Comparison of rapid enzyme-linked immunosorbent assay and immunomagnetic separation methods for detection of Escherichia coli O157 in fecal, hide, carcass, and ground beef samples

Escherichia coli O157 in ground beef from local retail markets in Pachuca, Mexico

Escherichia coli O157 strains have been recognized as pathogenic bacteria, of which raw beef is a known vehicle. An evaluation was done of the presence of E. coli O157 in ground beef from local retail markets in Pachuca, Hidalgo State, Mexico. A total of 120 ground beef samples (500 g) were tested for E. coli O157 by simultaneous application of the U. S. Department of Agriculture, Food Safety and Inspection Service (FSIS)'s Microbiology Laboratory Guidebook culture procedure 5.05, and two commercial kits, Reveal for E. coli O157:H7 and Visual Immunoprecipitate Assay (VIP) Gold for enterohemorrhagic E. coli. Two incubation times (8 and 20 h) were used with the commercial kits. Presence of stx1, stx2, and eaeA loci was determined by multiplex PCR. Of 360 subsamples (120 per procedure), 12 samples were found to be E. coli O157 positive by the FSIS culture method. With VIP, 73 subsamples were presumptive positive after 8 h of enrichment, and 60 were presumptive positive after 20 h of enrichment. Of these, only 6 (8 h) and 8 (20 h) subsamples were confirmed true positives with the FSIS method. With Reveal, 60 subsamples were presumptive positive after 8 h of enrichment and 50 were presumptive positive after 20 h of enrichment. Of these, only 6 (8 h) and 8 (20 h) subsamples were confirmed as true positives with the FSIS method. A total of 57 E. coli O157:H7 and 21 E. coli O157 strains were isolated. None of the O157 or O157:H7 strains had stx1 or stx2 loci, and only one had the eaeA locus. To our knowledge, this is the first report of the presence of E. coli O157 in commercial ground beef from Mexico, and the first report of isolation of a large number of stx-negative E. coli O157 and E. coli O157:H7 strains in Mexico.

Rapid detection and identification of bacterial pathogens by using an ATP bioluminescence immunoassay

Rapid identification of viable bacterial contaminants in food products is important because of their potential to cause disease. This study examined a method for microbial detection by using a combined ATP bioluminescence immunoassay. Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium were selected as target organisms because of their implication in foodborne illness. Various matrices containing the target cells were examined, including ground beef homogenate, apple juice, milk, and phosphate-buffered saline. Specific antibodies were immobilized on the surface of 96-well plates, and then the sample matrices containing target cells in the wells were incubated. Sample matrix (no cells) was used to establish background. The plates were washed, and the wells were incubated with BacTiter-Glo reagent in Mueller-Hinton II broth. Bioluminescent output was measured with the GloMax 96 luminometer. Signal-to-noise ratios were calculated, resulting in a limit of detection of 10(4) CFU/ml for both E. coli O157:H7 and Salmonella Typhimurium. The limit of detection for both species was not affected by the presence of nontarget cells. The various sample matrices did not affect signal-to-noise ratios when E. coli O157:H7 was the target. A weak matrix effect was observed when Salmonella Typhimurium was the target. A strong linear correlation was observed between the number of cells and luminescent output over 4 orders of magnitude for both species. This method provides a means of simultaneously detecting and identifying viable pathogens in complex matrices, and could have wider application in food microbiology.