Funding: the study was supported by the Russian Science Foundation (Project ID 20-15-00235).

Acknowledgements: the authors thank Polikarpova SV of Filatov City Clinical Hospital № 15 and Karaseva OV of the Research Institute of Emergency Pediatric Surgery and Traumatology for K. pneumoniae isolates.

Author contributionThe increasing use of colistin in the clinic has led to the emergence and spread of colistin resistance. According to the literature, antibiotic resistance can have a metabolic cost, resulting in poor adaptation and survival, i.e. reduced bacterial fitness. The aim of this study was to investigate molecular mechanisms underlying resistance to colistin and their effect on the bacterial fitness of carbapenem-resistant (carba-R) strains of K. pneumoniae isolated from the patients of Moscow hospitals in 2012–2017. Of 159 analyzed carba-R isolates, 71 (45%) were resistant to colistin (minimum inhibitory concentration over 2 mg/L). By conducting Sanger sequencing, we were able to identify the mechanisms underlying colistin resistance in 26 (37%) isolates. Growth curves were constructed by measuring optical density at 600 nm wavelength for 15 hours. The competitive growth of colistin-resistant (col-R) K. pneumoniae isolates was assessed relative to the colistinsusceptible (col-S) isolate. Col-R and col-S cultures harvested in the exponential phase were combined at the ratio of 1:1, incubated in the Luria-Bertani medium and plated onto Luria-Bertani agar plates with 10 mg/L colistin and without it. The competition index was calculated as the ratio of grown col-R and col-S colonies. Resistance to colistin did not affect the growth kinetics of K. pneumoniae, but did reduce the competitive ability of the bacteria as compared to the col-S isolates. However, some col-R isolates were more competitive than the col-S strains of the same sequence type. Further research is needed to elucidate the effects of colistin resistance on bacterial fitness.: Shamina OV planned and conducted the study, analyzed the literature, analyzed and interpreted the obtained data, and wrote the manuscript; Kryzhanovskaya OA, Lazareva AV, Alyabieva NM planned and conducted the study; Mayanskiy NA planned, conducted and supervised the study, analyzed the literature, collected, analyzed and interpreted the obtained data, and wrote the manuscript.