Detection of Salmonella spp. in eggs: DNA analyses, culture techniques, and serology

A polymerase chain reaction (PCR) method for direct detection of Salmonella spp. in whole-shell eggs is described. The method does not require isolating strains. Preenrichment, rapid DNA isolation, and a nested PCR system targeting the invA gene enabled detection of the genus Salmonella. The specific nested PCR product of 283 base pairs was formed from all 21 serovars, including 43 Salmonella strains tested. No PCR product was formed from 56 non-Salmonella enterobacteriacea and other bacterial strains tested. Experiments with artificially contaminated eggs showed a detection limit of about 10(3)-10(4) colony-forming units (cfu)/egg before and about 1-10 cfu/egg after preenrichment. In analysis of 180 single eggs from 4 flocks and 36 pools of 5 eggs each from another 4 flocks from the same producer, Salmonella spp. were detected in 5 of 90 eggs from 2 different flocks. Determination of anti-Salmonella antibodies in eggs yielded positive results for 2 additional and the 2 PCR-positive flocks. In contrast, classical selective culture detected Salmonella spp. in only 1 of 100 eggs in one flock when 100 eggs from each flock were analyzed.