Synergistic activity of colistin plus rifampin against colistin-resistant KPC-producing Klebsiella pneumoniae.

Infections caused by carbapenem-resistant KPC-producing Klebsiella pneumoniae are responsible for high rates of mortality and represent a major therapeutic challenge, especially when the isolates are also resistant to colistin. We used the checkerboard method to evaluate the synergistic activity of 10 antibiotic combinations against 13 colistin-resistant KPC-producing K. pneumoniae isolates (colistin MIC range of 8 to 128 mg/liter). Colistin plus rifampin was the only combination that demonstrated consistent synergistic bacteriostatic activity against 13/13 strains tested, reducing the colistin MIC below the susceptibility breakpoint (MIC ≤ 2 mg/liter) in 7/13 strains at rifampin concentrations ranging from 4 to 16 mg/liter. Bactericidal synergistic activity was also documented for 8/13 tested strains. Other antimicrobial combinations with carbapenems, gentamicin, and tigecycline showed variously synergistic results. Colistin plus rifampin also exhibited bacteriostatic synergistic activity against 4/4 colistin-susceptible KPC-producing K. pneumoniae isolates (colistin MIC range of 0.5 to 2 mg/liter) and 4/4 ertapenem-resistant extended-spectrum beta-lactamase (ESBL)-producing K. pneumoniae isolates (ertapenem MIC range of 16 to 32 mg/liter). Collectively, our data suggest that colistin plus rifampin is the most consistently synergistic combination against KPC-producing K. pneumoniae isolates, including colistin-resistant strains. Colistin-rifampin combinations may have a role in the treatment of multidrug-resistant K. pneumoniae and may possibly slow the selection of heteroresistant subpopulations during colistin therapy.