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

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 5 h enrichment of alfalfa sprouts contaminated at 0.2 cfu/g, there was significant improvement in isolation on the selective plates following acid treatment of two types of IMS beads. Following 24 h 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 30 min or IMS beads for 7 min 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.