TSI

Here is one of my all time favourite media, triple sugar iron. Used to differentiate between coliforms on the basis of utilization of glucose, lactose and sucrose, gas production and H2S production. The medium is an agar slant dispensed in 10 ml amounts. It is pale red in colour before inoculation.

It contains 3 carbohydrates or sugars, glucose, lactose and sucrose. Glucose is found in the butt (bottom of slant) and lactose & sucrose at the slant. utilization of any sugar results in a change in colour from pale red to yellow, which indicates acid production. An alkaline reaction is indicated by a deeper red colour.

H2S production is seen by a blackening of the medium. Gas production can be detected by any one of 3 ways:

bubbles along the tube

cracks in the medium

the entire medium moves up in the tube.

All coliforms utilize glucose. Actually if orgs had a choice between many sugars they would use glucose first, since it is easier to digest (degrade). Once the glucose has been used up, the org moves to the slant to the lactose and sucrose. If the org is a LF it utilises the lactose and sucrose; if the org is a NLF, it does not.

You can deduce the reaction an org would give on another medium based on its reaction on TSI. Try it yourself.

next time: expected reactions for coliform on TSI

format of reporting results of TSI