Cheeses made with pasteurized milk have been implicated in foodborne illness outbreaks, mainly due to post-processing contamination. The FDA Food Code specifies that food with pH<4.2 or water activity (aw) <0.88 is exempted from time/temperature control for safety (TCS). Many natural cheeses that do not support the growth of pathogenic bacteria have compositional values outside these boundaries and require TCS. This research investigated the influence of cheese compositional factors and storage environment upon the growth of pathogenic bacteria on cheeses, to establish a guideline that assists in evaluating cheeses' ability to support pathogen growth under extended non-refrigerated holding. Behavior of Escherichia coli O157:H7, Listeria monocytogenes (LM), Salmonella spp., and Staphylococcus aureus on a variety of cheeses held at 25°C were investigated. Cheese pH, in combination with % moisture, aw, or %SMP (salt-in-the-moisture phase) were found significant (P<0.05) in predicting pathogen growth, with pH and %moisture as the most significant factors. To mimic scenarios in retail food establishments, growth/no-growth of pathogens on cheeses held with temperature cycling (4 and 25°C every 12 h) and under aerobic conditions were investigated, with cheeses held at isothermal (25°C) and anaerobic conditions (vacuum-packaged) as the control. The growth/no-growth outcomes of pathogenic bacteria on cheeses were not significantly different (P>0.05) with aerobic vs. anaerobic and isothermal vs. temperature-cycling holding conditions. The results suggest that natural cheeses with pH ≤5.1 and any %moisture, or pH >5.1 and %moisture ≤39% will not support pathogen growth outside refrigeration at up to 77°C for up to 15 days. Among all cheese varieties, Queso Fresco (QF) has been most implicated in foodborne illness outbreaks. Post-processing LM contamination of QF can lead to high LM growth even under refrigeration. Transcriptomics analysis of LM-Scott A grown on QF and in broth (control) at 7°C was conducted to understand the major metabolic pathways and virulence gene expression. Results suggest that LM-Scott A on QF diversified carbohydrate intake, utilized ethanolamine and 1, 2-propanediol, increased iron intake, and activated agmatine deiminase system and virulence expression, whereas flagellar synthesis and putrescine/spermidine transport were reduced. Results could facilitate development of antimicrobial strategies and enhance regulatory risk assessment.
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