A SELECTIVE PLATING AGAR FOR DIRECT ENUMERATION OF SALMONELLA IN ARTIFICIALLY CONTAMINATED DAIRY PRODUCTS1

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Abstract
A selective plating medium was developed that allows direct enumeration of salmonellae in dairy products such as nonfat dry milk, and Cheddar and cottage cheese. The agar medium developed was a modification of the Lysine-Iron-Cystine broth of Hargrove et al., 1971. Strains of species of the genera Escherichia, Enterobacter, Citrobacter, Proteus, Shigella, Pseudomonas, and Bacillus were easily differentiated from salmonellae by colony color and color of surrounding area or absence of growth. The antibiotic, novobiocin, used as a selective agent, inhibited growth of some Proteus, Shigella, and Escherichia; however the antibiotic was most effective against Bacillus, without having any observable effect on salmonellae. The medium was sufficiently sensitive and selective to permit detection of as few as 1–2 salmonellae per gram of product in the presence of naturally occurring bacteria and should be of considerable value in following the Salmonella content of artificially contaminated foods during processing and also during storage.

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