Deoxycholate-Citrate Agar Medium 14 (In accordance with IP 2007)

Principle And Interpretation

Desoxycholate Citrate Agar is prepared as per the modified formula of Leifson (1) and is also recommended by Indian Pharmacopoeia 2007 (2). This medium is used for the isolation and maximum recovery of intestinal pathogens belonging to Salmonella and Shigella groups from foods and pharmaceutical products (3). However, it is recommended to use less inhibitory medium when Shigellae have to be isolated (4). Salmonella, major causative agent of enteric disease especially food borne toxic infection and typhoid was first observed by Eberth in 1880. This medium is routinely used to check the presence of Salmonella in food and pharmaceutical products.

Proteus and other Gram positive organisms are inhibited due to higher concentration of both citrate and deoxycholate salts in this medium. Sodium desoxycholate at pH 7.3 to 7.5 is inhibitory for gram-positive bacteria. Sodium thiosulphate also helps in reactivation of sulphur containing compounds and prevents the desiccation of these compounds during storage. It also forms the substrate for enzyme thiosulphate reductase, which breaks it; to form H2S. H2S then reacts with Fe ions in the medium and produces black FeS precipitate. This gives the indicative appearance of colonies with black center. Sodium thiosulphates are also inactivators of halogens and can minimize its toxicity in the testing sample, if any during microbial limit tests. Citrate salt, in the concentration included in the formulation, are inhibitory to gram-positive bacteria and most other normal intestinal organisms. Combination of beef extract and peptone supplies nitrogen, mineral, vitamin factors required for enhanced growth. Lactose monohydrate supplies fermentable carbohydrate source in this medium. Neutral red acts as indicators, in presence of which lactose fermenters like coliform bacteria give pink colonies while lactose non-fermenters give colourless colonies. Salmonella gives either colourless and opaque colonies with or without black center, while Shigella gives colourless colonies without black center indicating absence of H2S production. Precipitation of deoxycholate by acid produced by lactose fermenters may give a zone of precipitation around the colony. This medium provides essential growth factors for growth of several auxotrophic strains of Paratyphi and Typhi. The selectivity of this medium permits the use of fairly heavy inocula without danger of overgrowth of the Shigella and Salmonella a by other microflora. For the routine examination of stool and urine specimens, it is suggested that other media such as MacConkey Agar (MM082), Bismuth Sulphite Agar (MM027) etc. be used in conjunction with this medium.