Lactose Pre-enrichment Method for Isolation of Salmonella from Dried Egg Albumen Its Use in a Survey of Commercially Produced Albumen

A review of the literature has revealed a scarcity of attempts to compare directly various methods proposed for the detection of Salmonella organisms in food products. Unfortunately, often it has been assumed that methods useful in clinical or public health laboratories, engaged in identifying cases and carriers, are also suitable for the examination of food products. Furthermore, in many instances new methods have not been adequately proved in effectiveness nor have they been subjected to comparisons with noninhibitory media to determine how well they will support the growth of Salmonella. Productivity, as well as selectivity, is very important where interest lies in quantitative determination or in the detection of small numbers of Salmonella in a food product. The medium selected should not only be noninhibitory for Salmonella, either alone or when combined with the product under examination, but should also be capable of supporting growth of small numbers of organisms especially in higher dilutions where any possible nutritional or inhibitory substances from the food under test would have been diluted out. In a previous paper, North and Bartram (1953) demonstrated the favorable effect of L-cystine on the productivity of selenite F broth. These findings were confirmed by Byrne, Rayman, and Schneider (1955) and more recently by Taylor, Silliker, and Andrews (1958). Among other things, Taylor et al. compared selenite cystine (SC) broth with broths prepared with dulcitol or mannitol instead of lactose. They found no statistical advantage by these substitutions and thereby confirmed Leifson's (1936) claim that there was no improvement when a carbohvdrate other than lactose was used. Also, they found that the selenite brilliant green sulfapyridine enrichment broth of Osborne and Stokes (1955) and Stokes and Osborne (1955) did not produce more isolations of Salmonella and in some instances was markedly inferior to the SC broth. Comparative studies by the author (unpublished) on various enrichment broths showed that these modifications which contain brilliant green, with or without sodium sulfapyridine, were inferior to SC broth in productivity. In another paper, Silliker and Taylor (1958) called attention to "skips" occurring in the most probable numbers (MPN) determinations which involved the testing of 10-, 1. 0-, and 0.1-g portions of dried egg white, using SC and tetrathionate (TT) as enrichment media. They found that, by centrifuging, a soluble inhibitory substance could be decanted without appreciable loss of microorganisms. The sediment cultured in either SC or TT enrichment broth permitted detection of small numbers of Salmonella frequently missed when large amounts of egg white were cultured directly in the enrichment broth. The author has confirmed the superiority of this method over direct enrichment in either SC or TT broth for the examination of egg whites. We have encountered difficulty in the detection of small numbers of Salmonella in other foods where relatively large portions are cultured directly in selective enrichment media. In testing a variety of dried food products, we have found that reconstitution in water or broth and incubation for several hours prior to inoculation of selective enrichment media significantly enhances the recovery of Salmonella, presumably attenuated by prolonged storage in the dry state. The purpose of this paper is to describe (a) a procedure employing a nonselective medium, lactose broth, for pre-enrichment of dried foods prior to inoculation of selective media, which eliminates "skips" in quantitative estimations of Salmonella, and _(b) the results of this procedure in the examination of commercially produced dried egg white.