Development of a Solid-Phase Fluorescence Immunoassay for the Detection of Salmonella in Raw Ground Turkey

A solid-phase fluorescence sandwich immunoassay was developed to detect Salmonella spp. and tested for sensitivity on pure cultures as well as on spiked and naturally contaminated raw ground turkey. The solid support was a soft-glass capillary tube to which a polyclonal antibody against Salmonella spp. was adsorbed. The adsorbed antibody captured cellular antigens from the sample solution. The same polyclonal antibody was also biotinylated and, in combination with an avidin-Cy5 dye conjugate, served to allow detection of the immunologically captured cells. The minimum detectable number of Salmonella cells spiked into ground turkey was approximately 2.5 CFU/25 g. Fifty-one raw ground turkey samples were tested for the presence of Salmonella spp. using both this immunoassay and a modified Bacteriological Analytical Methods enrichment method. Fluorescence intensity values were normalized as a ratio of the sample fluorescence to the fluorescence of a standard. Fluorescence intensity ratios of 0.50 or greater were scored as Salmonella positive. The solid-phase fluorescence immunoassay yielded 10% false positives and no false negatives when results were compared with those from the modified enrichment method. Isolates recovered from the samples that produced false-positive results were identified as Citrobacter freundii and Enterobacter cloacae , both of which are known to cross-react with the antiserum used.