TSI contd....

I forgot to mention yesterday that the sugars in TSI are in different quantities, for a reason. Glucose is incorporated at 0.1% while lactose and sucrose are present in 1%. This is so that the orgs are forced to utilise lactose and/or sucrose after glucose is used up.

There is a systematic way of reporting the results. Remember that we are looking at and reading 4 different results, viz, acid or alkaline production in both the butt and slant, gas production and H2S production. We use the abbreviations A for acid and K for alkaline. Acid production is shown by a change in colour from red to yellow, while a change in colour to a deeper red is indicative of alkaline reaction.

All coliforms will give a positive result for glucose utilization, therefore an acid butt.

You must report acid or alkaline production in slant or butt, gas production and H2S production. An exemplar is A/A/no gas/no H2S.

Remember to give me some kind of indication of the order in which you are reporting.

see you!

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