Isolation of a Serogroup I-Specific Antigen from Legionella pneumophila

Abstract

Outer membrane material was extracted from a serogroup 1 strain of Legionella pneumophila using ethylenediaminetetraacetate. Ferritin-conjugated antiserum reacted only at the surface of the organism, as seen by electron microscopy. Outer membrane material was fractionated into five peaks by molecular sieve chromatography using a gel that had been equilibrated in a buffer containing sodium deoxycholate. One resolved peak (∼4 x 10⁴ daltons) was highly active serologically. When rechromatographed in the absence of sodium deoxycholate, material from this peak reaggregated to ∼10⁶ daltons. The serologic activity of this antigen was restricted to L. pneumophila serogroup 1, although minor cross-reactions with strains of serogroups 2 and 4 were detected using an enzyme-linked immunosorbent assay. The antigen was <10% carbohydrate, 15% protein, 1.1% phosphate, and the remainder lipid of unknown composition. Neither 2-keto-3-deoxyoctonate nor heptose was detected, and the antigen did not induce a Shwartzman reaction. Only one band was seen on sodium dodecyl sulfate-polyacrylamide gel electrophoresis.