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Abstract
Guillain-Barré Syndrome (GBS) is the leading cause of paralysis since the near eradication of polio, resulting from an autoimmune reaction where self-reactive antibodies are directed against gangliosides on the body’s own nerve cells. By mimicking GM1 gangliosides using its lipooligosaccharide (LOS) structures, Campylobacter jejuni is known to trigger GBS. The LOS structures also provide receptors for bacterially produced AB5 toxins, resulting in growth inhibition that is related to an increase in cell permeability. This selection pressure prompts C. jejuni LOS phase-variation and likely contributes to competition between gut pathogens. Through investigating this phenomenon, additional GM1-mimicking bacteria were detected in the chicken cecum by fluorescence microscopy and decreases in the Firmicutes phylum were observed after feeding AB5 toxins to chickens. From this we concluded that AB5 toxins may have a broader role in competition with the gut microbiota beyond C. jejuni. We also hypothesized that a consistent presence of GM1-mimicking gut microbes could influence the type of immune response directed against C. jejuni infections; especially if individuals are exposed to ganglioside-mimicking bacteria early in life during immune development. By screening 154 infant C. jejuni fecal isolates from endemic nations, we showed that this exposure does indeed occur. Experiments with human macrophage THP-1 cells showed that training with low doses of ganglioside-mimicking bacteria reduces the release of pro-inflammatory cytokines upon subsequent challenge with C. jejuni that display GBS antigens. In our attempts to identify commensal ganglioside-mimicking bacteria, we isolated three Enterococcus species from the chicken gut and determined that they produce glycoproteins reactive with α-GM1 antibodies. The full genomes of these isolates were sequenced and two of their glycoproteomes were analyzed. This revealed numerous glycosylated proteins in each species including several which contain HexNAc-Hex modifications possibly involved in asialo-GM1 ganglioside-mimicry. The discovery of these glycoproteins suggests a general O-linked protein-glycosylation system in the Enterococcus genus and identifies a new protein glycosylation system in Gram-positive bacteria. Overall, this project has shed new light on the complexity of ganglioside-mimicking bacteria and their interactions with competing organisms as well as the host.