TRIPLE SUGAR IRON AGAR

Triple Sugar Iron Agar, is recommended for identification and differentiation of Enterobacteria by British Pharmacopoeia. It was originally proposed by Sulkin and Willett and modified by Hajna for identifying Enterobacteriaceae. This medium complies with the composition as specified in British Pharmacopoeia. It can also be used for the examination of meat and food products, for the examination of milk and dairy products and for microbial limit test for confirming the presence of Salmonella and in the identification of gram-negative bacilli.
Organisms that ferment glucose produce a variety of acids, turning the colour of the medium from red to yellow. More amount of acids is 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. Gas production (CO2) is detected by the presence of cracks or bubbles in the medium, when the accumulated gas escapes. Thiosulphate is reduced to hydrogen sulphide by several species of bacteria and H2S combines with ferric ions of ferric salts to produce the insoluble black precipitate of ferrous sulphide. Reduction of thiosulphate proceeds only in an acid environment and blackening usually occurs in the butt of the tube. Triple Sugar Iron Agar should be used in parallel with Urea Agar/ Broth to distinguish between Salmonella and Proteus species. The reactions can be summarized as follows:
Alkaline slant / acid butt-only glucose fermented
Acid slant / acid butt-glucose and sucrose fermented or glucose and lactose fermented or all the three sugars, glucose, lactose and sucrose fermented.
Bubbles or cracks present-gas production
Black precipitate present- H2S gas production

for identi?cation of gram-negative enteric bacilli on the basis of dextrose, lactose and sucrose fermentation and H2S production.