Roles of crrAB two-component regulatory system in Klebsiella pneumoniae: growth yield, survival in initial colistin treatment stage, and virulence

Alternation of the colistin resistance-regulating two-component regulatory system (crrAB) is a colistin resistance mechanism in Klebsiella pneumoniae, but their roles in bacteria are not fully understood. Twelve colistin-susceptible K. pneumoniae clinical isolates were included in this study: six crrAB-positive and six crrAB-negative. We deleted the crrAB genes from two crrAB-positive isolates and complemented them. We measured the growth yields by determining growth curves in lysogeny broth and minimal media with or without Fe2+. In vitro selection rates for colistin resistance were determined by exposure to colistin, and survival rates against high concentrations of colistin (20 mg/L) at the early stage of growth (20 min) were investigated. Virulence was determined using a serum bactericidal assay and Galleria mellonella larval infection. The presence of crrAB was not associated with colistin resistance and did not increase the in vitro selection rate of colistin resistance after exposure. The growth yield of crrAB-positive isolates was higher in LB media and increased when Fe2+ was added to minimal media. The crrAB-positive isolates showed higher survival rates in the early stages of exposure to high colistin concentrations. Decreased serum resistance was identified in the crrAB-deleted mutants. More G. mellonella larvae survived when infected by crrAB-deleted mutants, and higher survival rates of bacteria were identified within the larvae infected with wild-type than crrAB-deletant isolates. Through rapid response to external signals, crrAB would provide advantages for K. pneumoniae survival by increasing the final growth yield and initial survival against colistin treatment. This may partly contribute to the bacterial virulence.