ABSTRACT
A colorimetric DNA hybridization-based assay has been evaluated against two conventional culture methods (FDA and USDA) for detection of Listeria spp. in dairy, Meat, and seafood products. A total of 1,300 samples representing 15 food types were analyzed in parallel by both the DNA hybridization (DNAH) and culture methods (FDA for dairy products and seafoods, USDA for meats). Samples included inoculated and naturally contaminated products and uninoculated controls. Fifteen strains representing five species of Listeria were used as inocula. Of 660 dairy and seafood samples tested, the FDA culture method detected 354 positives and the DNAH method detected 393 positives, 391 of which were confirmed. The DNAH method was statistically equivalent to the FDA method for eight of the nine products tested. In some trials, the DNAH method detected more positives than the FDA method for cheddar cheese and in some cases these differences were statistically significant. Of 540 meat samples tested, the USDA culture method detected 261 positives and the DNAH method detected 378 positives, all of which were confirmed. The DNAH method was statistically equivalent to the culture method for three of the six products tested. In some trials, the DNAH method detected more positives than the USDA method for roast beef, hot dogs, and fermented sausage. In some cases, these differences were statistically significant. Of 100 naturally contaminated products tested, the DNAH method detected 86 positives and the culture methods detected 84 positives. The DNAH method was statistically equivalent to the culture methods for these samples. The DNAH method gives a negative result 48 h after the start of sample enrichment, whereas the FDA and USDA methods require 3 to 4 days or longer. It is concluded that the DNAH assay is a rapid, accurate, and objective alternative to the culture procedures for the detection of Listeria spp. in foods.
ABSTRACT
A colorimetric DNA hybridization assay has been developed for the rapid detection of Escherichia coli in foods. The method employs two oligonucleotide probes which are specific for the 16S ribosomal RNA of E. coli . Probes are added to lysates of test cultures and allowed to hybridize to target rRNA if present. The probe-target complex is captured via hybridization to a polystyrene dipstick. The immobilized target is detected using an antibody-horseradish peroxidase conjugate which binds to the immobilized probe-target complex. The probe-target-antibody complex generates a colorimetric signal when exposed to a substrate/chromogen mixture. A total of 233 E. coli isolates representing typical, toxigenic, invasive, hemorrhagic serotype 0157:H7, and other pathogenic strains all resulted in a positive assay signal. Dose-response experiments indicate the sensitivity of the assay is approximately 1 × 106 CFU/ml. Specificity of the assay was determined by testing 207 strains of non- E. coli species at 109 CFU/ml. All of the non- E. coli organisms tested were negative with the exception of Escherichia fergusonii and Shigella species. A total of 345 enriched samples including inoculated, uninoculated, and naturally-contaminated foods was tested for the presence of E. coli by the hybridization assay and a conventional cultural method. The false-negative rate for the hybridization assay was 1.2%. By comparison, the false-negative rate for the culture method in these studies was 23.4%. Based on these data, the DNA hybridization method is significantly more accurate than conventional methods for the detection of E. coli in foods.
