Thin agar layer- versus most probable number-PCR to enumerate viable and stressed

A mid-log phase broth culture of Escherichia (E.) coli O157:H7 381 (final concentration 104 cfu/mL) was monitoredby conventional liquid- and solid-based enumeration techniques combined with PCR while it wassubjected to thermal stress in gradually more complex systems (i.e., Tryptone Soya Broth, pasteurized milkand during lab-scale productions of a pasta filata fior di latte cheese obtained from raw or pasteurizedmilk). Our results highlighted: i) the incapability of the selective medium, ii) the effectiveness of the thinagar layer-PCR method, and iii) the effectiveness of the most probable number (MPN)-PCR method (in comparisonwith both plating-based methods) in recovering and selectively counting viable and stressed orinjured E. coli O157:H7. Moreover, MPN-PCR was superior to both plating-based methods in terms of speedand easiness to get results. The thermal stresses herein applied (heating at 55 °C for 5 and 8 min) were lesseffective on the pasteurized milk than on the Tryptone Soya Broth and the pathogen was more protectedin the raw milk-based matrices than in the pasteurized ones. Moreover, given the contamination level(104 cfu/mL of milk) of the strain, the temperature/time of stretching and the hardening and brining conditionsherein used, the complete inactivation of the pathogen is not achievable.