Why are colonies develop on a heavily seeded plate smaller than those that appear on a sparsely seeded plate?

Answer 1

Hi:

When you plate bacterial cells at a high density in a liited area of a plate, and as they develop into colonies, there is a competition between the adjacent developing colonies for nutrients. Moreover, the metabolic byproducts that are secreted into the medium by the actively reproducing cells tends to interfere with the growth of the neighboring colonies, resulting in smaller colonies. This is absent in colonies that are far away from each other and hence they tend to grow better and larger.

Note that bacteria tend to keep growing so long as there is a supply of nutrients and till they are inhibited by, say compounds like metabolic byproducts. So if we are able to remove the waste from agar surfacces, theoretically, even E. coli should form huge colonies. This has been achieved in fed-batch fermentors.

Hope this helps.

Answer 2

The size depends on the organism and the statement is not always true. In some cases, and inhibitory effect from the metabolism of other colonies reduce size.

Answer 3

Because the nutrients have to shared by the organisms.