0 1993 The Pathological Society of Great Britain and Ireland The induction of trimethoprim resistance encoded by the type IV di hydrofolate reductase gene

Summary. The effect of plasmid pUKll23, which confers low level resistance to trimethoprim when tested on solid minimal medium, but almost no resistance when tested on IsoSensitest agar, was investigated in liquid media. The growth of Escherichia coli J62-2, harbouring pUKll23, was unaffected in liquid minimal medium containing trimethoprim 10 mg/L. However, in IsoSensitest broth, exposure to this drug concentration resulted in bacteriostasis. After an initial delay, resistance to trimethoprim was induced in IsoSensitest broth containing trimethoprim 10 mg/L, by the imposition of thymine starvation. This response was immediately reversible when trimethoprim was removed, confirming that resistance resulted from induction rather than selection of resistant mutants