A case-control study of nosocomial ampicillin-resistant enterococcal infection and colonization at a university hospital.

OBJECTIVE: To assess risk factors for colonization and nosocomial infection with ampicillin-resistant enterococci (ARE). DESIGN: Patients with ampicillin-resistant enterococci were compared retrospectively by logistic regression analysis with controls harboring susceptible strains. ARE were characterized by whole plasmid DNA analysis and restriction enzyme analysis of plasmid (REAP) DNA with EcoRI. SETTING: The study was done at a 1,125 bed, tertiary-care teaching hospital in North Carolina with patients from whom enterococci were isolated from June 1, 1989, to March 30, 1991. PATIENTS: The final study group comprised 44 cases with nosocomially-acquired colonization or infection with ARE and 100 controls with ampicillin-susceptible strains. Clinical and epidemiological risk factors for ARE were abstracted by chart review. RESULTS: After controlling for age and site of infection, patients with ARE were more likely to have been admitted previously to our hospital and to have received third-generation cephalosporins and clindamycin. However, only advanced age and clindamycin therapy were independently associated with presence of ARE. REAP with EcoRI showed 20 groups of enterococci on 19 different wards. CONCLUSIONS: These results suggest that ARE are endemic and multifocal in origin in our hospital and that advanced age and use of clindamycin are important selective risk factors for ARE colonization and infection.