Synergistic killing of NDM-producing MDR Klebsiella pneumoniae by two ‘old’ antibiotics—polymyxin B and chloramphenicol

Objectives: Combination therapy is an important option in the fight against Gram-negative 'superbugs'. This study systematically investigated bacterial killing and the emergence of polymyxin resistance with polymyxin B and chloramphenicol combinations used against New Delhi metallo-beta-lactamase (NDM)-producing MDR Klebsiella pneumoniae. Methods: Four NDM-producing K. pneumoniae strains were employed. The presence of genes conferring resistance to chloramphenicol was examined by PCR. Time-kill studies (inocula similar to 10(6) cfu/mL) were conducted using various clinically achievable concentrations of each antibiotic (range: polymyxin B, 0.5-2 mg/L; chloramphenicol, 4-32 mg/L), with real-time population analysis profiles documented at baseline and 24 h. The microbiological response was examined using the log change method and pharmacodynamic modelling in conjunction with scanning electron microscopy (SEM). Results: Multiple genes coding for efflux pumps involved in chloramphenicol resistancewere present in all strains. Polymyxin B monotherapy at all concentrations produced rapid bacterial killing followed by rapid regrowth with the emergence of polymyxin resistance; chloramphenicol monotherapy was largely ineffective. Combination therapy significantly delayed regrowth, with synergy observed in 25 out of 28 cases at both 6 and 24 h; at 24 h, no viable bacterial cells were detected in 15 out of 28 cases with various combinations across all strains. No polymyxin-resistant bacteriawere detected with combination therapy. These results were supported by pharmacodynamic modelling. SEM revealed significant morphological changes following treatment with polymyxin B both alone and in combination. Conclusions: The combination of polymyxin B and chloramphenicol used against NDM-producing MDR K. pneumoniae substantially enhanced bacterial killing and suppressed the emergence of polymyxin resistance.