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Francisella tularensis , the Gram-negative bacterium that causes tularemia, produces a high molecular weight capsule that is immunologically distinct from  Francisella  lipopolysaccharide but contains the same O-antigen tetrasaccharide. To pursue the possibility that the capsule of  Francisella  live vaccine strain (LVS) has a structurally unique lipid anchor, we have metabolically labeled  Francisella  with [ 14 C]acetate to facilitate highly sensitive compositional analysis of capsule-associated lipids. Capsule was purified by two independent methods and yielded similar results. Autoradiographic and immunologic analysis confirmed that this purified material was largely devoid of low molecular weight LPS and of the copious amounts of free lipid A that the Francisellae accumulate. Chemical hydrolysis yielded [ 14 C]-labeled free fatty acids characteristic of  Francisella  lipid A but with a different molar ratio of 3-OH C18:0 to 3-OH C16:0 and different composition of non-hydroxylated fatty acids (mainly C14:0 rather than C16:0) than that of free  Francisella  lipid A. Mild acid hydrolysis to induce selective cleavage of KDO-lipid A linkage yielded a [ 14 C]-labeled product that partitioned during Bligh/Dyer extraction and migrated during thin-layer chromatography like lipid A. These findings suggest that the O-antigen capsule of  Francisella  contains a covalently linked and structurally distinct lipid A species. The presence of a discrete lipid A-like molecule associated with capsule raises the possibility that  Francisella  selectively exploits lipid A structural heterogeneity to regulate synthesis, transport, and stable bacterial surface association of the O-antigen capsular layer.

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Evidence Suggesting That Francisella tularensis O-Antigen Capsule Contains a Lipid A-Like Molecule That Is Structurally Distinct from the More Abundant Free Lipid A