Molecular and epidemiologic study of multiresistant Serratia marcescens infections in a spinal cord injury rehabilitation unit.

Between March 1984 and February 1986, ten patients admitted to a spinal cord injury/stroke rehabilitation unit became bacteriuric with a strain of Serratia marcescens resistant to ampicillin, cephalothin, cefoxitin, ticarcillin, cotrimoxazole, gentamicin, and tobramycin. All the patients were catheterized, and in most, bacteriuria was asymptomatic. The organism was also recovered from their hospital environment (sinks, toilets, urine-collecting basins). Analysis of total plasmid content of multiresistant isolates revealed the presence of two plasmids (7 kilobase, 25.5 kilobase), not found in aminoglycoside susceptible strains of Serratia marcescens. Restriction endonuclease analysis and Southern hybridization (DNA probe: 25.5 kilobase plasmid) verified that these plasmids were identical. The 25.5 kilobase plasmid was purified, introduced by transformation into an Escherichia coli strain C recipient, and was found to mediate resistance to gentamicin and tobramycin. The emergence of multiresistant Serratia marcescens coincided with an increase in antibiotic usage on the ward. The reservoir seemed to be the urinary tracts of asymptomatic catheterized patients and their contaminated hospital environment.