An Alanine Racemase Gene as a New Index for Detecting Escherichia coli in Foods

A gene of alanine racemase, a typical prokaryotic enzyme, was evaluated as a new index for detecting Escherichia coli in foods. An alanine racemase gene fragment containing a non-conserved sequence of the gene was amplified from genomic DNA of E. coli by a polymerase chain reaction, and then labeled with digoxigenin as a probe for detecting E. coli. Food samples and bacteria were each treated as at 25°C for l0 min in 0.1 N NaOH containing 0.5% SDS, before being directly spotted on to nylon membranes for DNA hybridization. The probe was specific for E. coli; all 48 strains of E. coli examined, including such pathogenic strains as E. coli O157: H7, showed positive signals, whereas all 59 strains of non-E. coli species, except for one strain (Shigella sonnei), did not show a signal. Various foods inoculated with E. coli K-12 showed positive signals, whereas no uninoculated foods showed any signal. Quantification of E. coli cells in the death phase by the hybridization method showed good correlation with that by the plate culture method. The alanine racemase gene could prove useful as an index for detecting E. coli in foods.