Aerobic microbes grow in the environment rich in oxygen. For this, a large surface of the medium can create a condition of increased exposure to aeration. For this purpose, microbes in their respective broth media are kept on a rotary shaker. Most of the microbes are aerobic and grow in the presence of oxygen.
There are some other microbes like Clostridium tetani, which can grow in the complete absence of oxygen and hence, are called obligate anaerobes.
There are other rmicrobes that can adapt themselves to either condition at least partially, if not wholly (facultative). An oxygen free environment can be created by two methods: (l) mechanical expulsion of free oxygen and (2) use of some reducing agents.
Most of the operations involve a combination of both the procedures. However, the roll-tube method is a simple method practised in the laboratory for characterizing microbes in respect of their response to oxygen.
Roll tube method
Air should be expelled first from the medium by it boiling for 10 min. Then, the molten and cool medium (45°C) is inoculated with micro organisms.
The, inoculum is distributed throughout the medium by rolling the tube between the palms. Obligate anaerobes will grow at the bottom and will not appear anywhere near surface or Roll-tube method of characterizing microbes in respect of their response to oxygen, (a) uniformly distributed microbes; (b) obligate anaerobe; (c) facultative anaerobe and (d) aerobic microbe.
Below the surface Organisms having some tolerance to oxygen (facultative anaerobes) will grow below the surface the aerobic organisms will grow on the surface of the nutrient medium
Under proper culture conditions, cultures exhibit their characteristic features. These features give useful information for their identification. However, these are phenotypic only and subject to variations depending on the set of cultural environment such as nutritional status of the media, crowdedness, physical and chemical environment, etc.
Various descriptive terms are used to describe the culture in agar plate, agar slant and nutrient broth.
Agar plate colonies
i) Size- Diameter of colonies may vary from a pin-point to a few mm. Even after prolonged incubation, size of colonies remains unchanged.
ii) Margin or edge – Circular, filamentous, irregular or with certain projections.
iii) Elevation – Raised, convex or leaf like.
iv) Pigmentation – Coloured or colorless.
v) Optical feature – Transparent, opaque or translucent.
vi) Texture – Soft, slimy or dry and brittle.
Culture in agar slants
i) Culture density – Moderate, scanty or abundant.
ii) Margin – uniform or irregular
iii) Textrue – Soft, slimy or dry and brittle.
iv) Pigmentation – Coloured or colourless.
Culture in Nutrient broth or liquid medium
i) Culture density – Scanty, moderate or abundant.
ii) Distribution – Uniformly distributed throughout the medium (turbid); on the surface as ring, scum or film or with granular sedimentation.
iii) Odor – May not be detected, may be purified, fruity or aromatic.