Comparison of Culture Methods for Detection of Salmonella in Dried Active Yeast

Abstract

Abstract An improved procedure has been developed for the isolation of Salmonella from dried active yeast. Examination is presently made with a procedure employing pre-enrichment in 1% tryptone broth using a sample-broth ratio of 1:5, with subsequent transfers to selenite cystine (SC) broth and tetrathionate (TT) broth before streaking onto selective agars. The new method employs trypticase soy broth as the pre-enrichment medium using a samplebroth ratio of 1: 10, and subsequent transfers to lauryl sulfate tryptose (LST) broth and TT broth before streaking onto selective agars. The most significant points of change are samplebroth ratio and substitution of LST broth for SC broth. After a 24 hr pre-enrichment period, most probable numbers of the 5 Salmonella serotypes tested with the present procedure ranged from 13 × 103 to 7 × 105/ml compared with 13 × 106 to 49 × 107/ml using the new procedure. An incompatibility between active yeast and SC broth resulted in marked inhibition of 4 of the 5 Salmonella serotypes tested. LST broth was found to be highly productive for Salmonella growth and can be used in place of SC broth because of the absence of a large bacterial flora in dried active yeast. TT broth is retained as one of the selective broths in the event gross contamination is encountered and Salmonella becomes overgrown by non-salmonellae in the LST broth.