Question

SIM media is a type of biochemical test that allows you to identify three different characteristics of a bacterium.

What does a positive test look like for each test? What information about the bacterium does a positive test give?

Answer 1

SIM medium is a type of media that can test for the hydrogen sulphide production, indole production and also motility of bacterium. The media contains ferrous ammonium sulfate and sodium thiosulphate, which together work as indicator for hydrogen sulphide producing microbes. When a microbe produces hydrogen sulphide gas in the media, it reacts with the ferrous ammonium sulphate and forms a black precipitate of ferrous sulphide which is a positive test for the H2S gas producing microbes.

When a microbe have tryptophanase enzyme in them they tend to convert tryptophan into indole. In SIM media, the casein peptone has tryptophan in it and when microbe has tryptophanase, tryptophan is converted into indole which is detected as a red band at top of the media by reacting with Kovacs reagent. The motility of the microbe is also tested as the SIM media's agar is semi-solid in nature and lets the microbe to move around the media. This motility is indicated by the organism producing turbidity or a line inside the media when inoculated only on the surface.

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