Direct detection by polymerase chain reaction (PCR) of Escherichia coli in water and soft cheese and identification of enterotoxigenic strains

PCR was used to develop a method to detect Escherichia coli in surface water and soft cheese, which does not require cultivation of bacteria. DNA sequences from the ma/B operon of E. coli were amplified to specifically detect this bacterium. Samples of surface water and soft cheese naturally contaminated with E. coli from less than 100 cells per g up to several times 105cells per g were analysed by both the classical culture method and the PCR assay. Comparable results were obtained with both methods. Soft cheese samples artificially contaminated with various levels of enterotoxigenic E. coli were analysed with a second PCR test specific for the heat‐labile enterotoxin type I (LTI) of E. coli. The detection limit was about 1000 bacteria per g of soft cheese. In addition, two soft cheese samples naturally contaminated with 2 times 105and 6 times 105E. coli per g as determined by the culture method were analysed by LTI‐PCR and found to contain low levels of enterotoxigenic E. coli.