Gene Amplification

Gene amplification is an increase in the number of copies of a gene or genomic region within a cell. The resulting higher gene dosage usually increases production of the corresponding RNA or protein, which can allow a pathogenic microorganism to survive an otherwise inhibitory antimicrobial concentration. It is a biological copy-number change, not laboratory amplification by PCR.

How increased gene dosage produces resistance

Amplification raises the quantity of the encoded product, strengthening a pre-existing resistance mechanism rather than necessarily creating a new biochemical function.

• Extra copies of a drug-target gene can cause target overproduction, so a given antimicrobial dose cannot inhibit enough target molecules.
• Amplification of genes encoding drug-inactivating enzymes can increase antimicrobial degradation or chemical modification.
• More copies of transporter genes can increase efflux pump production, lowering the intracellular antimicrobial concentration.
• Amplification of pathway genes can strengthen a metabolic bypass or compensate for the antimicrobial-blocked step.

Origin and selection

Copy-number increases may arise through replication errors, recombination or changes in plasmid copy number. During antimicrobial exposure, natural selection favours cells carrying an advantageous amplification, allowing them to multiply within the pathogen population.

• Amplified chromosomal regions commonly occur as tandem repeats, while resistance genes may also be multiplied on plasmids.
• Amplifications may contract when drug pressure is removed, especially if maintaining extra DNA and protein production imposes a fitness cost.

Evolutionary and clinical significance

Amplification can produce low-level or intermediate resistance sufficient for survival under treatment. It may also create heteroresistance, in which only a subpopulation carries enough copies to withstand the drug.

• Continued survival gives the pathogen time to acquire more stable resistance mutations, making amplification an evolutionary stepping-stone.
• Because copy number can change rapidly, susceptibility may vary with antimicrobial pressure even without a new target-site mutation.