are used to determine if bacteria can ferment glucose and/or lactose
and if they can produce hydrogen sulfide or other gases. (If an organism can ferment glucose, it is "glucose positive". If it ferments lactose, it is "lactose positive".) In addition, TSI detects the ability to ferment sucrose. These characteristics help distinguish various Enterobacteriacae, including Salmonella and Shigella, which are intestinal pathogens.
TSI contains three sugars: glucose, lactose and sucrose. Lactose and sucrose occur in 10 times the concentration of glucose (1.0% versus 0.1%). Ferrous sulfate, phenol red (a pH indicator that is yellow below pH 6.8 and red above it), and nutrient agar are also present. The tube is inoculated by stabbing into the agar butt (bottom of the tube) with an inoculating wire and then streaking the slant in a wavy pattern. Results are read at 18 to 24 hours of incubation.
Reading the Results
A yellow slant on TSI indicates the organism ferments sucrose and/or lactose. On KIA a yellow slant indicates the organism ferments lactose. (Because KIA does not contain sucrose, sucrose fermentation is not detected with KIA tests.) Other results are the same for TSI and KIA. A yellow butt shows that the organism fermented glucose. Black preciptate in the butt indicates hydrogen sulfide production. Production of gases other than hydrogen sufide is indicated either by cracks or bubbles in the media or the media being pushed away from the bottom of the tube.
Understanding the Results
If an organism ferments glucose only, the entire tube turns yellow due to the effect of the acid produced on phenol red. Because there is a minimal amount of glucose present in the tube, the organism quickly exhausts it and begins oxidizing amino acids for energy. Ammonia is thus produced and the pH rises. Within 24 hours the phenol red indicator reverts to its original red color on the slant. Because TSI/KIA media is poured as a deep slant, the butt has limited oxygen and bacteria are unable to oxidize amino acids there. The butt thus remains yellow.
If an organism can ferment lactose and/or sucrose, the butt and slant will turn yellow (as they do from glucose fermentation). However, they remain yellow for at least 48 hours because of the high level of acid products produced from the abundant sugar(s).
KIA resembles TSI in all respects except that KIA contains two sugars (lactose and glucose) while TSI contain three sugars (lactose, glucose and sucrose). Like TSI media, KIA contains 10 times as much lactose as glucose. Thus KIA tests for an organism's ability to ferment glucose or lactose but not sucrose.
If the gas being produced is hydrogen sulfide (H2S), it reacts with the ferrous sulfate and preciptates out as a black precipitate (ferric sulfide) in the butt. Organisms producing large amounts of hydrogen sulfide (e.g. Salmonella and Proteus) may produce so much black precipitate that it masks the yellow (acid) color of the butt.
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| 1. Reading results | 2. Interpreting results on TSI |
3. Interpreting results on KIA |
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© 1998