Influence of lipopolysaccharide outer-core in the intrinsic resistance to antimicrobial peptides and virulence in Edwardsiella ictaluri.

The genus Edwardsiella consists of bacteria with an intrinsic resistance to cyclic cationic antimicrobial peptides (CAMPs). Edwardsiella ictaluri, a pathogen of the catfish (Ictalurus punctatus) and the causative agent of a systemic infection, is highly resistant to CAMPs. Previously, we determined that the oligo-polysaccharide (O-PS) of the lipopolysaccharide (LPS) does not play a role in the E. ictaluri CAMP resistance and an intact core-lipid A structure is necessary for CAMPs resistance. Here, we evaluated the influence of the outer-core in the CAMPs resistance and fish virulence. E. ictaluri wabG, a gene that encodes for the UDP-glucuronic acid transferase that links the lipid A-inner-core to the outer-core-oligopolysaccharides, was deleted. Deletion of ΔwabG caused a pleiotropic effect, influencing LPS synthesis, CAMPs resistance, growth, and biofilm formation. E. ictaluri ΔwabG was attenuated in zebrafish indicating the important role of LPS during fish pathogenesis. Also, we evaluated the inflammatory effects of wabG LPS in catfish ligated loop model, showing a decreased inflammatory effect at the gut level respects to the E. ictaluri wild type. We conclude that E. ictaluri CAMPs resistance is related to the molecules present in the LPS outer-core and that fish gut inflammation triggered by E. ictaluri is LPS dependent, reinforcing the hypothesis that fish gut recognizes LPS in an O-PS dependent fashion.

Inflammatory effects of Edwardsiella ictaluri lipopolysaccharide modifications in catfish gut.

Bacterial lipopolysaccharides (LPS) are structural components of the outer membranes of Gram-negative bacteria and also are potent inducers of inflammation in mammals. Higher vertebrates are extremely sensitive to LPS, but lower vertebrates, like fish, are resistant to their systemic toxic effects. However, the effects of LPS on the fish intestinal mucosa remain unknown. Edwardsiella ictaluri is a primitive member of the Enterobacteriaceae family that causes enteric septicemia in channel catfish (Ictalurus punctatus). E. ictaluri infects and colonizes deep lymphoid tissues upon oral or immersion infection. Both gut and olfactory organs are the primary sites of invasion. At the systemic level, E. ictaluri pathogenesis is relatively well characterized, but our knowledge about E. ictaluri intestinal interaction is limited. Recently, we observed that E. ictaluri oligo-polysaccharide (O-PS) LPS mutants have differential effects on the intestinal epithelia of orally inoculated catfish. Here we evaluate the effects of E. ictaluri O-PS LPS mutants by using a novel catfish intestinal loop model and compare it to the rabbit ileal loop model inoculated with Salmonella enterica serovar Typhimurium LPS. We found evident differences in rabbit ileal loop and catfish ileal loop responses to E. ictaluri and S. Typhimurium LPS. We determined that catfish respond to E. ictaluri LPS but not to S. Typhimurium LPS. We also determined that E. ictaluri inhibits cytokine production and induces disruption of the intestinal fish epithelia in an O-PS-dependent fashion. The E. ictaluri wild type and ΔwibT LPS mutant caused intestinal tissue damage and inhibited proinflammatory cytokine synthesis, in contrast to E. ictaluri Δgne and Δugd LPS mutants. We concluded that the E. ictaluri O-PS subunits play a major role during pathogenesis, since they influence the recognition of the LPS by the intestinal mucosal immune system of the catfish. The LPS structure of E. ictaluri mutants is needed to understand the mechanism of interaction.