Chapter 7: Bacterial Genetics & Gene Transfer

Bacterial genetics is a foundational field of microbiology that explores how genetic information is maintained, altered, and shared among microorganisms, often using Escherichia coli as a primary research model. The bacterial genome typically consists of a singular, circular, double-stranded DNA chromosome that carries essential genetic information, alongside auxiliary small DNA circles called plasmids that often contain nonessential but beneficial genes, such as those for antibiotic resistance or toxin production. A critical aspect of microbial survival and clinical impact is horizontal gene transfer, which allows the movement of genetic material—including virulence factors found on pathogenicity islands—between bacteria without direct descent. This transfer occurs through three primary mechanisms: conjugation, requiring cell-to-cell contact via a sex pilus; transduction, which involves viral vectors known as bacteriophages; and transformation, the uptake of naked DNA from the environment by competent cells. Bacteriophages play a dual role, as they can either cause immediate cell death through the lytic cycle or integrate into the host genome as a dormant prophage in the lysogenic cycle, sometimes conferring new properties to the bacteria via lysogenic conversion. Genetic variability is further introduced through spontaneous mutations in the DNA sequence and the activity of mobile genetic elements called transposons. These "jumping genes" are instrumental in the spread of antibiotic resistance, which bacteria achieve through mechanisms such as enzymatic destruction of the drug, active efflux of toxic substances, or modification of the antibiotic's target sites. To manage their metabolic efficiency, bacteria have evolved complex gene regulation systems, such as the lac operon, which utilizes both negative control through repressor proteins and positive control through catabolite activator proteins to ensure enzymes are only produced when specific nutrients like lactose are available. Additionally, bacteria can adapt to extreme environmental stress by modifying their RNA polymerase specificity through alternative sigma subunits, allowing for the rapid activation of survival-related gene sets.