A Selective Medium and Presumptive Procedure for Detection of Salmonella in Dairy Products

Abstract

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.

Improved Salmonella recovery from moderate to highly polluted waters

“…The findings are in agreement with previous results reported for sodium novobiocin addition to the enrichment broth (Jeffries 1959) or to the plating media (Hargrove et al 1971 ;Boothroyd & Baird-Parker 1973;Hoben et al 1973;Restaino et al 1977). The method described in the We especially thank Manuel Serra for his generous contributions to the statistical analysis.…”

“…Several authors (Jeffries 1959;Hargrove et al 1971;Boothroyd & Baird-Parker 1973;Hoben et al 1973;Restaino et al 1977) This antibiotic inhibits the growth of Proteus, Pseudomonas and Escherichia coli which are usually present in large numbers in surface waters. A much modified Rappaport medium (R10/43"C) recently described (Vassiliadis et al 1978) permits the recovery of a wide range of salmonellas.…”

A Rapid, Presumptive Procedure for the Detection of Salmonella in Foods and Food Ingredients

“…Cultures of various known Salmonella and known type species of non-Salmonella were used in this study to evaluate the modified LICNR broth and to verify the results obtained by Hargrove et al (6). Freshly grown cultures were inoculated into the modified LICNR presumptive broth.…”

“…Medium. The medium, lysine-iron-cystine-neutral red broth (LICNR), of Hargrove et al (6) was modified as follows: (i) mannitol was substituted for lactose, (ii) a combination of L-lysine mono-and dihydrochlorides (8:2) was used instead of the monohydrochloride, and (iii) novobiocin was used at two concentrations, either zero added or at concentrations of 10 to 15 ug/ml of broth. The modified LICNR broth had the following composition: 10 g of L-lysine mono-and dihydrochlorides (8:2), 5 g of tryptone, 3 g of yeast extract, 5 g of mannitol, 1 g of glucose, 1 g of salicin, 0.5 g of ferric ammonium citrate, 0.1 g of sodium thiosulfate, 0.1 g of L-cystine, 0.025 g of neutral red, and 1 liter of distilled water.…”

“…A simpler culture method for the rapid detection of Salmonella was described by Hargrove et al (6). This procedure was developed primarily for detecting Salmonella in dairy products in a single enrichment step and was tested extensively with known Salmonella standards.…”