Nosocomial outbreak caused by a multiresistant clone of Acinetobacter baumannii: results of the case-control and molecular epidemiologic investigations.

OBJECTIVE: To describe a nosocomial outbreak caused by multiresistant Acinetobacter baumannii.
DESIGN: Descriptive and case-control study. Antibiotic susceptibilities and pulsed-field gel electrophoresis (PFGE) of genomic DNA digested with SfiI and also with ApaI were used as markers of strain identity.
SETTING: A large medical school-affiliated hospital in the city of Houston, Texas.
RESULTS: During a 10-week period, A baumannii was isolated from 25 patients admitted to the intensive care unit (ICU). The attack rate was 14.3 per 100 ICU admissions. Case patients were intubated more frequently and for longer periods, and had longer ICU and hospital stays (P < 0.05 for each of these characteristics). Multivariate logistic regression analysis identified the length of ICU stay and the use of third-generation cephalosporins as associated with the acquisition of A baumannii. Patients infected with A baumannii had a higher mortality rate than colonized patients and control patients (P < 0.01). Sixteen isolates recovered from these 25 patients were susceptible only to imipenem/cilastatin, and PFGE confirmed that a single clone was the cause of this outbreak. Nine isolates of A baumannii from patients infected or colonized in two other hospitals in Houston during the same period, differed from the outbreak isolates by their susceptibility to ciprofloxacin. However, PFGE results were identical, indicating unsuscepted genetic relatedness among isolates from three different hospitals.
CONCLUSIONS: A baumannii is an important nosocomial opportunistic pathogen and can adversely affect the outcome of ICU patients who acquire this organism. This outbreak was caused by a single clone that was isolated concurrently from three hospitals.