Survey of Retail Alfalfa Sprouts and Mushrooms for the Presence of Escherichia Coli O157

Survey of Retail Alfalfa Sprouts and Mushrooms for the Presence of Escherichia coli O157:H7, Salmonella, and Listeria with BAX, and Evaluation of this Polymerase Chain Reaction-Based System with Experimentally Contaminated Samples

February 2003

Journal of Food Protection  Volume: 66 Number: 2 Page: 182 — 187

Christine M. Strapp ; Adrienne E. H. Shearer ; Rolf D. Joerger


Abstract: BAX, a polymerase chain reaction (PCR)-based pathogen detection system, was used to survey retail sprouts and mushrooms for contamination with Escherichia coli O157:H7, Salmonella, Listeria spp., and Listeria monocytogenes. No Salmonella or E. coli O157:H7 was detected in the 202 mushroom and 206 alfalfa sprout samples screened. L. monocytogenes was detected in one sprout sample, and seven additional sprout samples tested positive for the genus Listeria. BAX also detected Listeria species in 17 of the mushroom samples. Only 6 of 850 PCR assays (0.7%) failed to amplify control DNA, and therefore reagent failures and the inhibition of PCR by plant compounds were rare. The sensitivity of the detection system was evaluated by assaying samples inoculated with 10 CFU of each of the pathogens. One hundred seventy-two alfalfa sprout samples were inoculated with E. coli O157:H7, and two sets of 130 samples were experimentally contaminated with Salmonella Enteritidis and L. monocytogenes. The frequency of detection depended on the protocols used for inoculation and culturing. Inoculation of samples with approximately 10 CFU from frozen stocks yielded detection rates of 87.5 and 94.5% for L. monocytogenes and Salmonella Enteritidis, respectively, in mushrooms. The corresponding rates for alfalfa sprouts were 94.5 and 76.3%. The E. coli O157:H7 detection rate was 100% for mushrooms but only 48.6% for sprouts when standard BAX culture protocols were used. The substitution of an overnight incubation in modified E. coli medium for the 3-h brain heart infusion incubation increased the rate of E. coli O157:H7 detection to 75% for experimentally contaminated sprouts. The detection rate was 100% when E. coli O157:H7 cells from a fresh overnight culture were used for the inoculation. Test sensitivity is therefore influenced by the type of produce involved and is probably related to the growth of pathogens in the resuscitation and enrichment media