Development of an enzyme-linked antibody hydrophobic grid membrane filter method for the detection of Salmonella in foods

Abstract

An enzyme-linked antibody hydrophobic grid membrane filter (ELA-HGMF) method was developed using M105 monoclonal antibody and EF-18 as a selective agar. Food was inoculated into nutrient broth and incubated overnight followed by enrichment in tetrathionate brilliant green broth incubated at 35°C (TBG 35) for 18-24 h. One ml of the 10 -3, 10 -4 and 10 -5 dilutions of the TBG 35 were filtered through HGMFs which were placed on EF-18 selective agar. Growth on the EF-18 medium was stained with the ELA, whether it was suspected to be Salmonella or not. Growth on a replicated filter that corresponded to the stained grid cells was then picked and confirmed biochemically and serologically. One hundred and twenty-one foods, feeds, animal by-products and feces were assayed using the ELA-HGMF method with the Health Protection Branch (HPB) standard cultural method as a reference procedure. Fifty-three samples (31 naturally contaminated and 22 artificially inoculated) were positive for Salmonella by both procedures, and strains were isolated from two samples by the ELA-HGMF method alone. An attempt to shorten the time for enrichment from 24 to 6 h decreased the sensitivity of the method and this modification was not pursued any further. The main advantage of the ELA-HGMF method is the relative ease of selection of Salmonella growth from the HGMFs. This, in turn, meant that few strains with atypical reactions needed to be further tested for determination as Salmonella or non-Salmonella. The use of ELA stain to detect the organisms on HGMFs enabled detection of a positive sample in five days, whereas, with few exceptions, a negative sample was detected within four days.