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Microbial genetics: general principles

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Last revised: Wednesday, March 29, 2000
Ch. 13 in Prescott et al, Microbiology, 4th Ed.
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 2000. Thomas M. Terry

Unique features of bacterial genetics


Mutation and mutation rate

Wild-type and mutant

Phenotype and genotype

  1. Phenotype = what is observed. Ex: red pigmented colony, resistant to antibiotic, requirement of leucine for growth
  2. Genotype = symbolic representation of gene responsible for phenotype. Ex: leu-, strR
    • Typically use 3-letter codes for genotype.

Naming bacterial strains

Mechanisms of Mutation

Nucleotide substitution (base change)

  1. "spontaneous error": can arise from diverse sources; don't know exact cause of any specific mutation. Errors in base pairing occur, even after proofreading, with frequency of 1 in 107 to 1 in 108.
  2. base analogs: compounds like 5-bromouracil look like Thymine, get incorporated into DNA during replication. However, when serving as templates, they don't always form the "correct" match of A, instead sometimes pair with C.
  3. nitrous acid: causes deamination; resulting bases can cause incorrect base pair matches
  4. UV radiation: causes fusion of adjacent thymine residues in same strand ---> thymine dimers. Can be repaired, but if not will cause inconsistent base insertions during replication.

Frameshifts (by insertion or deletion of a base pair)

Deletion mutations

Insertion mutagenesis

Types of Mutants and selection strategies

Colony morphology

Resistant mutants


Naive selection procedure for auxotrophs

Penicillin selection procedure for auxotrophs

  1. mutagenize culture to increase mutation frequency
  2. grow cells in minimal medium supplemented with 19 amino acids (but not serine), vitamins, other biosynthetic needs for which auxotrophs other than serine have needs. Result: all cells, both wt and other auxotrophs, will grow. But serine auxotrophs can't grow.
  3. when cells are happily in exponential growth, throw in penicillin (or other cell wall antibiotic). Will lead to wall damage in all growing cells, but not in serine auxotrophs.

  4. after lysing cells, plate survivors on plates containing serine, use two plate screening technique described above. Should enrich for serine auxotrophs considerably.
    • Note: same type of procedure can be used to select any auxotroph. In practice, try to find a lot of independently isolated mutants, then study and compare them (some will be in same gene).

Conditional lethal mutants

Ames test

An application of power of bacterial genetics to help screen for substances that might cause cancer.


Difficulties with carcinogen testing

Design of the Ames test

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