Friday, October 21, 2011

Why both slow and fast dividing subpopulations of bacteria are missed during routine culture?

During routine culture, the number of bacteria on solid agar medium that can form distinct colonies, separated from each other are usually around 100. Among 100 colonies, we may not notice slow or fast dividing subpopulations.

It is easy to explain why we miss slow dividing bacteria during routine culture- their number is very small and they divide slowly. By the time these subpopulation of bacteria start to grow and form colonies, the normal bacterial population would have already grown and covered the agar. Hence, to isolate those subpopulations of small colony variats (SCV), the normally dividing population needs to be eliminated, which can be done with the help of aminoglycosides (see the blogpost on Oct.5).

On the other hand, one would expect to notice fast dividing subpopulation more frequently. Even if their initial number is low, one would expect them to gradually dominate the population since their growth rate is higher than the normal. However, this may not happen if this fast dividing subpopulation undergoes senescence. As they divide, the growth rate of mother cell gradually reduces and becomes comparable to the normal population whereas the daughter cell may be a rejuvenated offspring. It may be due to the mother cell undergoing senescence that the fast dividing subpopulation does not dominate the culture. However, the fast dividing subpopulation can be selected by removing the normal population by repeatedly growing the culture in early exponential phase (as described in the previous blog on Sep.19).

Thus, bacterial senescence can explain why the fast dividing, hypervirulent subpopulation of bacteria that can be isolated by repeatedly growing the culture in early exponential phase does not dominate the whole population even if they have the growth advantage over the normal population.

Next- Shifting bacterial population distribution to the right or left

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