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Polymyxins including polymyxin B and colistin (polymyxin E) are considered the last resort for treating infections caused by carbapenem-resistant gram-negative bacteria. However,  in vitro  regrowth with the emergence of resistance during treatment is common. Polymyxin heteroresistance, particularly in  Acinetobacter baumannii  and  Klebsiella pneumoniae , has been widely reported. This study was primarily performed to evaluate the prevalence of colistin heteroresistance in carbapenem-resistant  K. pneumoniae  (CR-KP) and the association between  in vitro  regrowth and heteroresistance. The mechanisms of colistin resistance and the ability of combination therapies to suppress resistance selection were further investigated. A population analysis profile (PAP) analysis showed that 69 (71.9%) of 96 CR-KP strains had colistin heteroresistance. Time-kill assays revealed that the colistin monotherapy could quickly eliminate the bacterial cells in strains without heteroresistance within the first 6 h. Conversely, it could initially reduce the number of cells in heteroresistant strains, but then regrowth occurred rapidly. Resistance screening at 12 and 24 h in the time-kill assays indicated that susceptible populations were killed, and regrowth was the exact result of the continued growth of resistant subpopulations. Colistin resistance in the regrowth subpopulations was mainly due to the overexpression of  phoPQ  and  pmrD . Colistin combined with tetracyclines (tigecycline or minocycline) or aminoglycosides (amikacin or gentamicin) could effectively suppress the resistance selection and significantly elicit  in vitro  synergistic effects. These findings suggested that the combination therapy can be used to treat infections caused by CR-KP with colistin heteroresistance. Nevertheless, further  in vivo  studies considering drugs pharmacokinetics/pharmacodynamics are needed to confirm these findings.

Heteroresistance Is Associated With in vitro Regrowth During Colistin Treatment in Carbapenem-Resistant Klebsiella pneumoniae