Thin agar layer- versus most probable number-PCR to enumerate viable and stressed Escherichia coli O157:H7 and application in a traditional raw milk pasta filata cheese.
A mid-log phase broth culture of Escherichia (E.) coli O157:H7 381 (final concentration 10(4) cfu/mL) was monitored by conventional liquid- and solid-based enumeration techniques combined with PCR while it was subjected to thermal stress in gradually more complex systems (i.e., Tryptone Soya Broth, pasteurized milk and during lab-scale productions of a pasta filata fior di latte cheese obtained from raw or pasteurized milk). Our results highlighted: i) the incapability of the selective medium, ii) the effectiveness of the thin agar layer-PCR method, and iii) the effectiveness of the most probable number (MPN)-PCR method (in comparison with both plating-based methods) in recovering and selectively counting viable and stressed or injured E. coli O157:H7. Moreover, MPN-PCR was superior to both plating-based methods in terms of speed and easiness to get results. The thermal stresses herein applied (heating at 55 °C for 5 and 8 min) were less effective on the pasteurized milk than on the Tryptone Soya Broth and the pathogen was more protected in the raw milk-based matrices than in the pasteurized ones. Moreover, given the contamination level (10(4) cfu/mL of milk) of the strain, the temperature/time of stretching and the hardening and brining conditions herein used, the complete inactivation of the pathogen is not achievable.
National Research Council of Italy, Institute of Sciences of Food Production (CNR-ISPA), Bari, Italy. vincenzina.fusco@ispa.cnr.it
Vincenzina
V
Fusco
Maria
M
Riccardi
Grazia Marina
GM
Quero
D000362:Agar; D000818:Animals; D002611:Cheese / Q000382:microbiology; D015169:Colony Count, Microbial; D004926:Escherichia coli; D019453:Escherichia coli O157 / Q000235:genetics* / Q000302:isolation & purification*; D005506:Food Contamination; D005516:Food Microbiology; D008892:Milk / Q000382:microbiology; D009824:Ointments; D061352:Pasteurization; D016133:Polymerase Chain Reaction / Q000379:methods*; D011336:Probability
D009824:Ointments; D000362:Agar