Lipopolysaccharide biosynthesis and traffic in the envelope of the pathogen Brucella abortus

Pathogenic Brucella abortus, containing a mix of lipopolysaccharides with or without O-antigen, grows its envelope in a unipolar manner. Here, Servais et al, localize the LPS translocation machinery and identify the main O-antigen ligase in Brucella species, shedding light on the basic biology of this organism. Lipopolysaccharide is essential for most Gram-negative bacteria as it is a main component of the outer membrane. In the pathogen Brucella abortus, smooth lipopolysaccharide containing the O-antigen is required for virulence. Being part of the Rhizobiales, Brucella spp. display unipolar growth and lipopolysaccharide was shown to be incorporated at the active growth sites, i.e. the new pole and the division site. By localizing proteins involved in the lipopolysaccharide transport across the cell envelope, from the inner to the outer membrane, we show that the lipopolysaccharide incorporation sites are determined by the inner membrane complex of the lipopolysaccharide transport system. Moreover, we identify the main O-antigen ligase of Brucella spp. involved in smooth lipopolysaccharide synthesis. Altogether, our data highlight a layer of spatiotemporal organization of the lipopolysaccharide biosynthesis pathway and identify an original class of bifunctional O-antigen ligases.