Abstract
Five types of Escherichia coli strains were obtained and sequenced: colistin-susceptible (CL-S) strains, in vitro induced colistin-resistant (CL-IR) strains, mcr-1-negative colistin-resistant strains from livestock (CL-chrR), mcr-1-positive colistin-resistant strains (CL-mcrR), and mcr-1-transferred transconjugants (TC-mcr). Amino acid alterations of PmrAB, PhoPQ, and EptA were identified, and their mRNA expression was measured. Their growth rate was evaluated, and an in vitro competition assay was performed. Virulence was compared through serum resistance and survival in macrophages and Drosophila melanogaster. CL-IR and CL-chrR strains were colistin-resistant due to amino acid alterations in PmrAB, PhoPQ, or EptA, and their overexpression. All colistin-resistant strains did not show reduced growth rates compared with CL-S strains. CL-IR and CL-chrR strains were less competitive than the susceptible strain, but CL-mcrR strains were not. In addition, TC-mcr strains were also significantly more competitive than their respective parental susceptible strain. CL-IR strains had similar or decreased survival rates in human serum, macrophages, and fruit flies, compared with their parental, susceptible strains. CL-chrR strains were also less virulent than CL-S strains. Although CL-mcrR strains showed similar survival rates in human serum and fruit fly to CL-S strains, the survival rates of TC-mcr strains decreased significantly in human serum, macrophages, and fruit flies, compared with their susceptible recipient strain (J53). Chromosome-mediated, colistin-resistant E. coli strains have a fitness cost, but plasmids bearing mcr-1 do not increase the fitness burden of E. coli. Along with high usage of polymyxins, the no fitness cost of mcr-1-positive strains may facilitate rapid spread of colistin resistance.
Highlights
MATERIALS AND METHODSColistin is considered as a “last resort” antibiotic against multidrug resistant Gram-negative bacteria, despite its nephrotoxicity and neurotoxicity (Landman et al, 2008)
We compared fitness cost and virulence between colistin-resistant E. coli strains due to chromosomal gene mutations and plasmid-borne mcr-1 gene
Not all mutations are associated with colistin resistance
Summary
Colistin is considered as a “last resort” antibiotic against multidrug resistant Gram-negative bacteria, despite its nephrotoxicity and neurotoxicity (Landman et al, 2008). CL-mcrR strains EC006, EC019, and EC111 were used as donors of mcr-1-harboring plasmids and E. coli J53 pUHEgfp was employed as a recipient Both donor and recipient were incubated in LB broth until optical density at 600 nm reached 0.5. Identification of mcr-1-loss cells within each culture was determined by transferring 96 colonies from BAP to 4 mg/L colistin-containing MH broth in each well of a 96-well plate. MH agar with and without colistin (4 mg/L) were used for plating dilutions of the bacterial mixture to differentiate the colistinresistant strains and MG1655. Colonies were counted after overnight incubation and competition index (CI) values were determined based on (CFUs of colistin-resistant strain/CFUs of MG1655 or J53) ratio (Humphrey et al, 2012; Beceiro et al, 2014).
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