GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

19. BACTERIAL GENETICS: HEREDITY, VARIATION, AND TRAIT TRANSMISSION

19.2. MUTATIONS AND THEIR ORIGIN

19.2.3. Mutant Selection

On standard Agar media, only a few Mutations can be detected directly by changes in pigmentation, colony Morphology, or other phenotypic traits. Some traits become apparent upon The addition of indicators or Dyes to the medium. To isolate auxotrophic mutants (for example, those auxotrophic for The amino acid Histidine), Cell growth is compared on two media: one supplemented with histidine and one without it. Mutants will grow exclusively on the medium containing histidine.

As a rule, mutation frequency is low. Therefore, mutant enrichment Methods are employed to detect and select mutants. Obtaining an enrichment culture of mutants resistant to Antibiotics, toxic substances, and phages is relatively straightforward: only mutant strains will grow on a medium containing the corresponding agent. Methods for enriching auxotrophic mutants rely on the inverse principle—cell Suspensions are placed in conditions where mutants do not grow, while prototrophic Cells are eliminated or destroyed. Certain substances affect only actively growing cells without harming non-growing ones. Following the removal of the antimicrobial agent and the addition of necessary growth factors, the auxotrophic cells begin to proliferate. Penicillin is one such agent. The method for enriching auxotrophic mutants using penicillin is known as the "penicillin enrichment" or "penicillin method." Penicillin selectively kills growing cells. The enzyme penicillinase is used to inactivate and remove penicillin. If the cells are penicillin-resistant, Other Antibiotics with a similar mode of action—such as novobiocin, cycloserine, colistin, or kanamycin—can be utilized for enrichment.