INHIBITION OF LISTERIA MONOCYTOGENES AND OTHER BACTERIA IN SPANISH SOFT CHEESE MADE WITH LACTOCOCCUS LACTIS SUBSP. DIACETYLACTIS

The inhibitory effects of a lactose-negative single-strain starter culture of Lactococcus lactis subsp. diacetylactis were evaluated in Spanish soft cheese (Queso fresco, pH 6.5) against Listeria monocytogenes, Escherichia coli O157:H7 , Enterobacter cloacae, Pseudomonas aeruginosa and Pseudomonas fluorescens. Queso Fresco made without starter culture and stored at 3 and 7C supported growth of all bacteria tested, with the exception of E. coli and P. aeruginosa which were unable to grow at 3C. When cheese was made with the starter culture and stored at 7C , L. monocytogenes, P. aeruginosa and E. coli did not grow, the growth of E. cloacae was delayed, and P. fluorescens grew uninhibited. In cheese stored at 3C, only P. fluorescens was able to grow.     

INHIBITION OF LISTERIA MONOCYTOGENES AND OTHER BACTERIA BY SODIUM DIACETATE

Growth and survival patterns of Listeria monocytogenes strain Scott A were studied in brain heart infusion broth containing sodium diacetate. Minimum inhibitory concentrations decreased with decrease in temperature, from 35 and 32 mM at 35C and 20C, respectively, to 28 mM at 5C. Broth pH containing 35, 32, and 28 mM sodium diacetate was 5.25, 5.40 and 5.60, respectively. Sodium diacetate was more effective than acetic acid alone in inhibiting the organism over the pH range of 5.0-6.0. Addition of 21 mM (0.3%) sodium diacetate to ground beef or beef slurry suppressed total aerobic counts during refrigerated storage. Although the meat pH decreased from 5.6 to 5.2 by the addition of the compound, a major part of the antimicrobial effect was attributed to the diacetate and not just pH. Sodium diacetate suppressed growth of three additional L. monocytogenes strains and strains of Escherichia coli, Pseudomonas fluorescens, Salmonella enteriditis and Shewanella putrefaciens. P. fragi, Yersinia enterocolitica, Enterococcus faecalis, Lactobacillus fermentis and Staphylococcus aureus were insensitive to the compound. Sodium diacetate has potential for use in controlling growth of listeriae in meat, poultry and fish products and suppressing growth of certain Gram-negative spoilage and pathogenic microorganisms.     

INHIBITION OF LISTERIA MONOCYTOGENES BY LIQUID SMOKE AND ISOEUGENOL, A PHENOLIC COMPONENT FOUND IN SMOKE

The behavior of Listeria monocytogenes was monitored in wiener exudate or Tryptose Broth (TB) in the presence of liquid smoke (CharSol Supreme) or smoke ingredients (i.e., phenols and acetic acid). L. monocytogenes grew in untreated exudate held at 25C, but was inactivated in the presence of 0.2 and 0.6% liq'iid smoke (D-values of 36 and 4.5 h, respectively). Of 11 individual phenols tested, only isoeugenol exhibited antilisterial activity in TB at 37C. Growth of the pathogen in the presence of isoeugenol (100 ppm) was inhibited to a greater extent in TB acidified with acetic acid to pH 5.8 compared to pH 7.0. These studies confirm the potent antilisterial activity of liquid smoke and establish the potential of isoeugenol for controlling the growth of L. monocytogenes in certain processed meats .     

INHIBITION OF LISTERIA MONOCYTOGENES BY NATIVE MICROFLORA OF WHOLE CANTALOUPE1

Fresh‐cut cantalcupe has been recalled due to the possible presence of Listeria monocytogenes. Several studies have reported that naturally occurring microflora of vegetable surfaces may be antagonistic to pathogen attachment, growth or survival. To test this possibility for L. monocytogenes and cantaloupes, whole melon were treated with water, ethanol (70%) or chlorine (200 ppm) to reduce the native microflora on the melon surfaces. Treated or untreated melons were immersed in a six strain cocktail of L. monocytogenes (10⁷ CFU/mL) for 10 min and then allowed to dry for 1 h inside a biosafety cabinet followed by storage at 5, 10 and 20C for 15 days. Fresh‐cut pieces prepared from the treated or untreated melons and directly inoculated with L. monocytogenes (3.48 log CFU/g) were stored under the same conditions listed above. Populations of L. monocytogenes and five classes of native microflora were investigated. Growth of L. monocytogenes in sterile or nonsterile rind and fresh‐cut homogenates was also studied. The population of L. monocytogenes recovered from inoculated (10³ to 10⁸ CFU/mL) whole melons given no disinfection treatment or washed with water was significantly less (P < 0.05) than that recovered from melons treated with chlorine or EtOH. In general, populations of L. monocytogenes declined on the surface of treated and untreated whole melons and on fresh‐cut pieces over the 15 days storage period at the temperatures tested. However, the decline in pathogen populations was less rapid in the presence of reduced populations of native microflora. Higher populations of L. monocytogenes were attained in sterile tissue homogenates than in nonsterile homogenates. Addition of yeast and mold to sterile rind homogenates was highly inhibitory to growth and survival of the pathogen. The results of this study indicate that native microflora of whole cantaloupe inhibited attachment to rind surfaces as well as survival and growth of L. monocytogenes on cantaloupe surfaces and homogenized fresh‐cut pieces. Thus, L. monocytogenes recontamination of melons having a reduced level of native microflora following application of a disinfection treatment may be a food safety concern.     

SENSORY AND CHEMICAL EVALUATION OF MANCHEGO CHEESE AND OTHER CHEESE VARIETIES AVAILABLE IN THE SPANISH MARKET

Chemical and sensory characteristics of 28 different cheeses acquired in the Spanish market were evaluated. Principal Component Analysis (PCA) of the physico‐chemical results showed that the cheeses were separated into three groups: hard, soft and elastic cheeses, while PCA of the sensory results classified the same cheeses into four groups, distinguishing between two types of hard cheese (Manchego cheese with Appellation of Origin and other Spanish hard cheeses). Sensory Analysis differentiated between cheeses of very similar physicochemical characteristics, the attributes that contributed most to differentiation being fragility, adhesiveness, pasty and spicy. Some correlations were obtained between sensorial and chemical variables, the most significant being hardness versus total nitrogen (TN) and versus dry matter (DM), and fragility versus water activity.     

INHIBITION OF LISTERIA MONOCYTOGENES BY CARNOBACTERIUM PISCICOLA IN VACUUM-PACKAGED COOKED CHICKEN AT REFRIGERATION TEMPERATURES

Listeria monocytogenes inhibition by bacteriocinogenic Carnobacterium piscicola DX or nonbacteriocinogenic C. piscicola 2818 was examined in vacuum-packed minimally processed chicken cubes with gravy at 4, 8 and 15C. C. piscicola DX and C. piscicola 2818 at 10⁴ CFU/g were coinoculated individually with a pool of three Listeria monocytogenes strains at 10² CFU/g. At 4 and 8C , C. piscicola DX inhibited growth of L. monocytogenes by 3 logs, significantly (P < 0.05) more than did C. piscicola 2818. At 15C both C. piscicola strains inhibited L. monocytogenes by one log cycle. The pH of all inoculated systems decreased from an initial pH of 6.14 to final values ranging from 6.06 to 5.65 depending on the inocula used. Bacteriocin was detected in the systems coinoculated with C. piscicola DX. These studies demonstrate that lower temperatures and bacteriocin production enhanced L. monocytogenes inhibition by C. piscicola     

INCIDENCE OF LISTERIA MONOCYTOGENES IN RETAIL CHICKEN MEAT AND ESTABLISHING RELATIONSHIP WITH SOME BACTERIA BY LOGISTIC REGRESSION

ABSTRACT

The objective of this study was to determine the contamination of retail poultry meat in Erzurum (Turkey) by Listeria monocytogenes and to evaluate its relationship with indicator bacteria using logistic regression. The incidence of L. monocytogenes was 32.76%, found in 38 of 116 samples. The application of logistic regression showed Enterobacteriaceae, Pseudomonas spp. and Brochothrix thermosphacta populations have positive, and yeast and mold population have negative relationship with L. monocytogenes presence. The results of this study demonstrate that there is a serious risk in raw poultry meat for consumer health in Erzurum, because of the high incidence of L. monocytogenes in the samples of the present study. Hygienic conditions described in HACCP program should still be enforced in order to minimize L. monocytogenes in poultry meat during the processing.

PRACTICAL APPLICATIONS

Listeria monocytogenes and other Listeria species have been isolated from many different types of raw and processed food, but the main sources and routes of contamination are still not fully understood. There is a need for more knowledge, and data are needed for risk assessment and for improved preventive measures. In order to prevent and control contamination of the environment and food products with this pathogen, it is important to detect the most important sources of contamination and to understand the mechanisms of growth, including relationships with other bacteria. The main concern with raw poultry meat is the incidence of L. monocytogenes in raw chicken because of cross‐contamination with other foods in the home and the possibility of the microorganism surviving in processed chicken. Application of logistic regression was used to identify the main hazards associated with the presence of microorganisms, as well as for applications in predictive food microbiology, modeling binomially distributed data that involve the use of probability models, where the response variable is a “presence/absence” observation of whether or not growth will occur.     

THE INCIDENCE AND LEVEL OF LISTERIA SPP. AND LISTERIA MONOCYTOGENES CONTAMINATION IN PROCESSED POULTRY AT A POULTRY PROCESSING PLANT

To estimate the incidence and levels of Listeria spp. in an industrial poultry processing plant, samples of chicken breast meat, livers, surfaces of saws and tables, hands and gloves were analyzed. Forty percent of the breast samples presented Listeria: L. monocytogenes, L. innocua and L. grayi. Liver samples were contaminated by L. innocua and L. grayi. High levels of Listeria monocytogenes were found on saws (<30–2400 MPN/equipment) and tables (<30–11,000 MPN/equipment). Hands were contaminated by L. monocytogenes, L. innocua and L. grayi and gloves with L. innocua and L. grayi. The levels of Listeria on hands and gloves were low (<110 MPN/hand). L. monocytogenes serotypes were 1/2a, 1/2b, 1/2c and 4b. Overall, the study demonstrated the high prevalence of Listeria spp. and specifically L. monocytogenes in chicken breast meat, equipment and hands. Improvements and innovations at the poultry processing plant may effectively reduce final production contamination with Listeria.     

SPATIAL DISTRIBUTION OF POPULATION OF LISTERIA MONOCYTOGENES DURING MANUFACTURING AND RIPENING OF CAMEMBERT CHEESE

The spatial distribution of the survival and growth of Listeria monocytogenes during manufacturing and ripening of Camembert cheese was studied. All cheeses were artificially inoculated with L. monocytogenes and ripened in three stages: room temperature (～20C) and relative humidity (RH) of 60% for 36 h, 13C and RH of 93% for 2 weeks, and 7C and RH of 85% for 3 weeks. During ripening, different locations were analyzed for population of L. monocytogenes. The data were collected on days 1, 5, 10, 15, 20, 25, 30 and 35 of ripening. Results showed that the population of L. monocytogenes on day 1 was 6.06 log₁₀ cfu/g, which was 1.96 log₁₀ cfu/g increasing over the initial inoculation of 4.08 log₁₀ cfu/g. For the subsequent 20 days, the L. monocytogenes population declined to 5.33 log₁₀ cfu/g. Thereafter, the L. monocytogenes population increased to 6.07 log₁₀ cfu/g on day 35 of ripening. Generally, the growth of L. monocytogenes is faster in locations near the surface than in the center. There were no significant differences between batches and sections of cheese, whereas the ripening time and locations had a significant effect on the survival and growth of L. monocytogenes. The survival and growth data for L. monocytogenes during ripening of Camembert cheese can be used to develop dynamic predictive models for possible use in risk assessment and Hazard Analysis and Critical Control Point implementation.     

EFFECT OF ACID ADAPTATION AND DIFFERENT SALT CONCENTRATIONS ON SURVIVAL OF LISTERIA MONOCYTOGENES IN TURKISH WHITE CHEESE

ABSTRACT

Survival of acid‐adapted and nonacid adapted Listeria monocytogenes in Turkish white cheese with different salt concentrations (3.8 and 6.7% NaCl) was investigated. Cheese blocks were inoculated with L. monocytogenes and stored at 4C for 15 days. Samples were taken on different days and analyzed for the numbers of the pathogen, and the samples were challenged with simulated gastric fluid (SGF) for determining acid tolerance. There was no significant differences between the numbers of acid‐adapted and nonadapted L. monocytogenes cells regardless of the salt concentration on sampling days (P > 0.05). Challenge in SGF revealed that there was no significant difference between acid‐adapted and nonadapted bacteria in cheese with 3.8% NaCl throughout the storage period, while significant (P < 0.05) differences between those cells in cheese with 6.7% NaCl were found after day 5. These results suggest that acid‐adapted L. monocytogenes exposed to high salt concentration such as 6.7% may have an advantage in SGF.

PRACTICAL APPLICATIONS

Evaluation of potential stress responses of pathogens in food is important to understand the behavior of pathogens that can survive under stresses. As an environmental contaminant, Listeria monocytogenes may acquire adaptation to low acid conditions in cheese plants. Our results suggested that although there was no significant difference between acid‐adapted and nonadapted cells both in industrial (3.8% salt) and traditional (6.7% salt) white cheese, salt concentration significantly affected the survival of acid‐adapted cells in simulated gastric fluid (pH 2.3). These findings may indicate that L. monocytogenes, which is adapted to acid before contaminating high salt cheeses, may survive in stomach and cause infection. It can be said that increased salt concentration in cheese may not provide a barrier against acid‐adapted L. monocytogenes.

     

INHIBITION OF LISTERIA INNOCUA AND L. MONOCYTOGENES IN A LABORATORY MEDIUM AND COLD-SMOKED SALMON CONTAINING LIQUID SMOKE

Five commercial liquid smokes were tested in vitro and the most inhibitory to Listeria monocytogenes ATCC 19115 and L. innocua ATCC 33090 was Charsol Supreme. Chum salmon samples (100-g each) were brined, dipped for 15 s at varying concentrations of liquid smoke, inoculated with L. innocua, cold-processed and analyzed. Liquid smoke concentrations of 60–100% reduced L. innocua by 3-log₁₀/g in the final product. Dwell times of 15 s to 5 min using 60% liquid smoke gradually decreased Listeria survival with an optimum 5-min dip. Isoeugenol was antilisterial in vitro but lacked synergism with liquid smoke in cold-smoked salmon. An immunoassay kit detected low inoculum levels (< 100 CFU/g) of L. innocua in one of three samples that were treated with liquid smoke for two and four minutes. Charsol Supreme was antilisterial but could not be relied on to totally eliminate Listeria in cold-smoked salmon. Panelists found the 0 to 2-min dipped sockeye salmon slightly desirable with no significant (p < 0.05) differences. The 5-min treatment was significantly (p < 0.05) darker, scored lower in desirability and flavor and contained 93 ppm of phenolic compounds.     

EFFECT OF OXYRASETM ENZYME ON LISTERIA MONOCYTOGENES AND OTHER FACULTATIVE ANAEROBES1

The use of an oxygen-reducing membrane fraction to stimulate growth of some anaerobic bacteria has been reported. In this study, stimulated recovery of Escherichia coli 0157:H7, Salmonella typhimurium, Streptococcus faecalis, and heat-injured Listeria monocytogenes was achieved by using broth media supplemented with Oxyrase^TM (membrane-bound enzyme system derived from E. coli). The Oxyrase^TMenzyme allowed rapid growth of these facultatively anaerobic organisms in a nonselective broth. The activity of Oxyrase^TMin four different selective enrichment broths to support growth of L. monocytogenes also was evaluated. Among tested broths, FDA-approved Listeria Enrichment Broth containing Oxyrase^TMwas found to be substantially superior for facilitating the growth of L. monocytogenes. Oxyrase^TMpromoted a significantly greater number of L. monocytogenes than commonly used but nonspecific reducing agents, such as L-cysteine HCl and sodium thioglycolate. These findings indicated that all organisms tested can be grown well in liquid media using the oxygen-consuming membrane fragments to achieve and maintain oxygen-limited conditions. The Oxyrase ^TM enzyme system has potential applications in some selective media and other diagnostic tests specified for rapid detection of these facultatively anaerobic pathogens.     

INHIBITION OF LISTERIA MONOCYTOGENES BY CINNAMIC ACID: POSSIBLE INTERACTION OF THE ACID WITH CYSTEINYL RESIDUES

The effects of cinnamic acid (C₆H₅CH=CHCOOH) on L. monocytogenes strain Scott A and listeriolysin O (LLO) activity were studied. In addition, a possible mode of action of the acid was investigated. L. monocytogenes was more susceptible to cinnamic acid than to benzoic acid, and lowering the broth pH increased the inhibition. LLO activity was inhibited in the presence of 0.1% cinnamic or benzoic acid, and western blot analysis showed reduced intensity of listeriolysin bands in the presence of 0.2% of the acids. Addition of cysteine (5 mM) to cinnamic acid in PBS, pH 7.0, caused gradual reduction in UV absorption of the acid during incubation for 12 h. Cinnamic acid (>0.5%, pH 7.0) inhibited glucose uptake and ATP production in resting cells of L. monocytogenes. Results suggest a role of the cinnamate anion in the antilisterial and anti-LLO activity and potential applications in the food industry.     

INHIBITION OF LISTERIA MONOCYTOGENES BY LACTOCOCCUS LACTIS SUBSP. LACTIS ISOLATED FROM ITALIAN RAW HAM

A total of 168 strains of lactic acid bacteria were isolated from Italian raw ham and screened for antagonistic activity against Listeria monocytogenes by using an agar spot assay. Only one strain of Lactococcus lactis subsp . lactis produced antagonistic effects other than inhibition by low pH and hydrogen peroxide. The proteinaceous nature of the compound produced by L. lactis B10 was demonstrated by its inactivation by proteolytic enzymes. The cell-free culture super-natants (filtered and heat-treated) showed a bactericidal mode of action. Bacteriocin produced by L. lactis B10 was inhibitory to other lactic acid bacteria and one strain of Staphylococcus aureus, but not to the gram-negative bacteria tested .