4. Microbial mutability. Mutations. Mutagenic factors – chemical and physical, mechanism of action, practical use. Genetic transfer – transformation, conjugation, transduction – mechanisms. The importance of bacterial and phage genetics. Genetic engineering. Contemporary genetic methods in clinical microbiology. DNA probes. PCR – polymerase chain reaction.

Bacterial Changeability: Mutations

Types and Causes

• Genetic Variability in bacteria results from spontaneous mutations in individual genes as well as recombination processes.
• A mutation is defined as any change in the base sequence of DNA.
• Mutations are accidental changes in the bacterial DNA.
• Mutations are permanent alterations of the genome.

Types of Mutations:

• Point Mutations: Changes to a single nucleotide or base pair (bp). These are the most common type.
• Exchanges (substitution of a single nucleotide):
• Transition: One purine is replaced by another purine, or one pyrimidine is replaced by another pyrimidine.
• Transversion: A purine is replaced by a pyrimidine, or vice versa.
• Types of Point Mutations (based on effect on amino acid sequence):
• Silent Mutation: A change in the DNA sequence that results in no change in the amino acid encoded. This occurs due to the degeneracy of the genetic code (multiple codons can code for the same amino acid). The new amino acid might also be in a nonactive position or have similar properties. Silent mutations also occur in viruses.
• Missense Mutation: A substitution that results in a different amino acid being coded for.
• Nonsense Mutation: A substitution that results in a stop codon. This leads to a shorter protein that is less likely to function.
• Frameshift Mutations: Caused by the addition or deletion of one or more nucleotides. If the number of added or deleted nucleotides is not divisible by 3, it changes the "reading frame" of the genetic code, resulting in a completely different translation from the original. This often results in the loss or gain of amino acids in the sequence. The earlier in the sequence the deletion or insertion occurs, the more altered the protein will be. Frameshifts can be caused by frameshift mutagens, such as polycyclic, flat molecules that insert between bases.
• Deletions: Loss of a DNA sequence.
• Insertions: Addition of a DNA sequence. Transposons can insert into genes and inactivate them.
• Inversions: Reversal of a DNA sequence.

Mutation Rate:

• The frequency of mutations is expressed as the mutation rate.
• The mutation rate is defined as the probability of mutation per gene per cell division.
• The rate varies depending on the gene, approximately 10⁻⁶ to 10⁻¹⁰.

Causes of Mutations:

• Spontaneous Mutation: Changes that occur randomly. Luria and Delbrück demonstrated that changes in bacterial populations resulted from rare, random mutations in individual cells which were then selected.