Development of a selective plating technique for the recovery of Escherichia coli O157:H7 after heat stress.

The use of Sorbitol MacConkey Agar supplemented with 4-methylumbelliferyl beta-D-glucuronide (MSMA), which is commonly used in the isolation of Escherichia coli O157:H7, has been shown to perform poorly when stressed cells of the pathogen are present. The incorporation of a resuscitation period (2 h at 25 degrees C) on Trypticase Soy Agar (TSA) before overlay with MSMA was found to significantly (P < or = 0.01) improve recovery of heat-stressed (52 degrees C/60 min) cells. Maximal recovery was, however, obtained by adding catalase (1000 U) to the TSA before overlaying with MSMA. This recovery protocol was shown not to result in the loss of the major known virulence factors of E. coli O157:H7 (genes encoding eae, VT1 and VT2).