Previously unrecognized stages of species‐specific colonization in the mutualism between X enorhabdus bacteria and S teinernema nematodes

Summary The specificity of a horizontally transmitted microbial symbiosis is often defined by molecular communication between host and microbe during initial engagement, which can occur in discrete stages. In the symbiosis between S teinernema nematodes and X enorhabdus bacteria, previous investigations focused on bacterial colonization of the intestinal lumen (receptacle) of the nematode infective juvenile ( IJ ), as this was the only known persistent, intimate and species‐specific contact between the two. Here we show that bacteria colonize the anterior intestinal cells of other nematode developmental stages in a species‐specific manner. Also, we describe three processes that only occur in juveniles that are destined to become IJs . First, a few bacterial cells colonize the nematode pharyngeal‐intestinal valve ( PIV ) anterior to the intestinal epithelium. Second, the nematode intestine constricts while bacteria initially remain in the PIV . Third, anterior intestinal constriction relaxes and colonizing bacteria occupy the receptacle. At each stage, colonization requires X . nematophila symbiosis region 1 ( SR 1) genes and is species‐specific: X . szentirmaii , which naturally lacks SR1, does not colonize unless SR 1 is ectopically expressed. These findings reveal new aspects of X enorhabdus bacteria interactions with and transmission by their S teinernema nematode hosts, and demonstrate that bacterial SR 1 genes aid in colonizing nematode epithelial surfaces.