Recirculating immunomagnetic separation and optimal enrichment conditions for enhanced detection and recovery of low levels of Escherichia coli O157:H7 from fresh leafy produce and surface water.

The objective of this study was to develop a rapid, simple method for enhanced detection and isolation of low levels of Escherichia coli O157:H7 from leafy produce and surface water using recirculating immunomagnetic separation (RIMS) coupled with real-time PCR and a standard culture method. The optimal enrichment conditions for the method also were determined. Analysis of real-time PCR data (C(T) values) suggested that incubation of lettuce and spinach leaves rather than rinsates provides better enrichment of E. coli O157:H7. Enrichment of lettuce or spinach leaves at 42 degrees C for 5 h provided better detection than enrichment at 37 degrees C. Extended incubation of surface water for 20 h at 42 degrees C did not improve the detection. The optimized enrichment conditions were also employed with modified Moore swabs, which were used to sample flowing water sites. Positive isolation rates and real-time PCR results indicated an increased recovery of E. coli O157:H7 from all samples following the application of RIMS. Under these conditions, the method provided detection and/or isolation of E. coli O157:H7 at levels as low as 0.07 CFU/g of lettuce, 0.1 CFU/g of spinach, 6 CFU/100 ml of surface water, and 9 CFU per modified Moore swab. During a 6-month field study, modified Moore swabs yielded high isolation rates when deployed in natural watershed sites. The method used in this study was effective for monitoring E. coli O157:H7 in the farm environment, during postharvest processing, and in foodborne outbreak investigations.

Selective Enterotoxin Production by a Staphylococcus aureus Strain Implicated in a Foodborne Outbreak.

More than 80 of 230 participants (>34.7%) at a literary conference became ill with acute gastroenteritis 3 to 14 h after a catered meal. Attack rate data implicated cheese tortellini as the suspect food (p=0.0087). Selective plating of partially prepared and finished tortellini produced Staphylcoccus aureus counts of 6.0 × 107 and 1.0 × 106 CFU per gram, respectively. Enterotoxin, phage typing, antibiotic sensitivity testing, and other biotyping studies were applied to S. aureus isolates from the suspect food and the single food-handler involved. All isolates reacted identically by all criteria, and each isolate produced both type A and C staphylococcal enterotoxins. Type A enterotoxin (0.90 ug/100 g) alone was detected in samples of the suspect food. The production of type C enterotoxin by the outbreak strain was delayed approximately 4 h relative to production of enterotoxin A when grown in Heart Infusion broth (pH 5.5). This study serves as an example of selective enterotoxin production by S. aureus in suspect foods which can be misleading to outbreak investigators.