Figure 2: Update to the winnowing model of colonization.

Initial colonization of the light organ involves a number of biochemical and biomechanical mechanisms to establish the partnership between the Hawaiian bobtail squid, Euprymna scolopes, and the bioluminescent bacterium, Vibrio fischeri. The habitat transition of V. fischeri from the bacterioplankton to light organ symbiont involves several features that help facilitate colonization. The nascent light organ is made up of ciliated fields (inset; 1/2 of a light organ as shown by a scanning electron micrograph) that are composed of metachronally and randomly beating cilia that create microcurrents (arrows) that help to focus bacteria-sized particles above three pores on either side of the light organ in a shelter zone. The host increases expression of endochitinase and secretes mucus that contains a number of biochemical factors including chitobiose, a symbiont chemoattractant and a suite of host immunity factors that may serve to inhibit other bacteria. This unique microenvironment selects for V. fischeri while preventing colonization by non-symbiotic bacteria. V. fischeri cells that enter the light organ must traverse a unique biogeography that includes ciliated ducts, an antechamber and a bottleneck. A single V. fischeri cell (or on occasion several cells) migrate into each of the three crypts where they become nonmotile and divide and grow until a cell density is reached that enables the induction of quorum sensing and light production (approximately 9–12 h). Colonization also initiates cellular changes in the host, including the induction of haemocyte migration into the host ciliated fields along with apoptosis and regression of the ciliated field four days after colonization. lipopolysaccharide binding protein, LBP; bacteriocidal permeability-increasing protein, BPI; nitric oxide synthase, NOS; nitric oxide; NO, peptidoglycan recognition protein 2, PGRP2. Image credit: REF¹¹⁷