Outbreak of ampicillin-resistant Enterococcus faecium--risk factors for faecal colonisation

Since January 1995 there has been a nosocomial outbreak at Haukeland University Hospital involving more than 330 patients with clinical infections caused by ampicillin-resistant Enterococcus faecium (ARE) (minimum inhibitory concentration > or =32 mg/l). Rectal carriage of ARE was initially observed on two medical wards only. Here the ARE colonisation rate has remained high. To assess risk factors for ARE colonisation we performed a case-control study including 37 rectal carriers of ARE and 83 non-carriers on these wards. Significant differences were found between cases and controls with respect to the mean number of days on antimicrobial treatment (13.3 for carriers, 5.5 for non-carriers, p<0.001), mean number of different antibiotics prescribed (2.8 for carriers, 2.1 for non-carriers, p= 0.008) and mean number of days in hospital (18.4 vs 10.2, p=0.001). Unadjusted statistical analysis showed that several antibiotics were risk factors for ARE carriage. Logistic regression analysis showed that fluoroquinolone prescription (OR=3.5, p=0.01) and more than 10 days of antibiotic use (OR= 3.3, p=0.01) were significant risk factors. An additional follow-up screening of previous carriers revealed no colonisation 8 to 36 (median 9) months after discharge from hospital (n=17). Prolonged antimicrobial therapy and broad-spectrum antibiotics seem to facilitate nosocomial ARE colonisation.