In vitro responses of Acinetobacter baumannii to two- and three-drug combinations following exposure to colistin and doripenem.

We compared in vitro killing of colistin, doripenem, and sulbactam by time-kill methods against Acinetobacter baumannii isolates collected from patients before and after colistin-doripenem treatment (initial and recurrent isolates, respectively). Colistin-doripenem bactericidal activity against recurrent isolates was attenuated (mean log10 kill, -5.74 versus -2.88; P = 0.01) but was restored by adding sulbactam. Doripenem MICs rather than colistin MICs correlated with the activity of colistin-doripenem. Among colistin-resistant isolates, colistin-doripenem-sulbactam combinations achieved greater killing than colistin-doripenem alone (-5.65 versus -2.43; P = 0.04).