Complete lipooligosaccharide structure of the clinical isolate Acinetobacter baumannii, strain SMAL

Acinetobacter baumannii is a pathogenic organism that posesses a serious health threat because of the occurrence of the large number of (multi)drug-resistant strains. It can persist for prolonged periods in the hospital environment, infecting debilitated or immune-compromised patients. In this context, the endotoxin portion of the lipopolysaccharide, the lipid A, plays an important role in the pathogenesis of this bacterium, because it triggers the innate immune response and contributes to the inflammation state of the patient. In this study, the complete structure of the lipooligosaccharide has been determined. The saccharide backbone of the molecule was disclosed through chemical and spectroscopic analysis, whereas the lipid A moiety required detailed MS spectrometry and chemical investigations. The oligosaccharide backbone was found to be similar to that of A. baumannii ATCC 19606, although the LOS from the SMAL strain presented an enhanced zwitterionic character. The lipid A moiety comprises a heterogeneous and complex mixture of molecules: it possesses a conserved diphosphorylated disaccharide backbone substituted by three to seven fatty acids. The hexaacylated species appeared as the most abundant, and its chemical features, namely the number and the types of fatty acids, indicates its potential endotoxic activity.