A cluster of nosocomial cross-infection due to multiple antibiotic-resistant Acinetobacter baumannii: Characterization of the strain and antibiotic susceptibility studies

A multiple antibiotic-resistant (MAR) strain of Acinetobacter baumannii cau sed nosocomial cross-infection among 3 patients of a surgical intensive car e unit. The isolates were of identical biochemical profile (77776 S-U-) and serotype (serovar 36) and identical in terms of pulsed-field gel electroph oresis macrorestriction (Smal, Apal) analysis. This MAR strain was suscepti ble only to netilmicin, tobramycin, imipenem, meropenem, polymyxin B, and t rovafloxacin. The minimal bactericidal concentrations of imipenem and merop enem were markedly higher than the corresponding minimal inhibitory concent rations against this strain. Combined fresh defibrinated human blood (65 vo l%) and antimicrobial drug assays yielded the following results: polymyxin was the most rapidly bactericidally effective antibiotic in the presence of blood and in broth. Tobramycin and netilmicin were efficacious in 65 vol% blood. Imipenem was slightly more effective than meropenem in broth, wherea s both carbapenems sterilized blood-containing assay tube contents. Trovafl oxacin failed to achieve bactericidal activity (to 99.9% kill) in the prese nce of blood, presumably because this strain was resistant to ciprofloxacin and borderline susceptible to ofloxacin. Trovafloxacin combined with eithe r imipenem or meropenem yielded an indifferent effect. However, the combina tion of trovafloxacin (2 mu g/ml) plus tobramycin(1 mu g/ml) achieved steri lization of tube contents in the presence of blood within 4 h after exposur e and in broth following extended (overnight) incubation. This MAR strain o f A. baumannii was high-level resistant to rifampin; thus the combination o f polymyxin B plus rifampin proved indifferent.