Triple Sugar Iron Agar

PRODUCT SUMMARY AND EXPLANATION
Triple Sugar Iron Agar was originally proposed by Sulkin and Willett and modified by Hajna for identifying Enterobacteriaceae. This medium complies with the recommendation of Indian Pharmacopoeia for the identification of Gram-negative bacilli.

Triple Sugar Iron Agar (TSI Agar) is a differential medium that isolates and identifies the members of Enterobacteriaceae family and other gram-negative enteric bacilli based on the capacity to ferment sugars and give a reaction for hydrogen sulphide (H₂S). The medium is extensively applied in clinical diagnosis, water quality testing, drug quality control, and food microbiology.

The medium has three carbohydrates—sucrose, lactose, and glucose—and phenol red for pH indicator, ferrous sulphate to identify hydrogen sulphide, and peptones to provide a medium for bacterial growth. Differential fermentation of the three sugars results in diagnostic colour change in the slant and butt areas of the medium, enabling unambiguous identification of patterns of fermentation. Glucose fermenters will generally present an acid butt (yellow) with an alkaline slant (red) when glucose is depleted, but lactose/sucrose fermenters produce an acid slant and butt.

Gas production is seen as bubbles, cracking, or agar displacement. Hydrogen sulphide production is seen as black precipitate on reaction of H₂S with ferrous sulphate. They are quick and specific tests for the identification of pathogens like Salmonella, Shigella, Escherichia coli, Proteus, and Klebsiella.

TSI agar is streaked in a slant in test tubes, which allows for the presence of aerobic and anaerobic growth conditions simultaneously in one test tube. The test tubes are incubated at 35–37°C for 18–24 hours after being inoculated by stabbing the butt and streaking the slant. Physical reactions and colour changes seen after incubation are interpreted.

PRINCIPLE
Peptone, yeast extract and beef extract provide nitrogenous compounds sulphur, trace elements and vitamin B complex etc. Sodium chloride maintains osmotic equilibrium. Lactose, sucrose and dextrose monohydrate are the fermentable carbohydrates. Sodium thiosulphate and ferric or ferrous ions make H2S indicator system. Sodium thiosulphate is also an inactivator of halogen and can minimize its toxicity in the testing sample, if any during microbial limit tests. Phenol red is the pH indicator.

QUALITY CONTROL SPECIFICATIONS
Appearance of Powder : Light yellow to pink homogeneous free flowing powder.
Appearance of prepared medium : Pinkish red coloured clear to slightly opalescent gel forms in Petri plates.
pH (at 25°C) : 7.4±0.2