In vitro evaluation of antibiotic synergy for polymyxin B-resistant carbapenemase-producing Klebsiella pneumoniae

Abstract

Since carbapenemase-producing Klebsiella pneumoniae strains were first reported in North Carolina, these highly resistant organisms have been isolated with increasing frequency, especially in the New York City area. Polymyxin B is one of the few antimicrobials that retain reliable activity against these organisms. However, polymyxin B MICs are elevated against K. pneumoniae isolates with increasing frequency, leaving clinicians with few therapeutic options. We investigated several antimicrobial agents for potential synergy with polymyxin B against 12 clinical strains of carbapenemase-producing K. pneumoniae. A broth microdilution assay using a 96-well plate was developed in which graded dilutions of polymyxin B and the study drug were incubated with resistant isolates in a checkerboard pattern. Polymyxin B was studied in combination with cefazolin, ceftriaxone, cefepime, imipenem, gentamicin, tigecycline, doxycycline, and rifampin. All K. pneumoniae strains tested positive for K. pneumoniae carbapenemase (KPC) genes by real-time PCR and had elevated polymyxin B MIC values ranging from 16 to 128 μg/ml. Synergy was observed with the combination of polymyxin B and rifampin as well as with polymyxin B and doxycycline, resulting in at least a 4-fold decrease in the polymyxin B MIC. For both combinations, this effect occurred at physiologically achievable concentrations. Less pronounced synergy was noted with tigecycline and polymyxin B. No synergy was observed at physiologic concentrations with the other antimicrobials studied. These results suggest that rifampin, doxycycline, and tigecycline may be useful additions to polymyxin B in the treatment of infections caused by highly resistant carbapenemase-producing K. pneumoniae. Further studies are warranted to determine if these in vitro findings translate into clinical efficacy.

FIG. 1.

Total fractional inhibitory concentrations of polymyxin B combined with rifampin (a) and imipenem (b). Black squares indicate bacterial growth in the wells. Gray squares indicate the wells for which total FIC values exceeded 0.5, which was considered the cutoff value for a synergistic effect. White squares demonstrate the antibiotic combinations for which bacterial growth was inhibited and total FIC values were <0.5 and which, consequently, were considered the combinations producing synergy. Data obtained for isolate 7 are presented.

FIG. 2.

Total fractional inhibitory concentrations of polymyxin B combined with doxycycline-resistant (a) and doxycycline-susceptible (b) isolates. Data obtained for isolates 35 (a) and 3 (b) are presented.

FIG. 3.

Total fractional inhibitory concentrations of polymyxin B combined with tigecycline-resistant (a) and tigecycline-susceptible (b) isolates. Data obtained for isolates 7 (a) and 3 (b) are presented.

FIG. 4.

Total fractional inhibitory concentrations of polymyxin B combined with gentamicin-resistant (a) and gentamicin-susceptible (b) isolates. Data obtained for isolates 7 (a) and 35 (b) are presented.