Investigation of an outbreak of multiresistant Enterobacter aerogenes infection in an intensive care unit

In February 1996 we have experienced an outbreak with a multiresistant E aerogenes strain in a 12-bed intensive care unit (ICU) separated in two rooms. Over a 3-day period, four patients were found to be colonized at multiple sites (endotracheal aspirate, urine) by E aerogenes. The medical and nursing charts were reviewed in order to define the sequence of events which led to colonization in these patients. It was found that one patient probably acted as the index-case since he was retrospectively found to be already colonized with E aerogenes in another ward before his admission to the ICU. The three other patients were housed in boxes immediately adjacent to the one of the index case but no strict isolation precautions were taken for these patients since the index case had not been reported as being colonized with a multiresistant organism. All four patients had previously received broad-spectrum antibiotics and three of them were intubated and mechanically ventilated. Colonization ultimately progressed to infection in three of the four patients [one pneumonia, two urinary tract infections (UTI)]. Three patients were treated with cefepime as single drug therapy (2 × 1–2 g iv/day); two of them died (one due to underlying disease, the other because of uncontrolled infection) whereas one was cured clinically. Bacteriological failure was observed in two patients and eradication failure was associated with the development of resistance to cefepime during treatment in one of them. All E aerogenes strains isolated in the four patients had the same identification characteristics and displayed a similar in vitro antimicrobial resistance profile (the strains were sensitive only to gentamicin, amikacin, imepenem and to cefepime). Molecular epidemiological typing studies using random amplification of polymorphic DNA (RAPD) with two different 10-mer oligonucleotide primers yielded indistinguishable patterns for the four outbreak strains while these were clearly different from five non-outbreak associated strains which were included as controls.

Nosocomial outbreaks due to multiresistant E aerogenes is an emerging concern in ICU. Infections caused by this organism are often not detected at an early stage and are both difficult to control and to treat. In the present case, antibiotic treatment with cefepime did not prove very effective for controlling the outbreak which terminated only following the reinforcement of handwashing and isolation contact precautions. RAPD fingerprinting proved useful in this outbreak for discriminating the outbreak strains from epidemiologically unrelated strains. Further clinical trials are needed to optimize the therapy of multiresistant E aerogenes infection.