Polymyxin B in combination with meropenem againstcarbapenemase-producing Klebsiella pneumoniae: pharmacodynamicsand morphological changes

ABSTRACT

Combination therapy provides a useful therapeutic approach to overcome resistance until new antibiotics become available. In this study, the pharmacodynamics, including the morphological effects, ofpolymyxin B (PMB) and meropenem alone and in combination against KPC-producing Klebsiella pneumoniaeclinical isolates was examined. Ten clinical isolates were obtained from patients undergoing treatmentfor mediastinitis. KPCs were identified and MICs were measured using microbroth dilution. Time–killstudies were conducted over 24 h with PMB (0.5–16 mg/L) and meropenem (20–120 mg/L) alone or incombination against an initial inoculum of ca. 106 CFU/mL. Scanning electron microscopy (SEM) was employedto analyse changes in bacterial morphology after treatment, and the log change method was usedto quantify the pharmacodynamic effect. All isolates harboured the blaKPC-2 gene and were resistant tomeropenem (MICs ≥8 mg/L). Clinically relevant PMB concentrations (0.5, 1.0 and 2.0 mg/L) in combinationwith meropenem were synergistic against all isolates except BRKP28 (polymyxin- and meropenemresistant,both MICs >128 mg/L). All PMB and meropenem concentrations in combination were bactericidalagainst polymyxin-susceptible isolates with meropenem MICs ≤16 mg/L. SEM revealed extensive morphologicalchanges following treatment with PMB in combination with meropenem compared with thechanges observed with each individual agent. Additionally, morphological changes decreased with increasingresistance profiles of the isolate, i.e. increasing meropenem MIC. These antimicrobial effects maynot only be a summation of the effects due to each antibiotic but also a result of differential action thatlikely inhibits protective mechanisms in bacteria...