Microbiology - what is SIM test?

Answer 1

Its a test where a semisolid agar called Sulfide-Indole-Motility medium (or SIM medium) is inoculated with a bacteria to test for hydrogen Sulfide, Indole, and Motility of the organism. The medium is inoculated by a swab and stab type method (rub some bacteria on the surface of the medium and stab a straight hole through the medium using a straight wire with the bacteria on it). Incubate the bacteria for about 24 hours and then begin testing.... If hydrogen sulfide is present, it will react with the sodium thiosulfate in the medium and the indicator, ferric ammonium citrate, to produce ferrous sulfide which falls out of solution as a blackish precipitate. The presence of hydrogen sulfide typically means that the bacteria produces the enzyme cysteine desulfanase which breaks up the cysteine in the medium into, among other components, hydrogen sulfide. The Indole portion of the test is performed by adding Kovac's reagent to the inoculated medium. The Kovac's reagent reacts with the indole(if indole is present) to produce a pinkish-red or redish-purple ring around the top of the test tube. If indole isn't present, there will be no color change. The presence of indole means that the bacteria produces tryptophanase, an enzyme which breaks down tryptophan into smaller components, one of which being indole. The Motility aspect of the test is done by checking the medium for turbidity, or "fuzziness". If the medium has become fairly turbid throughout the medium, then the bacteria is motile. If the medium is clear and the only turbid appearance is in the stab line, then the bacteria is non-motile. Unfortunately, the motility aspect of this test typically gives false negative results. Sometimes the temperature that the bacteria was incubated at wasn't optimum for the species, sometimes the bacteria only have weak motility, sometimes the bacteria's flagella can get damaged which would impair motility, etc... The point is, this test is good if you want to know whether or not the bacteria you're testing produces tryptophanase or cysteine desulfanase. The motility aspect of the test is suspect to question, at least if the test result was negative for motility(a large amount of turbidity in the medium is a definite sign of motility and is hard to refute though).

Answer 2

SIM test is a test performed to test for sulfur reduction, Indole production and motility.
Black color formed= positive for sulfur reduction
Red color after adding Kovacs reagent= positive for Indole production
Microorgaism growth all over the tube= motile