PHASE-VARIABLE EXPRESSION OF LIPOPOLYSACCHARIDE CONTRIBUTES TO THE VIRULENCE OF LEGIONELLA-PNEUMOPHILA

With the aid of monoclonal antibody (mAb) 2625, raised against the lip opolysaccharide (LPS) of Legionella pneumophila serogroup 3, subgroup OLDA, we isolated mutant 811 from the virulent wild-type strain RC1. T his mutant was not reactive with mAb 2625 and exhibited an unstable ph enotype, since we observed all in vitro and in vivo switch of mutant 8 11 to the mAb 2625-positive phenotype, thus restoring:he wild-type LPS , Bactericidal assays revealed that mutant 811 was lysed by serum comp lement components, whereas the parental strain RC1 was almost serum re sistant. Moreover, mutant 811 was not able to replicate intracellularl y in macrophage-like cell Line HL-60. In the guinea pig animal model, mutant 811 exhibited significantly reduced ability to replicate, Among recovered bacteria, mAb 2625-positive revertants were increased by fo urfold. The relevance of LPS phase switch for pathogenesis of Legionel la infection was further corroborated by the observation that 5% of th e bacteria recovered from the lungs of guinea pigs infected with the w ild-type strain RC1 were negative for mAb 2625 binding. These findings strongly indicate that under in vivo conditions switching between two LPS phenotypes occurs and may promote adaptation and replication oil. I,pneumophila. This is the first description of phase-variable expres sion of Legionella LPS.