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The only known required component of the newly described Type XI secretion system (TXISS) is an outer membrane protein (OMP) of the DUF560 family. TXISS OMPs  are broadly distributed across proteobacteria, but properties of the cargo proteins they secrete are largely unexplored. We report biophysical, histochemical, and phenotypic evidence that  Xenorhabdus nematophila  NilC is surface exposed. Biophysical data and structure predictions indicate that NilC is a two-domain protein with a C-terminal, 8-stranded β-barrel. This structure has been noted as a common feature of TXISS effectors and may be important for interactions with the TXISS OMP . The NilC N-terminal domain is more enigmatic, but our results indicate it is ordered and forms a β-sheet structure, and bioinformatics suggest structural similarities to carbohydrate-binding proteins.  X. nematophila  NilC and its presumptive TXISS OMP  partner NilB are required for colonizing the anterior intestine of  Steinernema carpocapsae  nematodes: the receptacle of free-living, infective juveniles and the anterior intestinal cecum (AIC) in juveniles and adults. We show that, in adult nematodes, the AIC expresses a Wheat Germ Agglutinin (WGA)-reactive material, indicating the presence of  N -acetylglucosamine or  N -acetylneuraminic acid sugars on the AIC surface. A role for this material in colonization is supported by the fact that exogenous addition of WGA can inhibit AIC colonization by  X. nematophila . Conversely, the addition of exogenous purified NilC increases the frequency with which  X. nematophila  is observed at the AIC, demonstrating that abundant extracellular NilC can enhance colonization. NilC may facilitate  X. nematophila  adherence to the nematode intestinal surface by binding to host glycans, it might support  X. nematophila  nutrition by cleaving sugars from the host surface, or it might help protect  X. nematophila  from nematode host immunity. Proteomic and metabolomic analyses of wild type  X. nematophila  compared to those lacking  nilB  and  nilC  revealed differences in cell wall and secreted polysaccharide metabolic pathways. Additionally, purified NilC is capable of binding peptidoglycan, suggesting that periplasmic NilC may interact with the bacterial cell wall. Overall, these findings support a model that NilB-regulated surface exposure of NilC mediates interactions between  X. nematophila  and host surface glycans during colonization. This is a previously unknown function for a TXISS.

A Surface Exposed, Two-Domain Lipoprotein Cargo of a Type XI Secretion System Promotes Colonization of Host Intestinal Epithelia Expressing Glycans