Effects of regulatory network organization and environment on PmrD connector activity and polymyxin resistance in Klebsiella pneumoniae and Escherichia coli

Polymyxins are a class of cyclic peptides with antimicrobial activity against Gram-negative bacteria. Resistance to these compounds is often mediated by pathways that also confer resistance to host antimicrobial peptides. In Enterobacteriaceae, the PhoQ/PhoP and PmrB/PmrA two-component systems regulate many of the genes associated with these resistance mechanisms. In K. pneumoniae, spontaneous polymyxin resistance is frequently acquired through inactivation of the gene mgrB, which encodes a negative regulator of PhoQ. However, this resistance mechanism has not been reported in other genera of Enterobacteriaceae, despite the presence of mgrB among many members of this family. In addition, the frequency of developing spontaneous resistance to the antimicrobial peptide polymyxin through chromosomal mutations is much higher in Klebsiella compared to Salmonella or E. coli. Here we show that in K. pneumoniae, PmrD is not required for polymyxin resistance arising from inactivation of mgrB. In addition, we show that in E. coli, the protein PmrD can activate PmrA under certain conditions. Our results suggest that the importance of PmrD connector activity in polymyxin resistance depends on both the network organization and on the environmental conditions associated with PmrB stimulation.