TRIPLE SUGAR IRON AGAR (as per USP)

Triple Sugar Iron Agar Medium was originally proposed by Sulkin and Willett and modified by Hajna for identifying Enterobacteriaceae. This medium is in accordance with United States Pharmacopoeia and is recommended in pharmaceutical testing for identification of Gram-negative bacilli.
Organisms that ferment dextrose produce a variety of acids, varying the colour of the medium from red to yellow. More amounts of acids are liberated in butt region (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 dextrose fermenter but is unable to ferment lactose and/or sucrose. Bacteria that ferment lactose or sucrose (or both), in addition to dextrose, 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.

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