Antimicrobial Resistance and Selection Pressure

Antimicrobial selection pressure is the environmental filtering created when a drug suppresses susceptible microorganisms but allows resistant variants to survive or reproduce. It drives antimicrobial resistance by changing the genetic composition of a pathogen population, rather than by purposefully creating resistance in individual organisms. The principle applies to antimicrobials such as antibiotics and antifungals.

Heritable variation on which selection acts

Pathogen populations contain heritable differences arising through random genetic change or the acquisition of resistance determinants. Antimicrobial exposure then operates through natural selection on this variation.

• A random chromosomal mutation may alter the drug target, drug uptake or another relevant cellular function.
• Bacteria may obtain resistance genes through horizontal gene transfer, often involving plasmids or other mobile genetic elements.
• Antimicrobials do not direct pathogens to develop needed mutations; they preferentially remove susceptible competitors.

From individual survival to population resistance

At a given drug concentration, resistant organisms have a relative fitness advantage because susceptible organisms are killed or their growth is inhibited. Survivors reproduce and pass resistance to their descendants, increasing its frequency until resistant lineages may dominate the population.

• Repeated or unnecessary antimicrobial exposure repeatedly favours resistant lineages.
• In bacteria, resistance genes can spread through conjugation, transformation or transduction, including across some species boundaries.
• Drug gradients within patients or environments may permit partially resistant variants to survive and undergo further selection.
• Antimicrobial use can also select resistant commensal bacteria, which may serve as reservoirs of resistance genes.

Reducing selection and spread

The objective is not to eliminate necessary treatment but to reduce avoidable exposure and pathogen transmission through antimicrobial stewardship and public-health measures.

• Antimicrobials should be used only when indicated, with an appropriate drug, dose and duration guided by diagnosis wherever feasible.
• Vaccination, sanitation and infection prevention reduce infections and therefore antimicrobial demand.
• Surveillance and a One Health approach address selection and transmission across humans, animals and the environment.