Antibiotic resistance in a hospital isolate
A hospital microbiology lab tests a bacterial isolate from a patient with a persistent wound infection. The isolate is resistant to three antibiotics but sensitive to a fourth, Drug X. The patient has already received two weeks of Drug X without improvement. A junior scientist proposes that the bacteria must have a mutation conferring resistance to Drug X, and suggests increasing the dose. A senior scientist notes that the patient's wound contains a biofilm and that the lab tests used free-floating cells, not biofilm. The lab also finds the resistance genes are located on a plasmid shared with other species in the wound.
1. Explain why the junior scientist's conclusion may be wrong, and give alternative explanations for the treatment failure.
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The junior scientist assumes that treatment failure equals genetic resistance. Several alternatives fit the evidence better.
- Biofilm tolerance. Lab sensitivity tests use free-floating cells, but the wound contains a biofilm. Cells in a biofilm can survive antibiotic concentrations that kill planktonic cells because the matrix limits diffusion and cells enter a slow-growing state. This is tolerance, not inherited resistance, and it would not appear in a standard test.
- Pharmacokinetics. Poor blood supply to a chronic wound may mean Drug X never reaches effective concentration at the site, so the bacteria are sensitive in the lab but not exposed in the patient.
- Plasmid transfer. The resistance genes sit on a shared plasmid, so other species in the wound may be protecting the pathogen or contributing to the community's survival, complicating the single-species picture.
- Dose increase risk. Raising the dose assumes a resistance mechanism and may cause toxicity without benefit if the problem is poor penetration or biofilm tolerance.
A genetic mutation is possible but is not the only or best explanation on this evidence.
2. Describe how you would test whether biofilm tolerance rather than genetic resistance explains the failure.
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Laboratory tests. Grow the isolate as a biofilm, for example on a plate or bead assay, and measure the minimum concentration of Drug X that kills biofilm cells compared with planktonic cells. A large difference supports tolerance. Also sequence or PCR-test for known Drug X resistance genes to check for a genetic mechanism, and test whether resistance is lost after curing the plasmid.
Clinical sampling. Sample bacteria directly from the wound before and after treatment and compare their Drug X sensitivity with the lab stock. If wound cells remain sensitive when dispersed, tolerance or poor penetration is more likely than resistance.
Penetration check. Measure Drug X concentration in wound fluid or tissue, where feasible, to see whether the drug reaches the site.
Prediction. If biofilm cells survive high Drug X concentrations but dispersed cells are killed, and no resistance gene is found, biofilm tolerance is supported. If a resistance mutation appears in wound isolates after treatment, genetic resistance is supported. The results determine whether the response should be biofilm disruption, improved delivery or a change of drug.