AN ENZYME-LINKED-IMMUNOSORBENT-ASSAY FOR THE RAPID DETECTION OF STAPHYLOCOCCUS-AUREUS IN PROCESSED FOODS

A sandwich enzyme-linked immunosorbent assay (ELISA) was developed for the rapid detection of Staphylococcus aureus in foods. The assay was based on the detection of protein A which is a specific protein secreted by S. aureus. Following a 24-h incubation in a staphylococcal selective broth containing mannitol as the carbon source, the culture supernatant was added to the microtiter plate coated with anti-protein A immunoglobulin G (IgG). After incubation, peroxidase-labeled anti-protein A IgG was used to produce the signal of antigen-antibody reaction. The sensitivity of the assay for protein A was 0.1 ng/ml. For 37 strains of S. aureus studied, all produced protein A, and the amount (13-1,100 ng/ml) of protein A secreted by different strains varied to a large degree. For another 57 strains (including 19 Staphylococcus spp.) of bacteria tested, two strains (S. capitis subsp. capitis CCRC 12161 and S. lentus CCRC 12926) produced very low amounts of protein A (0.6-1 ng/ml) after 24-h incubation. Staphylococcus aureus was detected by the ELISA in all of six samples of precooked foods naturally contaminated with the bacterium. Twenty-two processed foods artificially inoculated with S. aureus at levels of < 2 CFU/g and 10 to 20 CFU/g, respectively, were all positive by the ELISA. As compared to the conventional culture methods which take 5 to 6 days to complete, the ELISA can detect low numbers of S. aureus in processed foods with a total analytical time of only 28 h.