for identification of gram-negative enteric bacilli on the basis of dextrose, lactose and sucrose fermentation and H2S production.
The ingredients included in the medium such as, Peptones, Beef extract and Yeast extract provide the nitrogen, carbon, and vitamins required for organism growth. Medium consists of three carbohydrates, Dextrose, Lactose and Sucrose. When the carbohydrates are fermented, acid production is detected by the Phenol Red pH indicator. Organisms that ferment glucose monohydrate produce a variety of acids, turning the colour of the medium from red to yellow. More amounts of acids are liberated in butt (fermentation) than in the slant (respiration). Growing bacteria also form alkaline products from the oxidative decarboxylation of peptone and these alkaline products neutralize the large amounts of acid present in the butt. Thus the appearance of an alkaline (red) slant and an acid (yellow) butt after incubation indicates that the organism is a glucose fermenter but is unable to ferment lactose and/or sucrose. Bacteria that ferment lactose or sucrose (or both), in addition to glucose, produce large amounts of acid enables no reversion of pH in that region and thus bacteria exhibit an acid slant and acid butt. Sodium thiosulphate is reduced to hydrogen sulphide, and hydrogen sulphide reacts with an iron salt yielding the typical black iron sulphide. Ferric ammonium citrate is the hydrogen sulphide (H2S) indicator. Sodium chloride maintains the osmotic balance of the medium. Agar is used as a solidifying agent.