In vitro and in vivo activity of colistin-nitroxoline combination against polymyxin heteroresistant carbapenem-resistant Klebsiella pneumoniae

BACKGROUND: Carbapenem-resistant Klebsiella pneumoniae (CRKP) has emerged as a critical global health threat, often causing life-threatening infections with limited therapeutic options. Polymyxins are often used as the last-line agent, but its efficacy is limited by heteroresistance-where resistant subpopulations emerge during treatment. To combat this, we evaluated the synergistic activity of colistin combined with nitroxoline against polymyxin heteroresistant (PHR) CRKP subpopulations. RESULTS: The prevalence of PHR was 90.9% (10/11 isolates). While colistin or nitroxoline monotherapy failed to eradicate PHR subpopulations, the combination of colistin and nitroxoline achieved complete clearance at reduced concentrations in 3 out of 4 tested isolates. In adaptive laboratory evolution experiments, colistin monotherapy rapidly selected for resistant mutants, with MIC increasing 256-fold by the second passage. In contrast, the combination maintained stable susceptibility over 20 passages, with no detectable MIC increase. N-phenyl-1-naphthylamine and propidium iodide staining, along with scanning electron microscopy, revealed that the combination induced markedly enhanced membrane damage and cellular disruption compared to monotherapies. In a murine peritoneal infection model, combination therapy reduced mortality to 10% versus 30% with colistin monotherapy, although the difference was not statistically significant (p = 0.30). CONCLUSIONS: PHR is highly prevalent in CRKP and undermines colistin monotherapy. Colistin-nitroxoline combination exhibits potent synergy, suppresses resistance emergence, and shows promise for treating refractory CRKP infections.