Antibiotic resistance genes are commonly transferred between bacteria via:
Vertical inheritance through binary fission faithfully copies the single circular chromosomal DNA with high fidelity via DNA polymerase III holoenzyme but fails to explain extraordinarily rapid global dissemination of antibiotic resistance observed within decades of antibiotic introduction. Horizontal gene transfer provides direct mechanism for movement of accessory genes between genetically unrelated cells, even across genus barriers. Conjugation involves direct cell-to-cell contact via retractile sex pilus encoded by tra operon and transfer of conjugative plasmids and integrative conjugative elements carrying resistance cassettes through type IV secretion system. Transformation allows uptake of naked extracellular DNA from environment via natural competence machinery ComEA and ComEC in species like Streptococcus pneumoniae, Bacillus subtilis and Neisseria gonorrhoeae. Generalized and specialized transduction uses temperate bacteriophages that mistakenly package host DNA including resistance genes during lytic cycle and inject into recipient, as documented for transfer of mecA. Together these mechanisms allow mobilization of beta-lactamases blaCTX-M, carbapenemases blaKPC and blaNDM, erm methylases, tet and van operons across species and diverse habitats within hours, far outpacing mutation. Passive diffusion of small molecules and flagellar swimming motility do not transfer genetic information, distinguishing HGT as the primary driver of resistance pan-genome expansion and public health crisis.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Horizontal Gene Transfer and Resistance Spread.