The lipopolysaccharide core oligosaccharide of plays a critical role in maintaining a proper gut symbiosis with the bean bug .

Lipopolysaccharide, the outer cell-wall component of Gram-negative bacteria, has been shown to be important for symbiotic associations. We recently reported that the lipopolysaccharide O-antigen of enhances the initial colonization of the midgut of the bean bug, However, the midgut-colonizing symbionts lack the O-antigen but display the core oligosaccharide on the cell surface. In this study, we investigated the role of the core oligosaccharide, which directly interacts with the host midgut, in the symbiosis. To this end, we generated the core oligosaccharide mutant strains, Δ, Δ, Δ and Δ and determined the chemical structures of their oligosaccharides, which exhibited different compositions. The symbiotic properties of these mutant strains were compared with those of the wild-type and O-antigen-deficient Δ strains. Upon introduction into via the oral route, the core oligosaccharide mutant strains exhibited different rates of colonization of the insect midgut. The symbiont titers in fifth-instar insects revealed significantly reduced population sizes of the inner core oligosaccharide mutant strains Δ and Δ These two strains also negatively affected host growth rate and fitness. Furthermore, individuals colonized with the Δ and Δ strains were vulnerable to septic bacterial challenge, similar to insects without a symbiont. Taken together, these results suggest that the core oligosaccharide from symbionts plays a critical role in maintaining a proper symbiont population and in supporting the beneficial effects of the symbiont on its host in the symbiosis.