Salmonella Differential Medium (without Membrane Filter)

Principle And Interpretation

DriFilter Membrane Nutrient Pad Medium is ready to use sterile culture media in the form of a 50 mm biological inert absorbent pads impregnated with Sabouraud Dextrose Medium, then dried and sterilized in 55 mm petri plate. They eliminate the need of laborious media preparation and autoclaving procedures. The nutrient pads are to be just rewetted with sterile distilled water and are ready to use. Use of nutrient pads allows larger sample volumes to be tested at a time. Interpretation of results is directly by counting the CFUs and also quantifies the microbial load present in the sample. Salmonella Differential Medium is slight modification of original formulation of Rambach (1) used for differentiation of Salmonella species from Proteus species and other enteric bacteria. Production of acid from propylene glycol is a novel characteristic of Salmonella species and is utilized in these media. Many of the media such as SS Agar, XLD Agar recommended for the identification and differentiation of Salmonella species (2) are based on lactose fermentation and hydrogen sulphide production. Peptone special and yeast extract supports the luxuriant growth of bacteria by supplying nitrogen and carbon compounds, long chain amino acids, vitamins and other essential nutrients. Sodium deoxycholate inhibits gram-positive organisms rendering the medium selective for enteric microorganisms. The BC indicator turns pink in presence of acid produced from propylene glycol. Lactose fermenting ability is determined by using an indicator, which can detect the presence of enzyme B-galactosidase. Lactose fermenting (B-galactosidase producing) bacteria yield blue violet coloured colony (3). Salmonella produce acid from propylene glycol and on combining with the pH indicator gives typical pink red colonies. Other enteric gram-negative bacteria form colourless colonies. Salmonella Typhimurium and Salmonella Enteritidis produce pink to red colonies. Specimen should be enriched in an appropriate selective enrichment broth. This enriched culture is then inoculated on Salmonella Differential Agar, Modified and incubated at 35-37°C for 24-48 hours.