The outer membrane of Gram-negative bacteria: lipopolysaccharide biogenesis and transport

The cell envelope of Gram-negative bacteria consists of two distinct membranes, the inner (IM) and the outer membrane (OM) separated by an aqueous compartment, the periplasm. The OM contains in the outer leaflet the lipopolysaccharide (LPS), a complex glycolipid with important biological functions. In the host, it elicits the innate immune response whereas in the bacterium it is responsible for the peculiar permeability barrier properties exhibited by the OM. LPS is synthesized in the cytoplasm and at the inner leaflet of the IM. It needs to cross two different compartments, the IM and the periplasm, to reach its final destination at the cell surface. In this chapter we will first summarize LPS structure, functions and biosynthetic pathway and then review in more details the studies that have led in the last decade to elucidate the protein machinery that ferries LPS from the IM to its final destination in the O