Fate of Listeria monocytogenes, Salmonella typhimurium DT104, and Escherichia coli O157:H7 in Labneh as a pre- and postfermentation contaminant

Molecular identification of naturally occurring bacteriocinogenic and bacteriocinogenic-like lactic acid bacteria in raw milk and soft cheese

Lactic acid bacteria (LAB) are currently used by food industries because of their ability to produce metabolites with antimicrobial activity against gram-positive pathogens and spoilage microorganisms. The objectives of this study were to identify naturally occurring bacteriocinogenic or bacteriocinogenic-like LAB in raw milk and soft cheese and to detect the presence of nisin-coding genes in cultures identified as Lactococcus lactis. Lactic acid bacteria cultures were isolated from 389 raw milk and soft cheese samples and were later characterized for the production of antimicrobial substances against Listeria monocytogenes. Of these, 58 (14.9%) LAB cultures were identified as antagonistic; the nature of this antagonistic activity was then characterized via enzymatic tests to confirm the proteinaceous nature of the antimicrobial substances. In addition, 20 of these antagonistic cultures were selected and submitted to genetic sequencing; they were identified as Lactobacillus plantarum (n=2) and Lactococcus lactis ssp. lactis (n=18). Nisin genes were identified by polymerase chain reaction in 7 of these cultures. The identified bacteriocinogenic and bacteriocinogenic-like cultures were highly variable concerning the production and activity of antimicrobial substances, even when they were genetically similar. The obtained results indicated the need for molecular and phenotypic methodologies to properly characterize bacteriocinogenic LAB, as well as the potential use of these cultures as tools to provide food safety.

Listeria monocytogenes and Salmonella spp. in raw milk produced in Brazil: occurrence and interference of indigenous microbiota in their isolation and development

This study aimed to verify the occurrence of Listeria monocytogenes and Salmonella spp. in raw milk produced in Brazil. On account of the poor microbiological quality of this product, possible interference from the indigenous microbiota in these pathogens was also evaluated. Two-hundred and ten raw milk samples were collected in four important milk-producing areas in Brazil, tested for L. monocytogenes and Salmonella spp. presence, and for enumeration of indicator microorganisms: mesophilic aerobes, total coliforms and Escherichia coli. The interference of the indigenous microbiota in the isolation procedures was also tested, as well the frequency of naturally occurring raw milk strains with antagonistic activity against both pathogens. The pathogens were not isolated in any raw milk sample, but poor microbiological quality was confirmed by the high levels of indicator microorganisms. When present at high levels, the indigenous microbiota generated an evident interference in the methodologies of L. monocytogenes and Salmonella spp. isolation, mainly when the pathogens appeared at low levels. Three-hundred and sixty raw milk strains were tested for antagonistic activity against both pathogens, and 91 (25.3%) showed inhibitory activity against L. monocytogenes and 33 (9.2%) against Salmonella spp. The majority of the antagonistic strains were identified as Lactic Acid Bacteria species, mainly Lactococcus lactis subsp. lactis and Enterococcus faecium, known by antimicrobial substance production.

Acid-tolerant Listeria monocytogenes persist in a model food system fermented with nisin-producing bacteria

AIMS

To investigate the induction of the acid tolerance response (ATR) in Listeria monocytogenes and to assess the persistence of the pathogen in broth fermented using a nisin-producing starter culture.

METHODS AND RESULTS

Lactic, acetic and hydrochloric acids were used to induce the ATR in L. monocytogenes growing at early exponential phase. Cells were then challenged in medium acidified to pH 3.5 with the same acid. Only lactic acid induced a detectable ATR. ATR+ cells maintained their initial numbers after 1 h exposure while ATR- were reduced by c. 4 log10 CFU. ATR+ or ATR- cells were also inoculated in M17G broth fermented with nisin-producing (nis+) or control (nis-) Lactococcus lactis. When exposed to nisin, the numbers of ATR+ cells were c. 2 log10 CFU higher than non detectable ATR- cells at day 3. In the absence of nisin (nis- culture), L. monocytogenes was recovered from all ATR+ and ATR- samples after 30 days. In contrast, no L. monocytogenes were recovered from any nis+ATR- samples but four of five nis+ATR+ samples were positive for L. monocytogenes after 30 days.

CONCLUSIONS

The ATR confers cross-resistance to nisin for at least 30 days in a system fermented by nisin-producing bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

The cross-resistance induced by the ATR should be considered for the safety of foods fermented with bacteriocin-producing cultures.

Antimicrobial effects of garlic, clove and red hot chilli on Listeria monocytogenes in broth model systems and soft cheese

Antimicrobial activity of 1% (w/v) fresh garlic, ground clove and red dried chilli on Listeria monocytogenes was tested in broth systems at 37 degrees C and at 4 degrees C for 7 h. The initial cell concentration in the broth systems was between 2 x 10(6) and 4 x 10(6) CFU/ml. At 37 degrees C, growth to viable numbers of 3 x 10(8) CFU/ml in 7 h was measured. Clove had bacteriocidal activity and reduced the count to 1 CFU/ml. Garlic displayed bacteriostatic properties, and a count of 4 x 10(6) CFU/ml was maintained. Red chilli displayed an inhibitory effect and resulted in 50% lower counts than the control. L. monocytogenes had a slow growth rate at 4 degrees C and increased from an initial value of 3 x 10(6) to 5 x 10(6) CFU/ml during 7 h. The addition of garlic resulted in 3 x 10(6) CFU/ml, and clove reduced the viable cell concentration to 1 x 10(3) CFU/ml after 7 h. Two batches of soft cheese were produced in the laboratory using milk that was supplemented with L. monocytogenes. The final cheese containing L. monocytogenes with about 1 x 10(5) CFU/g. Half of each cheese batch was supplemented with either 1% garlic or 1% clove, whereby the other half served as a control. After 7 or 11 days incubation at 4 degrees C, the cheese was incubated at abuse temperature of 25 degrees C for 7 or 3 days, respectively. No antimicrobial effects of 1% (w/w) fresh garlic or clove powder on L. monocytogenes were observed in cheese after 1 or 2 weeks at the lower or higher temperature.

Laboratory-scale preparation of soft cheese artificially contaminated with low levels of Escherichia coli O157, Listeria monocytogenes, and Salmonella enterica serovars Typhimurium, Enteritidis, and Dublin

The production of cheese with incurred low levels of pathogenic microorganisms stressed by the production process was the aim of the study. A standard protocol for the preparation of artificially contaminated soft cheese on a laboratory scale was developed. Milk for cheese preparation was artificially contaminated with pathogenic target microorganisms at low levels, between 1 and 10 CFU/ml. Two strains of Escherichia coli OI157:H7, two strains of Listeria monocytogenes, and three Salmonella spp. (Salmonella enterica serovars Typhimurium, Enteritidis, and Dublin) were investigated. The food pathogens in the cheese were exposed to the entire production process. All three microorganism species survived the cheese production process and were detected in the final product at concentrations between 1 and 50 CFU/g. The cheese produced contains target microorganisms that have been exposed to curd formation, drainage, setting, and ripening. This cheese can be used to validate microbiological methods or to examine the target microorganisms in a natural food environment at low concentrations. It represents an alternative to artificial contamination of cheese by adding target microorganisms to a final cheese product.