D-089. Identification of htrB Genes in Moraxella catarrhalis Lipooligosaccharide Biosynthesis
Lipid A is the endotoxically active component of the lipooligosaccharide (LOS) of Moraxella catarrhalis. No other acyltransferases except UDP-GlcNAc acyltransferase responsible for lipid A biosynthesis in M. catarrhalis have been elucidated till now. By informatics,htrB1 and htrB2 genes for two of the late acyltransferases responsible for lipid A biosynthesis were selected, and each knockout mutant in M. catarrhalis serotype A strain O35E was constructed and named as O35EhtrB1 and O35EhtrB2. Structural analysis of lipid A from the parental and derived mutants showed that O35EhtrB1 and O35EhtrB2 were deprived of decanoic acid (C10:0) and dodecanoic (lauric) acid (C12:0), respectively, suggesting that htrB1 gene encoded decanoyl transferase while htrB2 gene encoded dodecanoyl transferase. Phenotypic analysis revealed that both mutants were similar to the parental strain in their toxicity in vitro. However, the O35EhtrB1 was sensitive to bactericidal activity of normal human serum and hydrophobic reagents. It had a reduced growth rate in BHI broth and an accelerated bacterial clearance at 3h (P <0.01) or 6h (P <0.05) after an aerosol challenge in a murine model of bacterial pulmonary clearance. Meanwhile, the O35EhtrB2 presented similar phenotypic properties to the parental strain except it was slightly sensitive to the hydrophobic reagents. These results revealed that the htrB genes, particularly htrB1, for two late acyltransferases responsible for the acyloxyacyl linked secondary acyl chains of the lipid A biosynthesis were important for some biological activities of M. catarrhalis.