Molecular and epidemiologic study of polyclonal outbreaks of multidrug-resistant Acinetobacter baumannii infection in an Israeli hospital.

OBJECTIVES: To perform a molecular and epidemiologic investigation of multidrug-resistant (MDR) Acinetobacter baumannii in an institution were polyclonal outbreaks have been observed and determine whether these polyclonal outbreaks had an endogenous origin or were caused by in-hospital patient-to-patient transmission.
DESIGN: Retrospective analysis of prospectively collected data.
SETTING: An epidemiologic and genotypic investigation of incident cases of MDR A. baumannii infection in an Israeli university tertiary care center. PATIENTS: Hospitalized patients with MDR A. baumannii isolated from clinical samples during a 13-week period, from April 15, 2003, through July 15, 2003. INTERVENTION: All patients with new MDR A. baumannii infections were recruited, and isolates were typed using pulsed-field gel electrophoresis. Data on in-hospital movements and consultations were extracted from computerized databases. Quantification of transmission opportunities (TOPs), defined as encounters between an established carrier and a future carrier of MDR A. baumannii, and analysis of ward clusters were performed.
RESULTS: We studied 96 MDR A. baumannii isolates, which belonged to 18 different pulsed-field gel electrophoresis clones. In 65% of cases, TOPs involving patients with the same clone were demonstrated, which is significantly greater than the number of TOPs involving patients with different clones (P=.01).
CONCLUSION: Although outbreaks of MDR A. baumannii infection may be polyclonal, we believe that patient-to-patient transmission explains most cases of transmission. Polyclonal local outbreaks reflect several clonal outbreaks occurring simultaneously. The cause of polyclonal outbreaks of A. baumannii infections clustered by ward and time remains to be explained.