INVESTIGATION OF OUTBREAKS OF ENTEROBACTER-AEROGENES COLONIZATION ANDINFECTION IN INTENSIVE-CARE UNITS BY RANDOM AMPLIFICATION OF POLYMORPHIC DNA

During a 3-month period, 41 isolates of Enterobacter aerogenes were cu ltured from different specimens from a 14-bed intensive care unit (ICU 1). These were obtained from 12 patients out of a total of 187 patient s admitted to the ICU. Sixteen E. aerogenes isolates were cultured fro m another ICU (ICU2) 6 months later. Six non-outbreak-associated strai ns were included as controls and all the isolates were compared by ran dom amplification of polymorphic DNA (RAPD), with three different 10-m er oligonucleotide primers. The six non-outbreak-associated strains we re distinguishable by RAPD with two of the three primers, RAPD fingerp rinting with primer AP12h was as discriminatory as the combined result s from all three primers and defined 22 different patterns for the 41 isolates from the ICU1. In nine instances, isolates with indistinguish able RAPD patterns were detected in two-to-five patients over a 3-15-d ay period, suggesting patient-to-patient transmission. During their st ay in ICU1, patients harboured one-to-12 distinguishable isolates. Iso lates from ICU2 were indistinguishable by RAPD analysis with the three different primers. These findings suggest that the cluster of colonis ations and infections in ICU1 was a 'false outbreak', consisting of su ccessive patient-to-patient transmission of different E. aerogenes str ains. In contrast, the outbreak on ICU2 probably involved the extensiv e spread of a single strain.