Characteristics of Serratia marcescens containing a plasmid coding for gentamicin resistance in nosocomial infections.

Strains of gentamicin-resistant Serratia marcescens (GRSM) that originated in a crowded neurosurgical close observation unit (COU) became established during a 2.5-year interval at several affiliated hospitals in Charleston, South Carolina. Most patients with GRSM had colonization or infection of the urinary tract associated with indwelling bladder catheters. Infected patients in the COU more often had pyuria and less often received systemic steroids than COU patients not harboring GRSM. However, length of stay, use of urinary catheters, exposure to systemic antibiotics, and exposure to gentamicin were not significantly different between the two groups. Of the strains of GRSM, 92% contained a conjugative 41-megadalton plasmid tht encoded resistance to ampicillin, carbenicillin, tetracycline, kanamycin, gentamicin, and tobramycin and elaborated similar aminoglycoside 3-acetyltransferases. Seven 41-megadalton plasmids from outbreak strains and a 41-megadalton plasmid from a 1973 isolate of GRSM gave identical DNA fragments after restriction endonuclease digestion.