From the aggregates, the V. fischeri cells migrate through LO pores, reaching ducts that ultimately lead into crypts, the sites of colonization (Fig. 1). In the ducts, the bacteria must move through mucus against an outward current generated by ciliated cells lining the passageway (McFall-Ngai and Ruby 1998). As a further barrier to colonization, the ducts contain high levels of nitric oxide, an anti-microbial agent that may function as a layer of defense against invasion by non-specific bacteria (Davidson et al. 2004). In the crypts, V. fischeri cells may encounter macrophage-like cells, a potential immune surveillance system (Nyholm and McFall-Ngai 1998). In addition, the bacteria may be exposed to toxic oxygen radicals such as hypohalous acid, produced by a halide peroxidase enzyme secreted by epithelial cells within the crypts (Weis et al. 1996; Small and McFall-Ngai 1999). Despite this plethora of potential host defenses, V. fischeri cells can enter the LO and grow to high cell density, approximately 1011 cells/cm3 (Visick and McFall-Ngai 2000). Thus, V. fischeri must possess mechanisms by which it can evade host defenses and thrive in the LO environment.
Growth to high cell density does not represent the endpoint of the symbiosis. Rather, the symbiosis is dynamic. Each morning the squid expels between 90 and 95% of the bacterial population from its LO (Lee and Ruby 1994a). During the day, the V. fischeri cells retained in the squid divide to repopulate the LO. Therefore, persistent colonization actually consists of cycles of expulsion and re-growth, requiring the symbiotic bacteria to adapt to changing environments within the LO.
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