Deoxycholate Citrate Agar, Modified (Hynes), 500 g

Deoxycholate Citrate Agar, Modified (Hynes) is a selective medium used for isolation and identification of Salmonellae and Shigallae. Leifson (1) developed Deoxycholate Agar as a differential medium containing pure chemicals, citrates and deoxycholate as inhibitors. Leifsons medium has been modified by many authors by several ways. Deoxycholate Citrate Agar, Modified (Hynes) is a differential medium modified by Hynes (2) for the isolation of Salmonellae and Shigellae. Deoxycholate Citrate Agar, Modified consist of more concentrations of inhibitors and is used in food microbiology (3). Peptic digest of animal tissue and beef extract provides carbon, nitrogen, vitamins and minerals. Coliform bacteria and grampositive bacteria are inhibited or greatly suppressed due to sodium deoxycholate, sodium citrate and ferric citrate. Lactose helps in differentiating enteric bacilli, as lactose fermenters produce red colonies while lactose non-fermenters produce colorless colonies. Coliform bacteria, if present form pink colonies on this medium. The degradation of lactose causes acidification of the medium surrounding the relevant colonies causing the pH indicator neutral red to change its colour to red. These colonies usually are also surrounded by a turbid zone of precipitated deoxycholic acid due to acidification of the medium. Sodium deoxycholate combines with neutral red in an acidic environment, causing the dye to go out of the solution with the subsequent precipitation of deoxycholate (1). The reduction of sodium thiosulphate to sulfide is indicated by the formation of black iron sulfide. Salmonella and Shigella species do not ferment lactose but Salmonella may produce H2S forming colorless colonies with or without black centers. Citrate and iron (Fe) combination has a strong hydrolyzing effect on agar when the medium is heated, producing a soft and unelastic agar. If autoclaved the agar becomes soft and almost impossible to streak (1). Surface colonies of non-lactose fermenters often absorb a little colour (pinkish) from the medium and organisms may be mistaken for coliforms (1). Storage and Shelf-life: Store below 30°C in tightly closed container and use freshly prepared medium. Use before expiry date on the label. References: 1. Leifson, 1935, J. Pathol. Bacteriol., 40:581. 2. Hynes M., 1942, J. Path. Bacteriol., 54, 193-207. 3. Speck M. (Ed.), 1984, Compendium of Methods for the Microbiological Examination of Foods, 2nd ed., APHA, Washington, D.C.