Chapter 8: Genetic Analysis and Mapping in Bacteria & Bacteriophages

Bacterial genomes are typically organized as single circular chromosomes and exist in haploid form, which makes them ideal for genetic studies. Combined with rapid reproduction rates and the ability to culture large populations easily, bacteria and their viral parasites have enabled researchers to develop sophisticated mapping techniques unavailable in eukaryotic systems. The chapter focuses on three major mechanisms of horizontal gene transfer in prokaryotes. Conjugation involves direct cell-to-cell contact through a sex pilus, mediated by the fertility factor, enabling genetic material to pass from donor to recipient cells. High-frequency recombination strains, formed when the fertility factor integrates into the bacterial chromosome, allow interrupted mating experiments that establish the linear order and relative distances between genes based on transfer timing. The discovery of F-prime factors, which carry chromosomal genes on plasmids, created merozygotes that are partially diploid and essential for studying gene regulation and dominance relationships. Transformation occurs when competent bacteria take up naked extracellular DNA and incorporate it into their genomes; the frequency of cotransformation indicates genetic linkage and enables chromosomal mapping. The chapter then explores bacteriophages, viruses that infect bacteria, distinguishing between lytic cycles that destroy host cells and lysogenic cycles in which phage DNA integrates as a prophage. Plasmids are discussed as autonomous genetic elements, including resistance plasmids that confer antibiotic immunity and bacteriocin plasmids involved in bacterial competition. Transduction, the phage-mediated transfer of bacterial DNA, is presented as a third mapping tool based on cotransduction frequencies. The chapter integrates ethical considerations throughout, addressing multidrug-resistant bacteria and the potential revival of bacteriophage therapy as an alternative to conventional antibiotics.