Comparison of Methods for Detection of Escherichia Coli O157H7 in Alfalfa Sprouts

Comparison of an Immunochromatographic Method and the TaqMan E. coli O157:H7 Assay for Detection of Escherichia coli O157:H7 in Alfalfa Sprout Spent Irrigation Water and in Sprouts After Blanching†

Journal of Industrial Microbiology & Biotechnology (2001) 27, 129-134.August 2001, Volume 27, Number 2, Pages 129-134

P M Fratamico and L K Bagi, Agricultural Research Service, Eastern Regional Research Center, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA

†Mention of brand or firm names does not constitute an endorsement by the U.S. Department of Agriculture above others of a similar nature not mentioned.

An immunochromatographic-based assay (Quix E. coli O157 Sprout Assay) and a polymerase chain reaction (PCR)-based assay (TaqMan E. coli O157:H7 Kit) were used to detect Escherichia coli O157:H7 strain 380-94 in spent irrigation water from alfalfa sprouts grown from artificially contaminated seeds. Ten, 25, 60, or 100 seeds contaminated by immersion for 15 min in a suspension of E. coli O157:H7 at concentrations of 106 or 108 cfu/ml were mixed with 20 g of non-inoculated seeds in plastic trays for sprouting. The seeds were sprayed with tap water for 15 s every hour and spent irrigation water was collected at intervals and tested. E. coli O157:H7 was detected in non-enriched water by both the TaqMan PCR (30 of 30 samples) and the immunoassay (9 of 24 samples) in water collected 30 h from the start of the sprouting process. However, enrichment of the spent irrigation water in brain heart infusion (BHI) broth at 37°C for 20 h permitted detection of E. coli O157:H7 in water collected 8 h from the start of sprouting using both methods, even in trays containing as few as 10 inoculated seeds. The TaqMan PCR assay was more sensitive (more positive samples were observed earlier in the sprouting process) than the immunoassay; however, the immunoassay was easier to perform and was more rapid. At 72 h after the start of the sprouting process, the sprouts were heated at 100°C for 30 s to determine the effectiveness of blanching for inactivation of E. coli O157:H7. All of the 32 samples tested with the TaqMan assay and 16 of 32 samples tested with the Quix assay gave positive results for E. coli O157:H7 after enrichment of the blanched sprouts at 37°C for 24 h. In addition, the organism was detected on Rainbow Agar O157 in 9 of 32 samples after 24 h of enrichment of the blanched sprouts. In conclusion, E. coli O157:H7 was detected in spent irrigation water collected from sprouts grown from artificially contaminated seeds by both the TaqMan and Quix assays. The data also revealed that blanching may not be effective to completely inactivate all the E. coli O157:H7 that may be present in sprouts.