Insights into flagellar function and mechanism from the squid-vibrio symbiosis

Abstract

Flagella are essential and multifunctional nanomachines that not only move symbionts towards their tissue colonization site, but also play multiple roles in communicating with the host. Thus, untangling the activities of flagella in reaching, interacting, and signaling the host, as well as in biofilm formation and the establishment of a persistent colonization, is a complex problem. The squid-vibrio system offers a unique model to study the many ways that bacterial flagella can influence a beneficial association and, generally, other bacteria-host interactions. Vibrio fischeri is a bioluminescent bacterium that colonizes the Hawaiian bobtail squid, Euprymna scolopes. Over the last 15 years, the structure, assembly, and functions of V. fischeri flagella, including not only motility and chemotaxis, but also biofilm formation and symbiotic signaling, have been revealed. Here we discuss these discoveries in the perspective of other host-bacteria interactions.

Keywords: Cellular microbiology; Symbiosis.

Fig. 1

‘Clock’ illustrating the temporal unfolding of initiation events during the first 4 days of symbiosis. Within 0.5 h of hatching, the host’s ciliated field responds to exposure to ambient bacterial PGN by shedding mucus (light green) in which V. fischeri (gray, flagellated cells) can attach, and then aggregate. Within 4–6 h, the aggregate migrates to the pores, and uses swimming motility and chemotaxis to follow a chemoattractant gradient (dark blue) to enter the surface pores, reaching the deep crypts by between 7 and 10 h. By 12–18 h, the bacteria have multiplied, filling the crypts and inducing bioluminescence (yellow). At dawn, 95% of the bacteria are expelled (red arrow) and the remaining 5% proliferate, starting a new day/night cycle. The presence of the symbionts triggers the development of the light organ, including the full regression of the ciliated field and appendages over the first four days

Fig. 2

Flagellar structure of V. fischeri. a Electron-microcopy image of the tuft of polar flagella of V. fischeri (b) Schematic diagram of the hook and basal body of a polar flagellum. The bacterial flagellum consists of three parts: the filament, the hook and the complex basal body embedded in the outer membrane (OM), inner membrane (IM), and peptidoglycan (PG). The rings present in the basal body that are specific to Vibrio spp. (T-ring and H-ring) are indicated in green, and the OM-derived sheath is depicted in dark gray

Fig. 3

Chemotaxis in the squid–vibrio symbiosis. a Representative methyl-accepting chemotaxis protein (MCP) topology, including the ligand-binding domain, the HAMP domain, and methylation domain. Triangles represent potential ligands. b Simplified scheme of the chemotaxis signal-transduction cascade in Vibrio spp. Changes in chemoeffector levels are detected by the transmembrane MCP chemoreceptors. The resulting signal cascade starts with the SH3-like adaptor protein CheW, and passes to the CheA histidine kinase. In response to decreased chemoeffector concentration, the chemoreceptors activate CheA autophosphorylation. Phosphorylated CheA (CheA-P) phosphorylates CheY. CheY-P binds to the flagellar motor and promotes a switch in the direction of rotation from counterclockwise to clockwise. Finally, CheZ dephosphorylates CheY-P, allowing rapid termination of the signal response and re-setting of the pathway. In the absence of chemoeffector, CheA-P favors counterclockwise rotation. c Chemoeffectors secreted by the squid tissues establish a gradient (blue) extending out from the pores.^, V. fischeri cells are attracted by this gradient, and migrate through the pores to their final site of colonization, the deep crypts

Fig. 4

The flagellum as a platform for host–microbe interactions. Features of the flagellum and flagellar activity that contribute to host–symbiont interaction. In the squid host, epithelial cells and hemocytes can phagocytose OMVs (red) containing LPS and PGN (gray), which will, in turn, trigger steps in the host’s developmental morphogenesis. Soluble, released PGN monomer (TCT for Tracheal CytoToxin; blue) is also presented here, because it is an alternative form in which to present PGN fragments to the host, inducing morphogenesis. At the same time, squid homologs of TLRs may detect flagellins in fragments of the filament (green), and stimulate the immune system via the NF-κB pathway