Dissemination of gentamicin-resistant plasmids among strains of Enterobacteriaceae.

The frequency of gentamicin-resistance among strains of many species of Enterobacteriaceae isolated from in-patients in our hospital has increased considerably since 1979. To clarify the reason for this increase, we tried to detect R-plasmids conferring gentamicin-resistance in all of the isolates in 1981 and found that most of them had conjugative R-plasmids. For epidemiological purposes we classified these into types I to X and 'others' by the following criteria: the nature of the resistances conferred; their incompatibility group; their molecular weight and the pattern of the DNA fragments after digestion with restriction endonucleasis. In the urology department, type-I plasmids conferring resistance to gentamicin, kanamycin, tobramycin and amikacin, and type-II plasmids conferring resistance to gentamicin, kanamycin and tobramycin were detected in the isolates belonging respectively to six species and twelve biotypes, and four species and six biotypes. In the second surgery ward, isolates belonging to six species and twelve biotypes were found to have type-III plasmids conferring resistance to gentamicin, kanamycin, tobramycin and chloramphenicol. Our investigation shows that not only antibiotic-resistant strains but also R-plasmids may be disseminated in hospitals.