Modeling the effect of temperature on survival rate of Salmonella Enteritidis in yogurt

A flow cytometric assay to detect viability and persistence of Salmonella enterica subsp. enterica serotypes in nuclease-free water at 4 and 25°C

Salmonella spp. is one of the most isolated microorganisms reported to be responsible for human foodborne diseases and death. Water constitutes a major reservoir where the Salmonella spp. can persist and go undetected when present in low numbers. In this study, we assessed the viability of 12 serotypes of Salmonella enterica subsp. enterica for 160 days in nuclease-free water at 4 and 25°C using flow cytometry and Tryptic Soy Agar (TSA) plate counts. The results show that all 12 serotypes remain viable after 160 days in distilled water using flow cytometry, whereas traditional plate counts failed to detect ten serotypes incubated at 25°C. Moreover, the findings demonstrate that 4°C constitutes a more favorable environment where Salmonella can remain viable for prolonged periods without nutrients. Under such conditions, however, Salmonella exhibits a higher susceptibility to all tested antibiotics and benzalkonium chloride (BZK). The pre-enrichment with Universal Pre-enrichment Broth (UP) and 1/10 × Tryptic Soy broth (1/10 × TSB) resuscitated all tested serotypes on TSA plates, nevertheless cell size decreased after 160 days. Furthermore, phenotype microarray (PM) analysis of S. Inverness and S. Enteritidis combined with principal component analysis (PCA) revealed an inter-individual variability in serotypes with their phenotype characteristics, and the impact of long-term storage at 4 and 25°C for 160 days in nuclease-free water. This study provides an insight to Salmonella spp. long-term survivability at different temperatures and highlights the need for powerful tools to detect this microorganism to reduce the risk of disease transmission of foodborne pathogens via nuclease-free water.

Antimicrobial activity of MccJ25(G12Y) against gram-negative foodborne pathogens in vitro and in food models

The use of bacteriocins is a promising alternative to improve food security through the biocontrol of food pathogens and spoilage microorganisms. Gram-negative produced microcin J25(G12Y), known as (MccJ25(G12Y)) is a variant of the well-studied and characterized antimicrobial peptide, microcin J25 (MccJ25). In the present work, we explored the activity of this microcin against Gram-negative bacteria linked to foodborne diseases. We evaluated the in vitro antimicrobial activity of MccJ25(G12Y) in solid medium against a collection of pathogenic and food-altering strains and studied its activity and stability in meat and dairy food systems. We show that MccJ25(G12Y) exhibited the same in vitro antimicrobial spectrum as its parental microcin (MccJ25) against different Gram-negative foodborne pathogens and spoilage strains. We highlight that low concentrations of MccJ25(G12Y) between 0.45 and 29.4 μM were able to inhibit a substantial number of pathogens, including Salmonella, Escherichia, Shigella and Enterobacter genus. We also demonstrate the antimicrobial effectiveness of the peptide against Escherichia coli O157:H7 NCTC 12900, Enterobacter cloacae CECT 194, and Salmonella enterica CECT 4396 in fish and beef burgers and yogurt. MccJ25(G12Y) was added or not to food matrices inoculated with the foodborne pathogens at 10⁵ CFU/g or mL. Afterward, food products were stored at 4 °C and selective media for the specific enumeration were used to determine the antimicrobial susceptibility of each pathogen to MccJ25(G12Y). The viability of the three pathogens was significantly reduced in the different food biological environments. In yogurt, the peptide decreased E. coli numbers on day 5 by about 4 log 10 CFU/mL as compared to non-treated samples. For S. enterica and E. cloacae no viable cells were detected at the end of the treatment. Adding MccJ25(G12Y) to fish burgers decreased E. cloacae numbers during storage 2 log₁₀ CFU/g on the first day, reaching a difference of about 5 log 10 CFU/g after 10 days compared to non-treated control. Finally, the peptide decreased E. coli O157:H7 numbers on the beef burgers samples during storage on day 10 by about 3 log 10 CFU/g as compared to non-treated samples. The stability analysis demonstrated that MccJ25(G12Y) is capable of remaining active in these food matrices for a considerable time during the storage at refrigeration temperatures. These results reinforce the studies on the potential applicability of this microcin as a biopreservative in the food industry.

Modelling survival of Salmonella Enteritidis during storage of yoghurt at different temperatures

The aim of this study was to evaluate the behaviour of Salmonella Enteritidis during the storage of yoghurt at different temperatures (4, 12, 20, and 25 °C), and to develop mathematical models to predict the behaviour of this bacterium as a function of storage temperature. Results indicated that Salmonella was able to survive longer during storage when temperature was low (e.g. 304 h at 4 °C, 60 h at 25 °C). The Geeraerd model with log-decrease and tailing was selected as the most suitable model to describe survival. To evaluate the effect of storage temperature on kinetic parameters such as death rate (k_max) secondary models were developed. The k_max was maximum at 25 °C and minimum at 4 °C with k_max = 0.28 and 0.039 h^-1, respectively. The residual population (N_res) ranged 0.5 and 1.8 log CFU/g but there was no temperature dependency of this parameter. A probabilistic example was conduced based on the developed model to assess the exposure to Salmonella by consumption of traditional Turkish yoghurt.

Modeling the effect of temperature on survival rate of Listeria monocytogenes in yogurt

The aim of the study was to (i) evaluate the behavior of Listeria monocytogenes in a commercially produced yogurt, (ii) determine the survival/inactivation rates of L. monocytogenes during cold storage of yogurt and (iii) to generate primary and secondary mathematical models to predict the behavior of these bacteria during storage at different temperatures. The samples of yogurt were inoculated with the mixture of three L. monocytogenes strains and stored at 3, 6, 9, 12 and 15°C for 16 days. The number of listeriae was determined after 0, 1, 2, 3, 5, 7, 9, 12, 14 and 16 days of storage. From each sample a series of decimal dilutions were prepared and plated onto ALOA agar (agar for Listeria according to Ottaviani and Agosti). It was found that applied temperature and storage time significantly influenced the survival rate of listeriae (p<0.01). The number of L. monocytogenes in all the samples decreased linearly with storage time. The slowest decrease in the number of the bacteria was found in the samples stored at 6°C (D-10 value = 243.9 h), whereas the highest reduction in the number of the bacteria was observed in the samples stored at 15°C (D-10 value = 87.0 h). The number of L. monocytogenes was correlated with the pH value of the samples (p<0.01). The natural logarithm of the mean survival/inactivation rates of L. monocytogenes calculated from the primary model was fitted to two secondary models, namely linear and polynomial. Mathematical equations obtained from both secondary models can be applied as a tool for the prediction of the survival/inactivation rate of L. monocytogenes in yogurt stored under temperature range from 3 to 15°C, however, the polynomial model gave a better fit to the experimental data.