EPIDEMIOLOGIC-STUDY OF AN OUTBREAK DUE TO MULTIDRUG-RESISTANT ENTEROBACTER-AEROGENES IN A MEDICAL INTENSIVE-CARE UNIT

In 1993, 63 isolates of Enterobacter aerogenes were collected from 41 patients in a medical intensive care unit (ICU). During the same perio d, only 46 isolates from 32 patients were collected in the rest of the hospital. All isolates were analyzed by antibiotic resistance phenoty pe, and 77 representative isolates were differentiated by plasmid rest riction analysis, ribotyping, and arbitrarily primed (AP)-PCR. The ext ended-spectrum beta-lactamases produced by 22 strains were characteriz ed by determination of their isoelectric points and by hybridization o f plasmid DNA with specific probes. The isolates were divided into 25 antibiotic resistance phenotypes, either susceptible (group I) or resi stant (group II) to aminoglycosides, and exhibited three phenotypes of resistance to beta-lactams: chromosomally derepressed cephalosporinas e alone or associated with either extended-spectrum beta-lactamases (m ainly of the SHV-4 type) or imipenem resistance. The results of the te sts divided the 77 representative isolates (group I, n = 21; group II, n = 56) into 15 plasmid profiles, 14 ribotypes, and 15 AP-PCR pattern s. Although the resistant isolates (group II) exhibited different plas mid profiles, ribotyping and AP-PCR analysis demonstrated an identical chromosomal pattern, indicating an epidemiological relatedness. They were mainly found in the medical ICU and occasionally in other units. The susceptible strains (group I) had various and distinct markers and were mainly isolated in units other than the medical ICU. In conclusi on, the presence of a nosocomial outbreak in an ICU and the spread of a multidrug-resistant epidemic strain throughout the hospital was conf irmed. Ribotyping and AP-PCR represent discriminatory tools for the in vestigation of nosocomial outbreaks caused by E. aerogenes.