Efficacy of a post enrichment acid treatment for isolation of Escherichia coli O157:H7 from alfalfa sprouts.

The enrichment, detection and isolation procedure in the current US FDA BAM have been shown effective for Escherichia coli O157:H7 in a wide variety of foods. Recently reported modifications to the enrichment protocol, including post-enrichment immunomagnetic separation (IMS) procedures have improved sensitivity of the method for alfalfa sprouts. However, cultural isolation on selective agar plates still presents a challenge in this food matrix. The focus of this study was to reduce levels of competing microflora and enhance isolation of E. coli O157:H7 on selective agars. We optimized the use of a short acid treatment after enrichment and with post-enrichment IMS beads. The optimized acid treatments were then evaluated in alfalfa sprouts artificially contaminated with E. coli O157:H7. After 5h enrichment of alfalfa sprouts contaminated at 0.2cfu/g, there was significant improvement in isolation on the selective plates following acid treatment of two types of IMS beads. Following 24h enrichment of alfalfa sprouts contaminated at low levels, E. coli O157:H7 was only recovered from 8/25 samples when no IMS or acid treatments were used. The use of only the acid treatment improved recovery to 19/25 samples. Following IMS of the enrichment broths, acid treatment increased isolation to 23/25 for Pathatrix™ and 24/25 for BeadRetriever™ IMS. Acid treatment (pH 2) of the enrichment broth for 30min or IMS beads for 7min is a simple and rapid way to greatly improve isolation of E. coli O157 from alfalfa sprout enrichments by reducing the interfering microflora on the selective media.