Question

Motility is one of the things that SIM medium tests for, so what is the purpose of running a separate test in motility agar?

Answer 1

SIM test is used to test ability for detection of hydrogen sulphide (H2S), indole production and motility. H2S production is detected by black precipitate formation. Indole production from tryptophan is detected by red ring with Kovacs reagent. Motility is detected by diffused cloudy growth in the medium starting from the stab of inoculation. However, as it is a combination of three tests, it is difficult to accurately understand the pattern of motility.

The motility agar is modified to contain tetrazolium salt (TTC). TTC is reduced by bacterial to formazan, which is a red colored insoluble precipitate. If the organism is motile, it will move away from the stab site and hence, entire medium will become pink/red. However, if the bacterium is non-motile, it will grow at the inoculation stab. Hence, red coloration will be seen only at inoculation point. Weakly motile bacteria will show slightly diffused growth from the site of inoculation. thus, red color is seen only till a short distance away from the inoculation stab point. Thus, it is better to visualize the motility pattern using motility agar, which is not possible with SIM test. This method is also able to detect motility of bacteria based on oxygen requirements as well. Aerobic motile bacteria will show motility at the top of the agar. In SIM test, the red coloration or black prpecipitate could obscure such identification.

Thus, motility agar will detect patterns of motility better than the SIM test, as it is a colorimetric detection method. Further, there is no interference from H2S production or red coloration of indole test.