Delayed secondary enrichment for the isolation of salmonellae from broiler chickens and their environment

Specimens collected from six broiler flocks were cultured for salmonellae by three methods. (i) For direct enrichment, the specimen was homogenized, and 1 ml of the homogenate was inoculated into tetrathionate-brillant green broth; (ii) for preenrichment, liquid specimens and homogenates were incubated at 37 degrees C, and on the next day 1 ml was inoculated into tetrathionate-brillant green broth; and (iii) for delayed secondary enrichment, incubated preenrichment cultures were held at room temperature for 7 to 10 days and then subcultured to fresh tetrathionate-brilliant green broth. All tetrathionate-brilliant green broth cultures were incubated at 42 degrees C for 24 to 48 h before plating. Significantly more isolations of salmonellae were obtained by delayed secondary enrichment than by direct enrichment or preenrichment. Salmonellae were isolated from 417 of 2,283 (18.3%) samples of litter, intestinal contents, and feces cultured by all three methods. Of these positive specimens, direct enrichment detected 208 (49.9%), preenrichment detected 282 (67.6%), and delayed secondary enrichment detected 373 (89.4%). Of 896 specimens of swabs and rinse fluids that were cultured by preenrichment and delayed secondary enrichment, 259 (28.9%) yielded salmonellae. Delayed secondary enrichment detected 254 (98.1%) of these, and preenrichment detected 147 (56.8%). A total of 23 serotypes of salmonellae were identified. The greater effectiveness of delayed secondary enrichment for the isolation of salmonellae was not likely due to the selection of certain serotypes or to an increased inhibition of competing flora.