Isolation of Salmonellae from food samples. V. Determination of the method of choice for enumeration of Salmonella.

Abstract

Comparison of three methods by which salmonellae may be isolated and enumerated from dried albumen, direct inoculation of enrichment media, centrifugation of samples, and pre-enrichment in noninhibitory media, reveals pre-enrichment to be the method of choice. The superiority of pre-enrichment manifests itself in replicate aliquots of the same sample by producing a statistically significant increase in numbers of isolations of salmonellae and in empirical use with various albumen samples by consistently higher values of most probable numbers (MPN). The primary factor involved in this superiority appears to be the greater ability of small numbers of salmonellae to initiate growth in the nonselective mannitol purple sugar broth than in the inhibitory enrichment media. The method of analysis recommended entails inoculation of mannitol broth pre-enrichment medium, transfer of 24-hr culture aliquots to tetrathionate broth, and streaking on brilliant green agar for isolation of salmonellae.