17.2.4—Antibiotic resistance and selection

Antibiotic resistance is a natural-selection outcome in a bacterial population. Bacterial cells may already differ genetically, and some may carry an allele that makes them less affected by an antibiotic.

• Variation first: resistance alleles arise through random mutation; antibiotic use does not create the mutation.
• Selection pressure: when the antibiotic is present, susceptible bacteria are more likely to be inhibited or killed, while resistant bacteria are more likely to survive.
• Differential reproduction: surviving resistant bacteria reproduce and pass the resistance allele to offspring, so its frequency increases over generations.
• Population outcome: repeated or inappropriate antibiotic use can make resistant strains more common, reducing the effectiveness of treatment.
• Spread: resistance alleles can be passed vertically during reproduction and, where supported by the source, can also spread between bacteria through plasmid-mediated horizontal transfer.

Use antibiotics only when appropriate and follow the prescribed course and guidance. This reduces unnecessary selection for resistance, but it does not reverse an established allele-frequency change immediately. Infection control, surveillance and development of new treatments can also matter; feasibility depends on biological, social and economic conditions.

Antibiotics select resistant bacteria; they do not deliberately make individual bacteria resistant. Resistance is a population change across generations, not an adaptation chosen by a bacterium. This card explains the selection mechanism and responsible-use boundary, not Hardy–Weinberg calculations, a full treatment protocol or a universal claim about every infection.