Phase Variation in Legionella pneumophila Serogroup 1, Subgroup OLDA, Strain RC1 Influences Lipid A Structure

A change in the lipopolysaccharide (LPS) upon phase variation in Legionella pneumophila serogroup 1, subgroup OLDA, wild-type strain RC1 was detected with the aid of LPS-specific monoclonal antibody MAb 2625. Chromosomal insertion and excision of a 30-kb instable genetic element of possibly phage origin was identified as the molecular mechanism for phase variation. The core of the LPS is a nonasaccharide that lacks heptose and phosphate, contains abundant 6-deoxy sugars, and is highly O- and N-acetylated. The MAb 2625 epitope is present in wild-type RC1 cells and is lost in the spontaneous mutant 811 upon phase variation. Lipid A was prepared by mild acid hydrolysis (0.1 M NaOAc-HOAc buffer, pH 4.4,100№C, 4 h) of LPS each of wild-type RC1 and phase-variant 811 followed by centrifugation and lyophilization of the pellet. LPS biosynthesis pathways involved in assembly of lipid A were also affected by phase variation, which resulted in a specifically altered lipid A structure with a modified profile of fatty acids of a particular type. Phase variation may affect a regulatory factor, which influences LPS biosynthesis, virulence, and serum resistance.