One-Day Fluorescent-Antibody Procedure for Detecting Salmonellae in Frozen and Dried Foods 1

Abstract

The indirect fluorescent-antibody technique was used to examine 422 food samples for the presence of salmonellae. A cultural phase involving a 16-hr preenrichment in buffered nutrient broth-milk medium followed by a 4to 5-hr subculture into fresh medium of the same composition was evaluated. This procedure yielded a sufficient population of salmonellae so that no false-negative results were obtained. Of the 31 false-positives obtained, 12 samples yielded positive cultural results upon extensive subculture of the original enrichment broths. Yeast cells and both vegetative and spore forms of bacilli were observed to fluoresce when stained with anti-Salmonella serum. Efforts to ascertain the cause of these cross-reactions and several alternate explanations are discussed.

A Comparison of the Fluorescent Antibody Method and a Standardized Cultural Method for the Detection of Salmonellas

“…There was little evidence in these studies of fluorescent staining of yeasts or Bacillus spp. as has been reported with these antisera (Goepfert et al 1970).…”

“…Other workers have obtained similar results (e.g. Goepfert et al 1970;Smyser & Snoeyenbos 1973). If reduction in the number of cultures to be examined in detail by the full isolation procedure is the prime consideration, however, this small percentage of false FA positives may be acceptable.…”

“…Fluoresceincoupled antisera with a very wide range of antigen coverage, both somatic and flagellar, are commercially available but this range of coverage was not found to be essential in Northern Ireland. Other workers using polyvalent 0 + H antisera do not appear to record the frequency with which flagella are stained, although they comment that the necessity for fluorescing cells to have attached fluorescent flagella to be considered as salmonellas, is too rigorous a criterion to be implemented (Goepfert et al 1970). A major advantage of the Spicer-Edwards H set of antisera was found to be the extreme dilution at which it may be used, and which undoubtedly reduced or eliminated heterologous staining.…”

“…The 7 vials of the Spicer-Edwards set were reconstituted as directed by the mnnufacturer, and equal volumes of each antiserum were mixed. The mixture was then diluted 1/500 with sterile PBS (Goepfert & Hicks 1969;Goepfert et al 1970) in sufficient quantity for one day's use only, as it was found that storing the diluted antiserum overnight at 4 OC gave very poor FA staining. The FA rabbit globulin antiglobulin (goat) was reconstituted as directed by the manufacturer and diluted 1/8 with sterile PBS for use in sufficient quantity for one day's use only.…”

The Use of Enrichment Serology for Salmonella Detection in Human Foods and Animal Feeds

“…No false positive reactions were obtained using the 24 h elective enrichment ES technique but 2 were detected by the 6 h technique. Thus, from this point of view, the ES technique was better than the fluorescent antibody technique where the usual level of false positives is &lo% (Georgala et al, 1965;Bantasia, 1969;Goepfert et al, 1970).…”