Science Buddies: "Ask an Expert"

the TSI test is to distinguish Enterobacteriaceae from other gram -ve intestinal bacilli, and to differentiate among different groups of Enterobacteriaceae. Does this work on hte basis of carb fermentation?

I also read up that : TSI agar detect reduction of sodium thiosulphate to hydrogen sulphide turn agar black.

Results given in my notes are
acid slant - acid butt - No H2S,
acid slant - acid butt - H2S produced,
alkaline slant - acid butt - No H2S,
alkaline slant - acid butt - H2S produced,
alkaline slant - alkaline/no change butt.

How come there's no acid slant - alkaline butt?

And which results show that the bacteria is Enterobacteriaceae or not? If Enterobacteriaceae, which species?

I'm also confused, Enterobacteriaceae is intestinal bacilli, right? And they are gram -ve, right? Then how to differentiate with other bacilli (as in first para)?

Thanks alot!!

Hi BobbyCx,

The TSI test stands for Triple Sugar Iron - so yes, it uses the basis of Carb fermentation (of dextrose, lactose and sucrose) to distinguish between strains tested.
Whether the medium ends up acidic or alkaline depends on if the bacteria growing on it are metabolising the acidic sugars and creating by-products of protein degradation that are alkaline.
I found this page that describes the test really well and it also explains (a little) how you can use it to differentiate between the different enterobacteria
http://biology.fullerton.edu/biol302/30 ... ochem.html

I think (I am not a microbe expert, I am just inetrpreting how the test is described) that the reason there is no acid slant and alkaline butt is because that if the bacteria are able to ferment sugars then they can do so throughout the whole tube. The protein degradation that causes the alkaline environment to return after all the sugars are used up and medium had turned acid, is only possible in the presence of air i.e in the slant. So you can get the acid butt and alkaline slant but the air is only present around the slant so only that can support anaerobic growth once fermentation stops. And if fermentation is possible it occurs in both the slant and the butt.

hope that makes sense,
caroline