Any change in the genotype of a bacterium or its phenotype is known as variation. Genotypic variation can occur as a result of changes in the genes by way of mutation, loss or acquisition of new genetic elements. These variations are heritable. Phenotypic variations are seen temporarily when bacteria are grown under certain environmental conditions. These variations are not heritable.
Mutation: A gene will mutate spontaneously, about once in a hundred million cell divisions. Such bacteria are called mutants. Most of these mutants die, but a when a mutant can adapt itself to the environment more readily; it may emerge as a new variant. Chromosomal mutations may lead to Emergence of drug resistance in bacteria. Examples include methicillin resistance in Staphylococcus aureus, Multi-drug resistance in Mycobacterium tuberculosis.
Transformation: Some bacteria have ability to uptake naked DNA fragment from the surrounding environment. When such a DNA confers new property to the bacterium, it is termed transformation. Change from R form of
Streptococcus pneumoniae to S form as demonstrated by Griffith is due to transformation.
Conjugation: Transfer of genetic material (usually plasmids) from one bacterium to another through the mediation of sex pili is known as conjugation. Any property that is coded on a transmissible plasmid can be transferred to a recipient bacterium. Properties such drug resistance mediated by beta-lactamases, bacteriocin production etc can be transferred by conjugation.
Transduction: Transfer of genetic material through mediation of bacteriophage is known as transduction. Only those strains of Corynebacterium diphtheriae that are infected by a beta phage are
toxigenic. Change in O antigen in Salmonella (S. anatum->S. newington-> S.minneapolis) is because of lysogenic phage.
Transposition: Variations in the flagellar antigens in Salmonella are due to transposons. Similar gene rearrangements may result in antigenic variations, as in Neisseria gonorrhoeae and Borrelia recurrentis.
A variation in the phenotype of a microorganism, where the genetic constitution remains unchanged is a non-heritable variation. Such variations are seen due to a change in environmental conditions and such variations are neither permanent nor heritable. They may revert back to normal state when the conditions are restored.
Some examples are:
- Loss of flagella in S.typhi when grown in phenol agar (H-O variation)
- Pleomorphism (variation in shape) in old cultures
- Lack of pigment production by S.aureus in anaerobic conditions
- Formation of spheroplasts and protoplasts
- V-W variation in Salmonella typhi that is characterized by loss of Vi antigen
- S-R variation in Salmonella typhi that is characterized by loss of O antigen and change in colony morphology to rough type.
- Production of flagella in Listeria monocytogenes occurs at temperature less than 20oC
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