Antibiotic resistance: evolution and control

Inheritance, variation and evolution • The development of understanding of genetics and evolution

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What feature of bacteria allows rapid evolution?

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Short generation times and large population sizes allow rapid accumulation and spread of genetic changes, producing observable evolutionary change in short periods.

Key concepts

What you'll likely be quizzed about

Definition of antibiotic resistance

Antibiotic resistance describes the ability of bacteria to survive exposure to an antibiotic that would normally kill them or stop their growth. Resistance arises when genetic changes alter bacterial structures or functions targeted by antibiotics, or when bacteria gain mechanisms that inactivate or expel the drug. Limiting factor: resistance does not mean a bacterium is invincible to all treatments; resistance is specific to particular antibiotics or classes.

Genetic mechanisms that produce resistance

Random mutations in bacterial DNA can change the target site of an antibiotic, reduce drug uptake, or increase efflux, producing resistance. Horizontal gene transfer moves resistance genes between bacteria via plasmids, transposons or bacteriophages, enabling rapid spread across strains and species. Plasmids act as mobile genetic elements that carry resistance genes and transfer them by conjugation, increasing genetic variation in bacterial populations .

Natural selection and rapid evolution

Antibiotic treatment imposes strong selection pressure on bacterial populations. Sensitive bacteria die under treatment, while individuals carrying resistance genes survive and reproduce. Successive rounds of selection increase the proportion of resistant bacteria in a few generations, producing rapid evolutionary change. Short bacterial generation times and large population sizes accelerate this process, allowing observable evolution within months or years in clinical and laboratory settings .

MRSA as an example and transmission risks

Methicillin-resistant Staphylococcus aureus (MRSA) carries genes that prevent killing by multiple antibiotics related to penicillin and methicillin. MRSA spreads in hospital environments where patients have wounds or weakened immune systems; these conditions remove normal barriers to infection and increase the chance of colonisation and disease. Cause → effect: high antibiotic use and frequent patient contact in hospitals increase selection and transmission of MRSA, producing outbreaks that are hard to treat and that raise morbidity and mortality .

Measures to slow development and spread

Antibiotic stewardship reduces unnecessary prescribing and restricts use to appropriate cases, lowering selection pressure. Completing prescribed antibiotic courses reduces the chance that partially resistant bacteria survive and spread. Infection-control measures such as hand hygiene, antiseptic alcohol treatments, isolation of infected patients and environmental cleaning reduce transmission opportunities. Limiting non-therapeutic antibiotic use in agriculture reduces environmental reservoirs of resistance genes. Development of new antibiotics remains slow and costly, so prevention of resistance is a primary control strategy .

Key notes

Important points to keep in mind

Resistance arises from genetic changes: mutations and horizontal gene transfer are key mechanisms .

Antibiotic treatment creates selection pressure that favours resistant bacteria, allowing rapid evolution in populations with short generation times .

MRSA demonstrates how clinical settings concentrate selection and transmission, producing difficult-to-treat outbreaks .

Antibiotic stewardship reduces unnecessary use and slows resistance by lowering selection pressure.

Completing prescribed antibiotic courses reduces survival of partially resistant bacteria and limits spread .

Infection-control measures (hand hygiene, antiseptics, cleaning, isolation) reduce transmission opportunities and outbreak size .

Limiting non-therapeutic antibiotic use in agriculture reduces environmental reservoirs of resistance genes .

Development of new antibiotics is slow and expensive, so prevention of resistance is essential .

Rapid detection and targeted treatment shorten exposure to broad-spectrum antibiotics and help protect microbial communities.

Public health surveillance tracks resistance trends and informs policy to reduce emergence and spread.