Role of Mrx Fimbriae of Xenorhabdus nematophila in Competitive Colonization of the Nematode Host

Abstract

ABSTRACTXenorhabdus nematophilaengages in mutualistic associations with the infective juvenile (IJ) stage of specific entomopathogenic nematodes. Mannose-resistant (Mrx) chaperone-usher-type fimbriae are produced when the bacteria are grown on nutrient broth agar (NB agar). The role of Mrx fimbriae in the colonization of the nematode host has remained unresolved. We show thatX. nematophilagrown on LB agar produced flagella rather than fimbriae. IJs propagated onX. nematophilagrown on LB agar were colonized to the same extent as those propagated on NB agar. Further, progeny IJs were normally colonized bymrxmutant strains that lacked fimbriae both when bacteria were grown on NB agar and when coinjected into the insect host with aposymbiotic nematodes. Themrxstrains were not competitively defective for colonization when grown in the presence of wild-type cells on NB agar. In addition, a phenotypic variant strain that lacked fimbriae colonized as well as the wild-type strain. In contrast, themrxstrains displayed a competitive colonization defectin vivo. IJ progeny obtained from insects injected with comixtures of nematodes carrying either the wild-type or themrxstrain were colonized almost exclusively with the wild-type strain. Likewise, when insects were coinjected with aposymbiotic IJs together with a comixture of the wild-type andmrxstrains, the resulting IJ progeny were predominantly colonized with the wild-type strain. These results revealed that Mrx fimbriae confer a competitive advantage during colonizationin vivoand provide new insights into the role of chaperone-usher fimbriae in the life cycle ofX. nematophila.