Basic Microbial Genetics

Bacteria typically possess a single circular double-stranded DNA chromosome, located in the nucleoid, containing the core genes essential for normal growth and metabolism.

• Small, circular, extrachromosomal DNA molecules, replicating independently of the chromosome, present in variable copy number per cell.
• Frequently carry nonessential but advantageous genes: antibiotic resistance genes and virulence genes are commonly plasmid-encoded, allowing rapid horizontal spread between bacteria independent of normal vertical (parent to offspring) inheritance.

• A permanent, heritable change in the DNA sequence, arising spontaneously (DNA replication errors) or induced by mutagenic exposure (radiation, certain chemicals).
• The fundamental source of new genetic variation within a bacterial population; a mutation conferring a survival advantage (for example, reduced antibiotic binding at a drug target) will be selected for and expand within the population under the corresponding selective pressure (for example, antibiotic exposure).

Bacteria can acquire new genetic material from other bacteria, not just through vertical inheritance from a parent cell, via three principal mechanisms of horizontal gene transfer.

Transformation:

• Uptake of free, naked DNA from the environment (released by lysed bacteria) directly across the recipient cell's membrane.
• Requires the recipient cell to be in a physiological state called competence, either naturally occurring (e.g. Streptococcus pneumoniae, Neisseria species, Haemophilus influenzae) or artificially induced in a laboratory setting.

Transduction:

• Transfer of bacterial DNA from one bacterium to another via a bacteriophage (a virus that infects bacteria), which accidentally packages bacterial DNA instead of, or alongside, its own genome during viral assembly, then injects this DNA into the next bacterium it infects.

Conjugation:

• Direct transfer of DNA (typically a plasmid) from a donor to a recipient bacterium via a physical connection, the sex pilus (see Bacterial Cell Structure), requiring direct cell-to-cell contact.
• The principal mechanism responsible for the rapid spread of antibiotic resistance plasmids between bacteria, including across different species, and is considered the single most clinically significant gene transfer mechanism for antimicrobial resistance dissemination.

• Virulence genes (toxin genes, adhesin genes, capsule synthesis genes) and antimicrobial resistance genes are both frequently located on mobile genetic elements (plasmids, transposons, integrated bacteriophage genomes), rather than solely on the core chromosome.
• This mobility means both virulence and resistance can spread horizontally between bacteria, including between different species, far more rapidly than would be possible through mutation and vertical inheritance alone.
• This principle underlies the rapid, global emergence and spread of antimicrobial-resistant organisms observed clinically, and is developed in full mechanistic and clinical detail in the dedicated Microbial Genetics and Molecular Microbiology section and the Antimicrobial Agents and Resistance section.