Mutation and adaptation

As with all organisms microorganisms have mechanisms to preserve their DNA genome so in which their genetic complement is stably   passed from   generation to generation. However in contrast with higher organisms Archaea and Bacteria have no method of replication-based recombination so have to rely on processes of direct genomic change to enable diversity in their populations. Microorganisms face several environmental factors which could possibly cause mutation in their genomic DNA. Permanent changes in the DNA caused through external factors are known as induced mutagenesis. Those changes caused by errors in the DNA replication machinery or other mistakes made through the cellular DNA metabolizing enzymes are known as spontaneous mutagenesis. The  rate  of error in DNA replication is among 10^-7  and  10^-11  per base  pair per round of replication equivalent to around 10^-4  to 10^-8  errors per gene  per generation in a bacterium the  size of E. coli. Spontaneous change can also include those genomic rearrangements that an organism may make to its own genome as exhibited through Streptomyces. These mutation hot spots on the genome are normally associated with an abundance of short inverted repeats. It is by that the repetitive nature of the sequence causes polymerase to stutter and make more errors at these particular points.

A strain isolated from its environment and held in pure culture in the laboratory is defined as the wild type of a particular strain. Any strain derived from the wild type with any change in its genomic make up compared with this wild kind is known as a mutant. This change can be referred to as a change in genotype. If the difference in genotype results in an observable change to the properties of the organism, such as a sudden inability to use a particular carbon source this is said to be a change in phenotype. Changes in dependence on medium components are frequently used in the elucidation of biochemical pathways or in the deduction of regulatory systems so the mutation of the wild type with a nutritional requirement is a common experiment. These strains are known as auxotrophs. Example for if E. coli is subjected to a chemical mutagen and as a result of mutation  now  requires the  vitamin B₁₂ to be present in its medium for the  strain to grow the strain is auxotrophic for B₁₂ or can be called a B₁₂ auxotroph.