lactic acid (LA) and hot water (HW) treatments. Hot water and different concentrations of lactic acid were evaluated for their reduction effects on Salmonella Typhimurium and Listeria monocytogenes on contaminated beef during refrigerated storage at 4 °C. The reductions were 0.05–1.19 and 0.09–1.14 log for S. Typhimurium and L. monocytogenes on day 0 respectively, while it was 0.43–1.78 and 1.69–3.84 log respectively on day 5 of storage. Results of this study suggest that LA and HW treatments can be used to reduce S. Typhimurium and L. monocytogenes which provide an additional measure of safety in production line.     

Inactivation of Listeria monocytogenes and Salmonella enterica serovar Senftenberg 775W inoculated into fruit juice by means of ultra high pressure homogenisation

The inactivation of Listeria monocytogenes and Salmonella enterica serovar Senftenberg 775 W by ultra high pressure homogenisation (UHPH) was evaluated in grape and orange juices inoculated at a concentration of approximately 7 log CFU/ml. The fluid inlet temperature used was 6 °C and the pressure levels assayed were 200, 300 and 400 MPa. Viable and injured bacterial counts were obtained 2 h after the UHPH treatments and after 5, 8, and 15 days of storage at 4 °C. Pressure level had a significant impact on the lethal effect of UHPH and complete inactivation of S. enterica serovar Senftenberg 775 W was achieved at 400 MPa. L. monocytogenes showed more resistance than S. enterica serovar Senftenberg 775 W to the UHPH treatments and no significant differences were observed between 300 and 400 MPa treatments in both juices. Sublethal injuries were not detected in any case. During the storage at 4 °C viable counts of both strains showed a decreasing trend. L. monocytogenes viable counts became undetectable in UHPH treated and also in control samples of grape juice which could be attributed to the presence of natural compounds with antilisterial effect.     

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.     

Effects of changes in pH and temperature on the inhibition of Salmonella and Listeria monocytogenes by Allyl isothiocyanate

The minimum inhibitory (MIC) and minimum bactericidal concentrations (MBC) of mustard allyl isothiocyanate (AITC) against five strains each of Salmonella and Listeria monocytogenes individually, and when combined by genus were studied in Mueller-Hinton broth at 21 or 37 °C as well as the interactive effects of pH (5.0–9.0) at temperatures of 4–21 °C on cell viability when held under these conditions for up to 10 d. The stability of 200 ppm AITC was monitored during these trials. The MIC and MBC values of AITC ranged from 60 to 100 ppm and 120–180 ppm, respectively, at 37 °C and ranged from 10 to 40 ppm and 200–600 ppm, respectively, at 21 °C against both pathogens. AITC had no antimicrobial activity at low temperatures (4 or 10 °C) and alkaline pH over 10 d, but at neutral pH, AITC reduced L. monocytogenes by 4.14 and 8.45 log₁₀ CFU/ml at 4 or 10 °C, respectively. At acidic pH, AITC was more effective against Salmonella which was reduced by 2.56 and 6.48 log₁₀ CFU/ml at 4 and 10 °C, respectively. However, AITC was more effective at combinations of 21 °C and neutral pH against L. monocytogenes (cells were not detected at and beyond 3 d) and at combinations of the higher temperature and acidic pH against Salmonella (cells were not detected at and beyond 6 d). Mustard AITC was more stable at low pH and temperature indicating that it can be an effective antimicrobial at combinations of low or neutral pH values and room or refrigerator temperatures (4–10 °C) against these foodborne pathogens.     

Efficacy of propionic acid against Listeria monocytogenes attached to poultry skin during refrigerated storage

This work evaluates the effect of propionic acid dip on the growth of Listeria monocytogenes on poultry legs stored at 4 °C for 8 days.Fresh inoculated chicken legs were dipped into either 1% or 2% propionic acid solution (v/v) or distilled water (control). Changes in mesophiles, enterobacteriaceae counts and sensorial characteristics (odour, colour and overall appearance) were also evaluated.The shelf life of the samples washed with propionic acid was extended by at least 2 days over the control samples washed with distilled water. Legs washed with 2% propionic acid showed a significant (p < 0.05) inhibitory effect on L. monocytogenes compared to control legs, with a decrease of about 2.72 log units after 3 days of storage. Sensory quality was not adversely affected by propionic acid.This study demostrates that while propionic acid did reduce growth of L. monocytogenes on meat, it did not completely inactivate the pathogen.     

Antibacterial mechanism of Myagropsis myagroides extract on Listeria monocytogenes

The ethanol extract of Myagropsis myagroides had antimicrobial activity against Gram-positive bacteria. The extract was fractionated through liquid–liquid extraction; the chloroform fraction had strong antimicrobial activity against Listeria monocytogenes (minimum inhibitory concentration (MIC) 0.063 mg/mL), and Clostridium perfringens (MIC 0.031 mg/mL). The chloroform fraction was separated into 22 sub-fractions using silica gel column chromatography, with the fourth fraction (CH4) possessing the strongest antimicrobial activity against Gram-positive bacteria. Leakage of 260 nm-absorbing material and ATP was observed in CH4-treated cells and morphological alterations were observed by electron microscopy. These results indicate that the cytoplasmic membrane may be a target of the CH4 fraction.     

高温高压对超深井钻井液密度的影响

井眼内钻井液密度是进行各种钻井施工和设计的必要的基础数据,高温高压环境下的超深井钻井液密度不再是一个常数,而是随温度和压力的变化而变化,因此有必要对超深井钻井中高温高压对钻井液密度的影响进行研究。利用高温高压钻井液密度模拟实验装置,采用胜科1井现场配制的超深井钻井液,测量了温度、压力对超深井水基钻井液密度的影响特性,根据测量结果,建立了温度、压力影响下的水基钻井液密度预测模型。结果表明,水基钻井液密度受温度变化影响比受压力变化影响大,随着温度、压力的增大,钻井液密度降幅较大,同时,高温高压下钻井液更具有可压缩性。建立的预测模型为合理确定现场钻井液密度范围提供了一种新方法。     

Antilisterial activity of a crude preparation of Lactobacillus sakei 1 bacteriocin and its lack of influence on Listeria monocytogenes haemolytic activity

The antilisterial activity of a semi-purified bacteriocin from the lactic acid bacterium (LAB) Lactobacillus sakei 1 bac⁺, was studied at 8 and 15 °C. L. monocytogenes was grown in the presence of bacteriocin and enumerated after 0, 5 and 10 days. The bacteriocin was detected by spot-on-the-lawn assay (on the first day) and quantified by critical dilution assay on the first day and on day 10. At 8 °C, L. monocytogenes population decreased in the presence of the bacteriocin, but resistant cells were selected and reached high counts. Despite this, the final populations of L. monocytogenes were still lower where bacteriocin was present than when absent. At 15 °C, the addition of bacteriocin did not affect the multiplication of L. monocytogenes. Moreover, it was observed that the haemolytic activity of L. monocytogenes was not affected by exposure to bacteriocin of L. sakei 1.     

Antibacterial effect of black seed oil on Listeria monocytogenes

Listeria monocytogenes is a major foodborne pathogen in the United States. Effective methods for reducing L. monocytogenes in foods would reduce the likelihood of foodborne outbreaks of listeriosis, and decrease economic losses to the food industry. Nigella sativa is a herbaceous plant, whose seeds (black seed) have been used as a spice and condiment in foods in the Middle East. The objective of this study was to determine the antibacterial effect of black seed oil on twenty strains of L. monocytogenes by disc diffusion method. A population of 7.0 log CFU of each strain of L. monocytogenes was inoculated on duplicate plates containing antibiotic medium one agar. The plates were allowed to dry at room temperature for 15 min. Three discs (6 mm diameter), each impregnated with 10 μl of black seed oil, vegetable oil (oil control), or gentamicin (positive control) were placed on each inoculated plate. The plates were incubated at 37 °C for 24 h, and were observed for zones of L. monocytogenes growth inhibition. Black seed oil exhibited a strong antibacterial activity against all the strains of L. monocytogenes, yielding a significantly (P<0.01) larger inhibition zone than that of gentamicin. The mean zones of inhibition produced by black seed oil and gentamicin were 31.50 ± 1.0 and 14.80 ± 0.50, respectively. The vegetable oil had no inhibitory effect on L. monocytogenes. Results indicate that black seed oil could potentially be used to inhibit L. monocytogenes, but appropriate applications in foods need to be validated.     

Antimicrobial activity of clove (Syzgium aromaticum) oil in inhibiting Listeria monocytogenes on chicken frankfurters

The ability of Listeria monocytogenes to survive and grow at refrigeration temperature in some ready to eat (RTE) poultry products is a public health concern. The inhibitory effect of clove oil (1% and 2%, v/w) applied to the surface of RTE chicken frankfurters was determined on seven strains of L. monocytogenes inoculated at low (10²–10³ cfu/g) or high cell numbers (10⁴–10⁶ cfu/g), and stored at 5 °C for 2 weeks or at 15 °C for 1 week. All strains of L. monocytogenes survived and grew on control frankfurters at 5 °C and 15 °C but growth was inhibited under both storage conditions in the presence of either 1% or 2% clove oil. Depending on the sensory considerations, the addition of clove oil to frankfurters may be an effective strategy to control L. monocytogenes in chicken frankfurters.