Bacterial genetics encompasses the study of genes, genomes, and inheritance in bacteria. Bacteria possess remarkable genetic plasticity, allowing them to adapt rapidly to environmental changes through mutation and horizontal gene transfer mechanisms.
Bacterial Genome Organization
The bacterial chromosome is a single, circular double-stranded DNA molecule typically 0.5–10 Mb in size, organized in a nucleoid region without a nuclear membrane — E. coli has a ~4.6 Mb genome encoding ~4,300 genes. Plasmids are small, circular, extrachromosomal DNA molecules that replicate independently, carrying accessory genes for antibiotic resistance, virulence factors, and metabolic capabilities. Transposons are mobile genetic elements that can move within or between DNA molecules, often carrying antibiotic resistance genes.
Mutation in Bacteria
Point mutations are single base substitutions (transition or transversion) that may alter an amino acid (missense), create a stop codon (nonsense), or have no effect (silent). Frameshift mutations are insertions or deletions of bases that shift the reading frame, usually producing a nonfunctional protein. Mutation rates in bacteria are approximately 10⁻⁶ to 10⁻⁹ per base pair per generation but can increase under stress via the SOS response. The Ames test uses Salmonella typhimurium mutants to detect chemical mutagens.
Horizontal Gene Transfer
Transformation involves uptake of free DNA from the environment by competent bacterial cells; natural competence occurs in Bacillus subtilis, Streptococcus pneumoniae, and Neisseria gonorrhoeae. Conjugation is direct cell-to-cell transfer of DNA via a conjugative pilus, encoded by the F (fertility) plasmid in E. coli, and can transfer plasmids and chromosomal genes between different species. Transduction is the transfer of bacterial DNA by bacteriophages — generalized transduction transfers any chromosomal fragment, while specialized transduction transfers specific genes adjacent to the phage integration site.
Gene Regulation
The lac operon in E. coli is the classic model of inducible gene regulation, controlled by the lac repressor and catabolite activator protein (CAP). The trp operon is a repressible system regulated by tryptophan levels through attenuation and the trp repressor. Quorum sensing allows bacteria to regulate gene expression in response to population density using autoinducer molecules such as N-acyl homoserine lactones.
Applications
Genetic engineering uses bacterial plasmids and restriction enzymes to clone genes and produce recombinant proteins such as insulin, growth hormone, and vaccines. Antibiotic resistance gene tracking is applied in clinical and environmental settings. CRISPR-Cas9, derived from bacterial adaptive immunity, has become a revolutionary tool for genome editing.
