Impact of the colistin resistance gene mcr-1 on bacterial fitness.

A Klebsiella pneumoniae isolate harbouring a 217 kb IncHI2-type plasmid (pKP2442) encoding the colistin resistance gene mcr-1 was isolated from a leukaemia patient. pKP2442 was mobilised by intragenus and intergenus transconjugation from the clinical isolate to Escherichia coli J53 (transconjugation frequency 6.86 × 10-8  ± 5.57 × 10-8 ) and K. pneumoniae PRZ (transconjugation frequency 4.04 × 10-8  ± 3.03 × 10-8 ), respectively. Since acquisition of resistance determinants often results in a loss of fitness, the impact of mcr-1 on the fitness of E. coli and K. pneumoniae was investigated. Escherichia coli J53 and K. pneumoniae PRZ transformants harbouring the TOPO expression vector encoding mcr-1 displayed significantly decreased growth rates compared with isogenic parental strains and controls. In contrast, competitive growth experiments revealed equal growth rates between E. coli J53 pKP2442 transconjugants (TcpKP2442 ) and the parental strain, whereas K. pneumoniae PRZ TcpKP2442 showed significantly reduced growth rates compared with their parental strain (selection rate constant -1.62 ± 0.49), indicating a decrease in fitness. Infection of A549 human lung epithelial cells with TcpKP2442 or mcr-1 transformants and controls revealed equal lactate dehydrogenase activities, indicating no significant impact of mcr-1 on cytotoxicity. Likewise, survival of Galleria mellonella larvae infected with mcr-1-expressing strains and isogenic controls was similar. These data indicate that expression of mcr-1 is able to cause a fitness cost when encoded on expression vectors and that acquisition of natural plasmid-borne mcr-1 does not impair fitness in E. coli J53 but negatively influences growth rates in K. pneumoniae PRZ.