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Triple sugar-iron agar test The triple sugar-iron (TSI)agar test is 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, and to distinguish the enterobacteriaceae from other gram negative intestinal bacilli. This differentiation is made on the basis of differences in carbohydrate fermentation patterns and hydrogen sulfide production by the various groups of intestinal organisms. To facilitate observation of carbohydrate utilization patterns, the TSI agar slants contain lactose and sucrose in 1% concentrations and glucose (dextrose) in a concentration of 0,1%, which permits detection of the utilization of this substrate only. The acid base indicator phenol red is 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. The slant is inoculated by means of a stab and streak procedure. This requires the insertion of a sterile, straight needle from the base of the slant into the butt. Upon withdrawal of the needle, the slanted surface of the medium is streakes. Following incubation, you will determine the fermentative activities of the organisms as described below. For you agar medium also contains sodium thiosulfate, a substrate for hydrogen sulfide (H2S) production, and ferrous sulfate for detection of this colorless end product. Following incubation, only cultures of organisms capable of producing H2S will show an extensive blackening in the butt because of the precipitation of the insoluble ferrous sulfide.Media Per designated student group; triple sugar iron agar slants; 7 for the short version, 14 for the long version.
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