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J. Exp. Med.,
Volume 188, Number 1, July 1, 1998 49-60
By
From the * Institut für Hygiene und Mikrobiologie, Universität Würzburg, 97080 Würzburg,
Germany; With the aid of monoclonal antibody (mAb) 2625, raised against the lipopolysaccharide (LPS)
of Legionella pneumophila serogroup 1, subgroup OLDA, we isolated mutant 811 from the virulent wild-type strain RC1. This mutant was not reactive with mAb 2625 and exhibited an unstable phenotype, since we observed an in vitro and in vivo switch of mutant 811 to the mAb
2625-positive phenotype, thus restoring the wild-type LPS. Bactericidal assays revealed that mutant 811 was lysed by serum complement components, whereas the parental strain RC1 was
almost serum resistant. Moreover, mutant 811 was not able to replicate intracellularly 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 fourfold. The relevance of LPS phase switch for pathogenesis of Legionella
infection was further corroborated by the observation that 5% of the bacteria recovered from
the lungs of guinea pigs infected with the wild-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 of L. pneumophila. This is
the first description of phase-variable expression of Legionella LPS.
Forschungszentrum Borstel, 23845 Borstel, Germany; § Institut für Medizinische
Mikrobiologie, Medizinische Hochschule, 30625 Hannover, Germany; and Gesellschaft für
Biotechnologische Forschung, 38124 Braunschweig, Germany
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