Describe the difference between the appearance of surface and subsurface colonies in a pour plate. if this is the same bacterial species, why do these differences in colonial growth occur

In a pour plate technique the diluted bacterial sample is directly mixed with the broth culture medium and incubated. For the same bacterial species different kinds of bacterial colonies can be observed both on the surface and subsurface of the medium. The colonies growing on the surface are usually smooth and circular in shape while those growing in the medium are slightly spindle shaped. This may be due to the difference in the availability of oxygen. In the presence of oxygen, the bacteria can grow comfortably on the surface of the medium while bacterial cells might compete with each for oxygen in the sub surface medium. 

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