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Name: Hussam
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: October 2002

When some bacteria are grown in lab conditions, they fail to develop flagella. Why is that the case?

Dear Hussam:

I do not know the exact bacteria that you're describing, but I would think that they might not produce flagella because their nutrient supply is readily available in culture. In nature, the primary function of the flagella is to drive the bacteria toward food sources. This ability would not be necessary under typical culture conditions, so the bacteria might be able to conserve energy by not producing flagella.

Thanks for the good question,

Jeff Buzby, Ph.D.
Children's Hospital of Orange County

When bacteria have the ability to switch off the production of outer structures such as flagella or fimbriae (pili) this is called 'phase variation'. The mechanisms behind this variation are various, but usually they are caused by mutations, often a single nucleotide change, or small deletion/insertion, or a reversible fragment of DNA (a flip-flop) during replication. Most of these mutations are reversible, for instance, a reversible fragment can reverse back to the 'expression on' direction. A single nt mutation can reverse back, which is frequently the case in nucleotide stretches like GGGGGGG (the number of nucleotides can change at high frequency in such stretches).

Such mutation hot spots are frequently found in the genes encoding the structural components of flagella or fimbriae, or in their regulatory genes. The result is an on-off switch at frequency higher than normal mutation rates (say, 1 in every 100 or 1000 generations, rather than 1 in a million).

This can be observed in the lab, since the mutation results in a visible colony morphylogy change. It is proposed that the same thing happens in nature, and that bacteria lacking such structures are at a disadvantage when colonizing a host. They may be at an advantage when surviving outside the body, though, and since the process is reversible one can see fluctuations in a bacterial populations with or without expression of such structures.

In addition, certain bacteria have outer structures whose expression is tightly regulated by external factors such as temperature. The fimbriae of E. coli are such an example, and by growth at certain temperatures they can either produce fimbriae or not.

Trudy Wassenaar

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