The impact of under-acylation of lipid A on the interaction between Klebsiella pneumoniae LPS and polymyxins B and E was examined with fluorometric and calorimetric methods, and by H-1 NMR, using a paired wild type (WT) and the Delta lpxM mutant strains B5055 and B5055 Delta lpxM, which predominantly express LPS with hexa-and penta-acylated lipid A structures respectively. LPS from B5055 Delta lpxM displayed a fourfold increased binding affinity for polymyxins B and E compared with the B5055 WT LPS. EC50 values were consistent with polymyxin minimum inhibitory concentration (MIC) values for each strain. Accordingly, polymyxin exposure considerably enhanced the permeability of the B5055 Delta lpxM OM. Analysis of the melting profiles of isolated LPS aggregates suggested that bactericidal polymyxin activity may relate to the acyl chains phase of the outer membrane (OM). The enhanced polymyxin susceptibility of B5055 Delta lpxM may be attributable to the favorable insertion of polymyxins into the more fluid OM compared with B5055. Molecular models of the polymyxin B-lipid A complex illuminate the key role of the lipid A acyl chains for complexation of polymyxin. The data provide important insight into the molecular basis for the increased polymyxin susceptibility of K. pneumoniae strains with under-acylated lipid A. Under-acylation appears to facilitate the integration of the N-terminal fatty-acyl chain of polymyxin into the OM resulting in an increased susceptibility to its antimicrobial activity/activities.