Investigation of outbreaks of Enterobacter aerogenes colonisation and infection in intensive care units by random amplification of polymorphic DNA.

During a 4-month period, 41 isolates of Enterobactor aerogenes were cultured from different specimens from a 14-bed intensive care unit (ICU1). These were obtained from 12 patients out of a total of 187 patients admitted to the ICU. Sixteen E. aerogenes isolates were cultured from another ICU (ICU2) 6 months later. Six non-outbreaks associated strains were included as controls and all the isolates were compared by random amplification of polymorphic DNA (RAPD), with three different 10-mer oligonucleotide primers. RAPD fingerprinting with primer AP12h was as discriminatory as the combined results from all three primers and defined 22 different patterns for the 41 isolates from the ICU1. In nine instances, isolates with indistinguishable RAPD patterns were detected in two-to-five patients over a 3-15-day period, suggesting patient-to-patient transmission. During their stay in ICU1, patients harboured one-to-12 distinguishable isolates. Isolates from ICU2 were indistinguishable by RAPD analysis with the three different primers. These findings suggest that the cluster of colonisations and infections in ICU1 was a 'false outbreak', consisting of successive patient-to-patient transmission of different E. aerogenes strains. In contrast, the outbreak on ICU2 probably involved the extensive spread of a single strain.