NilB is a surface-exposed outer membrane protein of Xenorhabdus nematophila. Expression of NilB is suppressed by NilR and growth in nutrient-rich medium. Members of this family exist in diverse bacteria and are common in the genomes of mucosal pathogens. Bioinformatic analyses reported that NilB is the only characterized member of a family of proteins distinguished by a conserved C-terminal domain of unknown function (DUF560) and N-terminal region tetratricopeptide repeats (TPR). Insertion and deletion mutational analyses revealed that NilB forms a ß barrel with 14 transmembrane strands and seven extracellular surface loops, and an N-terminal globular domain. The globular domain and surface loop 6 have a crucial role in the nematode colonization. Epifluorescence microscopy of these mutants revealed that NilB is necessary at early stages of colonization.