Deoxycholate Agar, 500 g

Deoxycholate Agar is prepared as per the formulation by Leifson (1). This media is used for the isolation and maximum recovery of intestinal pathogens belonging to Salmonella and Shigella species (2). The selectivity of medium permits the use of fairly heavy inocula without danger of overgrowth of the Shigella and Salmonella by other micro-flora. For the routine examination of stool and urine specimens, it is recommended that other media such as MacConkey Agar (M082), Bismuth Sulphite Agar (M027) etc. be used in conjunction with this medium. It can also be used to streak specimen from Selenite Broth cultures. This is particularly recommended for the detection of Shigella and Salmonella in the examination of rectal swabs and faeces. These organisms produce colourless colonies on this medium. Peptic digest of animal tissue provides carbon, nitrogen, vitamins and minerals. Coliform bacteria and gram-positive bacteria are inhibited or greatly suppressed due to sodium deoxycholate and sodium citrate. Sodium chloride maintains the osmotic balance of the medium while dipotassium phosphate buffers the medium. Lactose helps in differentiating enteric bacilli as lactose fermenters produce red colonies while lactose non-fermenters produce colourless colonies. Coliform bacteria if present form pink colonies on this medium. The degradation of lactose causes acidification of the medium surrounding the relevant colonies and the pH indicator neutral red changes 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). 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 the prepared medium at 2 - 8°C. Use before expiry date on the label. References: 1.Leifson, 1935, J. Path. Bacteriol., 40:581. 2.Speck M. L., (Ed.), 1984, Compendium of Methods for the Microbiological Examination of Foods, 2nd ed., APHA, Washington, D.C.