Modifications to methods for the enumeration and detection of injured Escherichia coli O157 : H7 in foods

Reliable methods are required for the detection and enumeration of potentia lly injured E. coli O157 in food in the presence of outnumbering competing bacteria. Selective agents can prevent or inhibit the recovery and subseque nt multiplication of injured cells and direct inoculation, either into sele ctive enrichment broths or onto selective agar plates is still used in many methods for E. coli O157 detection and enumeration. When compared with try ptone soya agar (TSA), sorbitol MacConkey agar (SMAC) was shown to underest imate the concentration of viable E. coli O157:H7 subjected to low pH and h igh NaCl concentration. Using a resuscitation stage on TSA followed by memb rane transfer to SMAC improved recovery to levels obtained on TSA, The memb rane method was used to monitor the numbers of artificially contaminated E. coli O157:H7 during the fermentation of a meat product and demonstrated be tter survival when compared to counts on SMAC. Six rapid methods for the de tection of E, coli O157 in food (BAX E. coli O157, Reveal 8 E. coli O157-H7 screening test, VIP EHEC, VIDAS E. coli O157 (ECO), EHEC-Tek and Tecra E. coli O157 visual immunoassay), were evaluated using beefburgers, parsley an d fermented meat artificially contaminated with injured cells. Methods usin g direct selective enrichment, with or without an elevated incubation tempe rature gave false-negative results. The incorporation of a non-selective pr e-enrichment medium improved the detection rates of these assays by up to t en fold. (C) 2000 Elsevier Science B.V. All rights reserved.