R Factor-Mediated Resistance to Gentamicin in a Clinical Isolate of Escherichia coli

R factor-mediated resistance to gentamicin was detected in Escherichia coli strain Will, a clinical isolate that required 12.5–25 µg of gentamicin/ml to inhibit its growth. Resistance to gentamicin and six other antimicrobial agents was transmitted serially to other strains of gram-negative bacilli. The MIC of gentamicin for progeny of matings of strain Will with other strains of E. coli or Shigella fiexneri varied from 1.5–100 µg/ml. These values correspond to a l5–l00-fold enhancement of resistance of the host strain. The degree of resistance to gentamicin mediated by the R factor in strain Will is likely to be clinically significant.