Antibiotic Resistance in a Hospital Outbreak
A hospital reports an outbreak of a wound infection caused by Staphylococcus aureus on a surgical ward. Over six weeks, 23 patients are affected. The isolate is resistant to methicillin, and also to three other antibiotic classes. The ward uses a standard hand-hygiene protocol, but an audit shows compliance of only 61% among staff. The hospital's infection-control committee proposes two options: (A) restrict all broad-spectrum antibiotic prescribing on the ward for three months and enforce hand hygiene; (B) continue current prescribing but introduce a new combination antibiotic as first-line treatment. A local newspaper runs a story blaming 'overuse of antibiotics in farming' for the outbreak.
1. Using your understanding of natural selection, explain how the ward's antibiotic prescribing practices could have contributed to the emergence of this resistant strain.
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Bacterial populations contain genetic variation, arising by mutation and, in S. aureus, also by horizontal gene transfer of resistance genes (e.g. on plasmids or transposons). When antibiotics are used, they kill susceptible bacteria but leave any individuals carrying resistance mechanisms alive. These survivors reproduce, and because bacteria reproduce rapidly and asexually, the resistant genotype increases in frequency in the population. Repeated or broad-spectrum antibiotic exposure intensifies this selection pressure, and the ward environment - many patients receiving antibiotics in close proximity - provides ideal conditions for resistant strains to spread between patients via staff hands and equipment.
The newspaper's farming claim is a plausible general contributor to resistance in the wider community, but it is not evidence for this specific outbreak; the audit data (61% compliance) and the ward's prescribing pattern are the more directly relevant factors. Correlation between agricultural use and resistance does not establish causation here.
2. Evaluate options A and B. Which is more likely to reduce resistance in the long term, and what evidence would you need to decide?
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Option A targets the selection pressure directly: reducing broad-spectrum prescribing removes the advantage held by resistant strains, and improving hand hygiene reduces transmission. Together these should reduce the frequency of resistant isolates over time, though the effect may take months and depends on whether resistant strains carry a fitness cost. It also addresses the audit finding of poor compliance, which is a modifiable cause of spread.
Option B risks accelerating the problem. Introducing a new combination antibiotic as first-line treatment applies strong selection for resistance to that combination, and combination therapy, while sometimes used to reduce resistance emergence, is not a substitute for infection control. Continuing current prescribing maintains the selection pressure that produced the outbreak.
Evidence needed: longitudinal resistance surveillance (proportion of methicillin-resistant isolates before and after), antibiotic consumption data, hand-hygiene compliance rates, patient movement and colonisation data, and ideally a controlled comparison (e.g. a ward implementing A versus a comparable ward). Without such data the choice is a judgement based on mechanism, not proof.
3. The committee wants to add a third measure: screening all incoming patients for MRSA colonisation. Discuss one benefit and one limitation of this measure.
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Benefit: screening identifies colonised patients who are asymptomatic carriers, allowing them to be isolated or decolonised before they spread the organism to others. This interrupts transmission at the point of entry to the ward and can reduce the number of new infections, particularly where compliance with hand hygiene is imperfect.
Limitation: screening detects colonisation at the time of swabbing only. A patient may acquire MRSA after admission (hospital-acquired colonisation), and a negative swab does not guarantee the patient is free of the organism, especially if sampling sites are limited. Screening also has costs - laboratory time, delayed bed allocation, and potential over-isolation of patients - and its effectiveness depends on the sensitivity of the test and on staff acting on results. It reduces transmission but does not remove the selection pressure from antibiotic use, so it complements rather than replaces option A.