Missense Mutations in the CrrB Protein Mediate Odilorhabdin Derivative Resistance in .

NOSO-502 is a preclinical antibiotic candidate of the Odilorhabdin class. This compound exhibits activity against pathogens, including carbapenemase-producing bacteria and most of the Colistin (CST)-resistant strains. Among a collection of CST-resistant strains harboring mutations on genes , , , and , only those bearing mutations in gene were found to be resistant to NOSO-502.CrrB is a histidine kinase which acts with the response regulator CrrA to modulate the PmrAB system, which finally induces the restructuring of the lipopolysaccharide present on the outer membrane and thus leading to CST resistance. Moreover, mutations also enhance the transcription of neighboring genes such as H239_3063, an ABC transporter transmembrane region; H239_3064, a putative efflux pump also known as KexD; and H239_3065, a -acetyltransferase.To elucidate the mechanism of resistance to NOSO-502 induced by CrrB missense mutations in , mutants of NCTC 13442 and ATCC BAA-2146 strains resistant to NOSO-502 and CST with single amino acid substitutions in CrrB (S8N, F33Y, Y34N, W140R, N141I, P151A, P151L, P151S, P151T, F303Y) were selected. Full susceptibility to NOSO-502 was restored in or deleted NCTC 13442 CrrB(P151L) mutants, confirming the role of CrrAB in controlling this resistance pathway. Deletion of (but no other neighboring genes) in the same mutant also restored NOSO-502-susceptibility. Upregulation of the gene expression was observed for all CrrB mutants. Finally, plasmid expression of in a strain missing the locus and significantly increased resistance to NOSO-502.