Isolation of Salmonellae from Food Samples

TAYLOR, WELTON I. (Swift and Company, Chicago), AND JOHN H. SILLIKER. Isolation of salmonellae from food samples. IV. Comparison of methods of enrichment. Appl. Microbiol. 9:484-486. 1961. A comparison of various methods of enhancing frequency of Salmonella isolations revealed that inoculation of a second enrichment broth, with culture from the first, wi-as no improvement over the single direct enrichment method. It was inferior to centrifugation. Selenite was observed to produce more positive isolations at 48 hr than at 24. No change occurred in tetrathionate. Reconstitution of dried albumen with water produced a significant increase in isolations over direct inoculation of enrichment broth in the case of tetrathionate but not selenite broth. Pre-enrichment in lactose broth before inoculation of enrichment media was vastly superior to reconstitution in water for both enrichment broths. A comparison of results obtained using dulcitol, mannitol, lactose and carbohydrate-free purple broths in pre-enrichment indicated that the carbohydrate added was immaterial. Potential solutions to the analytical problems posed by the needs for enumeration of small numbers of salmonellae from food samples have been presented in earlier publications of this series. Factors involved in the rationale governing the choice of media and both their experimental and empirical performances were discussed (Taylor, Silliker, Andrews, 1958), as were the merits of a methodology employing centrifugation (Silliker and Taylor, 1958). During the latter investigation, the inhibitory abilities of the enrichment broths were amply demonstrated. However, the possibility suggested itself that the debilitated condition of the salmonellae which have survived the rigors of the processing of eggs might make quantification impossible in inhibitory media. Several approaches for solution of the problem were considered. Would transfer of growth, from one enrichment broth to the other, aid in initiating growth for organisms static in the original broth? Would reconstitution in water add impetus to the initiation 'Present address: Children's Memorial Hospital, Chicago,