Genetic Variation and Gene Expression - Microbial Genetics

GENETIC VARIATION AND GENE EXPRESSION

Microorganisms may adapt rapidly to new environments and devise strategies to avoid or negate stressful or potentially harmful circumstances. Their ability to survive adverse conditions may result from the organism using genes it already possesses, or by the acquisition of new genetic information. The term ‘genotype’ describes the genetic composition of an organism, i.e. it refers to the genes that the organism possesses, regardless of whether they are expressed or not. It is not uncommon for a microbial cell to possess a particular gene but not to express it, i.e. not to manufacture the protein or enzyme that is the product of that gene, unless or until the product is actually required; this is simply a mechanism to avoid wasting energy. For example, many bacteria possess the genes that code for βlactamases; these enzymes hydrolyse and inactivate βlactam antibiotics (e.g. penicillins). In many organisms βlactamases are only produced in response to the presence of the antibiotic. This form of nongenetic adaptation is termed phenotypic adaptation, and there are many situations in which bacteria adopt a phenotypic change to counter environmental stress. But microorganisms may also use an alternative strategy of genetic adaptation, by which they acquire new genes either by mutation or by conjugation ; subsequently, a process of selection ensures that the mutant organisms that are better suited to the new environment become numerically dominant.

In bacteria, mutation is an important mechanism by which resistance to antibiotics and other antimicrobial chemicals is achieved, although the receipt of entirely new genes directly from other bacteria is also clinically very important. Spontaneous mutation rates (rates not influenced by mutagenic chemicals or ionizing radiation) vary substantially depending on the gene and the organism in question, but rates of 10⁻⁵–10⁻⁷ are typical. These values mean that, on average, a mutant arises once in every 100 000 to every 10 million cell divisions. Although these figures might suggest that mutation is a relatively rare event, the speed with which microorganisms can multiply means, for example, that mutants exhibiting increased antibiotic resistance can arise quite quickly during the course of therapy.