Endemic gentamicin resistance R factors on a spinal cord injury unit

Cleared lysates of gentamicin-resistant, gram-negative bacilli obtained during a prevalence survey and a subsequent prospective study on a spinal cord injury unit were analyzed. Of 105 strains obtained during the epidemiological study, 62 were analyzed for plasmid content. None of the 14 Acinetobacter strains carried plasmids. Of 20 strains from the initial prevalence survey, 9 carried a 36- or (in two cases) a 27-megadalton plasmid. Eight of the nine were Providencia strains; none were Pseudomonas strains. Of 28 nosocomial isolates obtained during the prospective survey, 22 carried plasmids of similar molecular weight (P less than 0.025, compared with the prevalence survey), including 20 of 22 isolates of members of the family Enterobacteriaceae and 2 of 6 Pseudomonas aeruginosa isolates. Conjugation, curing, and transformation indicate that these plasmids carry gentamicin, tobramycin, kanamycin, ampicillin, carbenicillin, cephalothin, and, variably, chloramphenicol resistance. Restriction endonuclease digestion of purified plasmid DNA suggests that the plasmids from multiple species of the family Enterobacteriaceae contain common sequences, whereas those from Pseudomonas spp. do not. This study suggests that an endemic conjugal 36-megadalton gentamicin resistance R factor exists in many nosocomial species of the family Enterobacteriaceae.