Concurrent outbreak of multidrug-resistant and susceptible subclones of Acinetobacter baumannii affecting different wards of a single hospital

BACKGROUND AND OBJECTIVE: Acinetobacter baumannii has emerged as an opportunistic pathogen among acutely ill patients, especially those with thermal injury. A prospective 8-month study was conducted to describe the clinical and molecular epidemiology of multidrug-resistant A. baumannii affecting a single hospital. METHODS: Univariate analysis comparing SmaI macrorestriction patterns of A. baumannii generated by pulsed-field gel electrophoresis (PFGE) versus clinical and demographic risk factors. RESULTS: A total of 200 isolates from 76 patients were collected, of which 185 isolates from 76 patients were analyzed by PFGE. A total of 17 distinct PFGE clonal types were identified. One clonal type (strain A) represented 129 isolates from 49 patients. A group of related clonal types (strain A variants) were identified as 40 isolates from 20 patients. The only risk factor other than geographic location associated with the presence of strain A was prior treatment with antibiotics active against gram-negative bacteria (P = .0015). The two clonal types differed in antibiotic resistance profiles: 25% of strain A isolates, the dominant strain in the burn unit, were susceptible to at least one antibiotic tested. In contrast, approximately 80% of the other strain types were susceptible to at least one antibiotic and were cultured from patients admitted elsewhere in the hospital. No combination of antibiotics was observed to yield additive or synergistic activity. CONCLUSION: Clonally related strains of Acinetobacter that differ in susceptibility patterns may coexist within a single hospital, dependent on the selective pressure related to antibiotic exposure.