The following points highlight the eight important types of microbial mutants. The types are: 1. Auxotrophic Mutants 2. Resistant Mutants 3. Cryptic Mutants 4. Conditionally Expressed Mutants 5. Antigenic Mutants 6. Metabolic Mutants 7. Regulatory Mutants 8. Temperature-Sensitive Mutants.
Microbial Mutant: Type # 1. Auxotrophic Mutants:
Auxotrophic mutants are those that are nutritionally deficient, i.e., they are unable to synthesize essential metabolites or growth factors (amino acid, purine, pyrimidine, vitamins, etc.) that are synthesized de novo by the parental or wild-type strains (prototroph) of the species.
An auxotrophic mutant will not grow when the essential metabolite (growth factor) is absent in culture medium; it grows and reproduces only when the required metabolite is supplied in the medium.
Microbial Mutant: Type # 2. Resistant Mutants:
Resistant mutants exhibit an increased tolerance to inhibitory agents, particularly antibiotics and phages. Microbes may develop such mutants spontaneously through a range of mechanisms, e.g., acquisition of enzymes able to metabolize antibiotics, and loss of cell surface components which act as phage-receptors.
Microbial Mutant: Type # 3. Cryptic Mutants:
Cryptic mutants are those that have lost a specific function but retain the intracellular activities to catalyze the reactions of the function, for example, loss of a permease.
Microbial Mutant: Type # 4. Conditionally Expressed Mutants:
These mutants remain as wild-type phenotype under one set of conditions but may express as mutant phenotype when these conditions alter.
Microbial Mutant: Type # 5. Antigenic Mutants:
Antigenic mutants show a change in the surface structure and composition of the microbial cell. Some antigenic mutants exhibit altered colonial shape due to effect on cell wall synthesis, and others lose the capacity to make specific cell components, e.g., capsular polysaccharides.
Microbial Mutant: Type # 6. Metabolic Mutants:
These mutants exhibit altered metabolic ability particularly the fermentation- ability of decreased or increased capacity to produce some end-product.
Microbial Mutant: Type # 7. Regulatory Mutants:
Regulatory mutants are those in which mutation affects either the regulatory region of the promoter gene or the activity of a regulatory protein. Mutants of Bacillus subtilis are grossly deficient in the enzymes needed to separate daughter cell after division and they grow at normal rates as very long chains or un-separated cells.
Microbial Mutant: Type # 8. Temperature-Sensitive Mutants:
Temperature-sensitive mutants grow at one temperature but not at another. Most of them are heat-sensitive, some, however, are cold-sensitive. The temperature sensitivity usually results from increasing heat or cold liability of the mutant gene-product.
For example, a temperature- sensitive mutant of E. coli fails to provide stability to enzymes involved in lactose utilization and does not grow on lactose at that temperature, while it retains the ability to grow on lactose at low temperature.