Antibiotic Resistance in a Hospital Outbreak
A hospital ICU reports 14 cases of bloodstream infection with Klebsiella pneumoniae over six weeks. All isolates are resistant to carbapenems. Genomic sequencing shows 12 of 14 isolates are near-identical, sharing a plasmid carrying a carbapenemase gene; the other two are genetically distinct but carry the same plasmid. Patients 1–3 were admitted from a long-term care facility; the rest were ICU patients who had been in adjacent beds. Hand hygiene audits show 68% compliance. Carbapenems were used in 80% of ICU patients in the preceding month. A clinician says, 'Resistance arose because each patient's bacteria mutated under antibiotic pressure, so this is 14 independent events.'
1. Evaluate the clinician's explanation using the genomic and epidemiological evidence.
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Evaluating the claim. The clinician is wrong: the data show one outbreak, not 14 independent mutation events.
Evidence.
- 12 of 14 isolates are near-identical genomically - consistent with clonal spread of a single strain from person to person, not independent mutation.
- The same plasmid appears in the two genetically distinct isolates. A plasmid is transferable by horizontal gene transfer (conjugation), so resistance can spread between unrelated bacteria without any new mutation. This is the strongest evidence against the mutation-only explanation.
- Epidemiology supports transmission: patients 1–3 came from a long-term care facility (a common reservoir), and the rest were in adjacent ICU beds - a spatial cluster consistent with contact transmission via hands or equipment.
- Hand hygiene at 68% provides a plausible transmission route.
Role of antibiotics. Carbapenem use in 80% of patients created selection pressure that favoured resistant strains and plasmid-carrying bacteria; it amplified the outbreak but did not generate 14 separate mutations.
2. Propose two interventions and explain the mechanism by which each would reduce resistance spread, distinguishing selection from transmission.
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Intervention 1 - Improve hand hygiene and cohort/isolate patients (transmission control). Mechanism: resistance genes, whether chromosomal or plasmid-borne, spread mainly by contact. Raising compliance above 90% and placing cases in cohorts with dedicated staff interrupts the chain of transmission, reducing new colonisations. This targets spread, not the bacteria's genetics.
Intervention 2 - Antibiotic stewardship: restrict carbapenem use, use targeted therapy and de-escalate (selection control). Mechanism: reducing carbapenem exposure removes the advantage that resistant strains and plasmid-bearing cells enjoy, so susceptible strains can compete again and plasmid carriage (which often carries a fitness cost) may be lost. This reduces the frequency of resistance in the population.
Distinction. Stewardship alone will not stop an ongoing outbreak because it does not prevent person-to-person transfer; isolation alone leaves a large resistant reservoir maintained by selection. Both are needed. Screening admissions from long-term care would add a third, earlier intervention.