Comparison of Escherichia coli O157:H7 enrichment in spiked produce samples

Abstract

Two strains of Escherichia coli O157:H7 were spiked into six varieties of produce at approximately 0.5 CFU g-1. Samples were enriched by using the U.S. Food and Drug Administration (FDA) Bacteriological Analytical Manual (BAM) method and by using an experimental method incorporating acid shock. Target colonies were detectable on selective agars after 30 of 48 analyses with BAM enrichment and 48 of 48 analyses with acid enrichment. Real-time PCR screening of 24-h enrichment broths revealed the presence of the diagnostic stx1 or stx2 genes after 27 of 48 analyses with BAM enrichment and 42 of 48 analyses with acid enrichment. The efficiency of the enrichment varied with strain and type of produce spiked but overall was better with the experimental enrichment method. Modifications of both the acid enrichment and BAM enrichment methods also were tested. The acid method with a modified incubation temperature consistently yielded high rates of recovery (>108 CFU ml-1), with no instances in which target cells could not be detected. Modification of the BAM procedure did not reproducibly improve enrichment efficiency.