The triple sugar-iron (TSI) agar slants are 1% lactose, 1% sucrose, 0.1% glucose, and phenol red to detect carbohydrate fermentation (medium changes from orange-reddish to yellow, positive). In addition, TSI slants contain sodium thiosulfate and ferrous sulfate for hydrogen sulfide production.
Inoculated slants should be observed between 18-24 hours.
Glucose fermentation is indicated when: slant is red; butt (bottom) of tube is yellow; with or without gas production (which would’ve appeared as tiny breaks/bubbles in the agar).
Lactose and/or sucrose fermentation is indicated when: slant is yellow; butt is yellow; with or without gas production (which would’ve appeared as tiny breaks/bubbles in the agar).
No carbohydrate fermentation when: slant is red (or no change); butt is red (or no change).
Hydrogen sulfide production is indicated by a black “trail”/blackening.
Clinical significance. The TSI test differentiates among types of Enterobacteriaceae (all are gram-negative, glucose fermenters w/acid production) and other gram-negative enteric bacilli based on the type of carbohydrate fermentors and hydrogen sulfide producers. TSI can separate out Proteus vulgaris, Proteus mirabilis (causes urinary tract infections, and is sensitive to ampicillin and cephalosporins), and Proteus penneri.
Reference
Cappuccino, J. G., & Welsh, C. (2018). Microbiology: A laboratory manual.
