MOTILITY
DETERMINATION- SOFT AGAR METHOD
AIM:
To observe the motility of the organism by soft agar method.
PRINCIPLE:
In semi-solid agar media, motile bacteria
‘swarm’ and give a diffuse spreading growth that is easily recognized by the
naked eye. The medium mainly used for this purpose is SIM medium (Sulphide
Indole Motility medium) which is a combination differential medium that tests
three different parameters, Sulfur Reduction, Indole Production and Motility.
This media has a very soft consistency that allows motile bacteria to migrate
readily through them causing cloudiness. The inoculum is stabbed into the
center of a semisolid agar deep. Bacterial motility is evident by a diffuse
zone of growth extending out from the line of inoculation. Some organisms grow
throughout the entire medium, whereas others show small areas or nodules that
grow out from the line of inoculation. The non-motile bacteria will only grow
in the soft agar tube and only the area where they are inoculated.
MATERIALS REQUIRED: SIM
Medium, Bacterial culture, Inoculation needle, Test tubes
PROCEDURE:
- Touch a straight needle to a
colony of a young (18- to 24-hour) culture growing on agar medium.
- Stab once to a depth of
only 1/3 to ½ inch in the
middle of the tube. Be sure to keep the needle in the same line it entered
as it is removed from the medium.
- Incubate at 35°-37°C and
examine daily for up to 7 days.
- Observe for a diffuse zone of
growth flaring out from the line of inoculation.
RESULTS:
- Positive: Diffuse, hazy growths that spread throughout the
medium rendering it slightly opaque.
- Negative: Growth that is confined to the stab-line, with sharply
defined margins and leaving the surrounding medium clearly transparent.
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