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.
A culture medium and testing procedure were developed to detect and differentiate Salmonella in pure culture study and for presumptive detection of Salmonella in dairy products. Most pure cultures of Salmonella were easily differtiated from other members of the Enterobacteriaceae group after 18 hr incubation at 37 C in a neutral red-lysine-iron-cystine broth. Salmonella changed neutral red in the medium from red to yellow and most strains turned the medium black through formation of a massive black precipitate. Species of Enterobacter , Citrobacter , Proteus , Shigella , and Pseudomonas intensified the red color of the medium and failed to blacken it. Species of Klebsiella and Escherichia usually changed the medium from red to yellow in 18 hr but none developed a black precipitate. The few nonhydrogen sulfide producing strains (no medium blackening) of salmonellae were differentiated from Escherichia and Klebsiella sp. by continued incubation to a total of 42 hr followed by the use of a second indicator, brom thymol blue. Only salmonellae gave an alkaline reaction and converted the medium from yellow to green or blue. The medium provided for rapid detection of most salmonellae after 18 hr incubation, as characterized by medium blackening or color change from red to yellow. Related enteric bacteria, other than Arizona strains were readily differentiated. For dairy products a slight modification in medium formula and use of novobiocin and trypsin were required. Novobiocin selectively inhibits growth of most interfering spore formers and gram-positive bacteria and also certain strains of Escherichia coli and Proteus . Trypsin was used to digest casein added with the dairy product sample. A positive presumptive test for Salmonella in dairy products was indicated in the medium by a color change from red to yellow and/or production of a massive black precipitate of iron sulfide after 24 hr incubation. Absence of salmonellae was indicated by no color change or no medium blackening. Results from testing several dairy products indicated that the procedure may be of value in the rapid screening of these foods for salmonellae. Although confirmation and serological identification are still essential, the test eliminates preenrichment and gives presumptive evidence of salmonellae contamination after 24 hr.