Vibrio splendidus lipopolysaccharide: an evolutionary compromise between virulence in oysters and resistance to grazers

While infectious agents might be expected to evolve to a non-virulent status for their hosts, this is often not the case. Virulence is frequently maintained because it is linked to adaptive advantages for the parasite, a situation associated with an evolutionary trade-off between transmission and virulence. Furthermore, in the environment, the virulence traits of nonobligate parasites are subject to various selection pressures. Various Vibrio species that colonize oysters are virulent. Here, we focused on the Vibrio splendidus species, which carries virulence factors, yet expresses moderate virulence in oysters. Daniel Oyanadel and his colleagues from the "Transmission, Resistance and Virulence" team of the IHPE, have identified during a national collaboration a genomic region (wbe) involved in the synthesis of lipopolysaccharide (LPS), a major component of the outer membrane, which contributes to this lower virulence. They show that certain LPS structures promote resistance to environmental grazers. However, these structures further activate the oyster immune system, resulting in reduced virulence in this host. These results indicate an evolution of V. splendidus towards moderate virulence, in a compromise between the ability to colonize the oyster as a host, and resistance to its environmental grazers. It is likely that this compromise contributes to the high genomic plasticity of the wbe region observed in this species.