Influence of Carbon Source, Bile Salts and Incubation Temperature on Recovery of Enterobacteriaceae from Foods Using MacConkey-type Agars

Abstract

The plea made many years ago (27} to replace the ill-defined coli-aerogenes ('coliform') bacteria as indicator organisms in foods processed for safety with the Enterobacteriaceae which are taxonomi-cally accurately defined and as a rule more abundant has of late been more generally accepted. This called for development of a rigorously standardized formula for violet red bile glucose agar and for assessment of the optimal incubation temperature. Four reference strains of Enterobacteriaceae. 120 samples of minced meat and 100 samples of frozen broiler chickens were used in these studies. Considerable differences in the performance of commercially available dried formulae, when used as poured plates were observed. These applied both to productivity and to the type of colony produced by a given pure culture. As expected, replacement of lactose plus glucose by an equimolar amount of glucose did not influence the performance of the medium. Intrinsic toxicity of some batches of medium to non-stressed Enterobacteriaceae appeared to be mainly responsible for substandard performance. It could be overcome by careful selection of the preparations of crystal violet and particularly the bile salts (,39) used in the formulae. Incubation at 30 C led to higher confirmed colony counts in minced meat than at 37 C. However. incubation temperature did not greatly influence similar counts in broiler drip. This observation could be substantiated by identification of the types of Enterobac-teriaceae isolated from the two commodities. Psychrotrophic species predominated in minced meats. which are often made from raw materials stored for some time under refrigeration, whereas mesophilic species were in the majority on frozen broilers, which are generally frozen shortly after slaughter.