Sensitive detection of Escherichia coli O157:H7 by conventional plating techniques

The direct detection and estimation of concentration of Escherichia coli O157:H7 down to 1 CFU/g of cheese was achieved by conventional plating techniques. Cheese was manufactured with unpasteurized milk inoculated with E. coli O157:H7 at 34 +/- 3 CFU/ml. The numbers of E. coli O15:H7 were monitored during cheese ripening by plating on sorbitol MacConkey agar supplemented with cefixime and potassium tellurite (CT-SMAC) and on CT-O15:H7 ID medium. Using the pour plate method, E. coli O15:H7 colonies could easily be distinguished from non-O157:H7 colonies on CT-O157:H7 ID medium but not on CT-SMAC. Higher numbers of E. coli O157:H7 were detectable with O15:H7 ID medium. Latex agglutination and PCR were used to confirm the identification of typical E. coli O15:H7 colonies, and nontypical colonies as not being E. coli O157:H7. As few as 1 CFU/g of cheese could be detected. E. coli O157:H7 also was detected in deliberately contaminated milk at concentrations as low as 4 CFU/10 ml.