Constitutive Phenotypic Modification of Lipid A in Clinical Acinetobacter baumannii Isolates

The degree of polymyxin B (PMB) resistance was measured in 40 clinical Acinetobacter baumannii isolates obtained from health care facilities. All of the tested isolates possessed a multidrug-resistant (MDR) phenotype against four classes of antibiotics (meropenem, doxycycline, gentamicin, and erythromycin), except for PMB. The blaOXA-23 gene was detected throughout the genetic analysis and experimental assay, indicating that all of the MDR strains were carbapenem-resistant A. baumannii strains. Multilocus sequence typing-based genotyping revealed that nine selected strains belonged to the international clone II lineage. When matrix-assisted laser desorption ionization–time of flight mass spectrometry was performed, intrinsic lipid A modification by phosphoethanolamine (PEtN) incorporation was noticeable only in the PMB-resistant (PMBR) strains. However, the presence of hexa- and penta-acylated lipid A due to the loss of the laurate (C12) acyl chain was noted in all PMB-susceptible strains but not in the PMBR strains. The reduction of negative surface charges in the PMBR strains was assessed by zeta potential analysis. Fluorescence imaging using dansyl-PMB revealed that, in the PMBR strains, PMB was less likely to bind to the cell surface. IMPORTANCE The widespread presence of MDR pathogens, including A. baumannii, is causing serious hospital-acquired infections worldwide. Extensive surveillance of MDR clinical A. baumannii isolates has been conducted, but the underlying mechanisms for their development of MDR phenotypes are often neglected. Either lipid A modification or loss of lipopolysaccharide in Gram-negative bacteria leads to PMBR phenotypes. The prevalence of intrinsic lipid A modification in PMBR clinical strains was attributed to high levels of basal expression of pmrC and eptA-1. Our findings suggest that new therapeutic strategies are warranted to combat MDR pathogens due to the emergence of many PMBR clinical strains.