7. You still do not have enough information to identify the pathogen, so you dec

ID: 123544 • Letter: 7

Question

7. You still do not have enough information to identify the pathogen, so you decide to run one more biochemical test. The test you run is called a Triple Sugar Iron (TSI) test. The TSI test has a pH indicator in it that turns the red-colored media to yellow if the inoculated bacterium uses certain sugars in the tube (see attached pictures) The test also shows if the bacterium makes hydrogen sulfide gas (H2S) if a black precipitate forms. After inoculating the TSI tube and incubating it overnight, you observed a red slant and a black butt (D). What sugar does this indicate that the bactería used? Did the bacteria produce H2S gas?

Explanation / Answer

During the process of Triple sugar Iron test three types of sugars glucose, Sucrose and lactose are used. By using this test we can easily differentiate different genera of enterobacteriaceae Gram Negative bacteria capable of cementing glucose with the production of acid. The enteric pathogen differenced on the different carbohydrate fermentation. The production of hydrogen sulphide is identified due to the formation of black precipitate in the back of the yube.

If red slant/yellow butt means only fermentation of glucose has occurred; yellow slant/yellow butt means lactose and/or sucrose fermentation has occurred. So there is no need of separate column showing the fermentation of sucrose and/ lactose. By simply seeing the color change in butt or slant one can identify which sugar fermented.

Organism

Acid from glucose (+/-)

Gas from Glucose (+/-)

H2S production (+/-)

MT1-Escherichia coli

MT2-Enterobacter aerogenes

MT3-Salmonella typhii

MT4-Proteus vulgaris

Addition of sucrose in TSI Agar permits earlier detection of coliform bacteria that ferment sucrose more rapidly than lactose. Adding sucrose also aids the identification of certain gram-negative bacteria that could ferment sucrose but not lactose.

The reaction in triple sugar iron Agar cannot be read after 24 hours of incubation because aerobic oxidation of fermentation products formed lactose of sucrose will occur on the slant will regain alkaline state. Moreover the production of hydrogen sulphide required acidic condition and is indicated by blackening of the butt of the medium in the tube.