Triple Sugar Test in Microbiology

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TRIPLE SUGAR-IRON AGAR TEST
Triple sugar-iron (TSI) agar test- designed to differentiate among the different groups or genera of the Enterobacteriaceae, which are all gram-negative bacilli capable of fermenting glucose with the production of acid a. Differentiation is made on the basis of differences in carbohydrate fermentation patterns and hydrogen sulfide production. To facilitate observation of carbohydrate utilization patterns- TSI agar slants contain lactose and sucrose (1%) concentrations and glucose (0.1%) concentration which permits detection of the utilization of this substrate only. The acid base indicator phenol red- also incorporated to detect carbohydrate fermentation that is indicated by a change in color of the medium from orange-red to yellow in the presence of acids. a. Method to inoculate slant: stab and streak procedure which requires insertion of sterile straight needle from the base of the slant into the butt. Upon withdrawal, the slanted surface of the medium is streaked. Determining activities of the organism:

a. Alkaline slant (red) and acid butt (yellow) with or without gas production (breaks in the agar butt): Only glucose fermentation has occurred. Glucose was degraded first. Since this substrate is minimally present, the small amount of acid produced on the slant surface is oxidized rapidly. Peptones in the medium are used in production of alkali. In the butt the acid reaction is maintained because of reduced O2 tension and slower growth of the organism. b.Acid slant (yellow) and acid butt (yellow) with or without gas production: Lactose and or sucrose fermentation has occurred. Since these substances are present in higher concentrations, they serve as the substrates for continued fermentative activities with maintenance of an acid reaction in both slant and butt. c.Alkaline slant (red) and alkaline butt (red) or no change (orange-red) butt- No carbohydrate fermentation has occurred. Instead peptones are catabolized...
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