Gentamicin resistance in Pseudomonas aeruginosa: R-factor-mediated resistance

By disk diffusion antimicrobial susceptibility testing, 11% of 313 consecutive strains of Pseudomonas aeruginosa, examined during July to October 1973, were resistant to gentamicin (minimal inhibitory concentration 12.5 to >100 mug/ml), and a further 31% were moderately resistant (6.25 to 12.5 mug/ml) to gentamicin at the University of Alberta Hospital in Edmonton, Canada. Of 45 gentamicin-resistant strains from that hospital, none possessed R-factors or gentamicin-inactivating enzymes. Eight of 13 strains obtained from three American sources, which contained gentamicin-acetylating (12 strains) or -adenylating (1 strain) activity, conjugally transferred both gentamicin resistance and antibiotic-inactivating activity. P. aeruginosa recipients were much more effective for detection of transferable gentamicin resistance than Escherichia coli recipients, although not all P. aeruginosa were equally as effective as recipients. One strain, POW 151, transferred resistance to both carbenicillin and gentamicin as well as to several other antibiotics. R-factors detected belonged to P-2 and P-3 (Com 6, C) incompatibility groups. Expression of gentamicin resistance due to acetylation of gentamicin was subject to marked phenotypic lag, especially in recipient strain P. aeruginosa 280. This was shown to result in the failure to detect gentamicin resistance transfer if the concentration of gentamicin in selection media was too high (>2.5 mug/ml for strain 280). Some but not all recipients were changed in pyocine type upon acquisition of R-factors.