Divining the essence of symbiosis: insights from the squid-vibrio model

Abstract

Biology has a big elephant in the room. Researchers are learning that microorganisms are critical for every aspect of the biosphere's health. Even at the scale of our own bodies, we are discovering the unexpected necessity and daunting complexity of our microbial partners. How can we gain an understanding of the form and function of these "ecosystems" that are an individual animal? This essay explores how development of experimental model systems reveals basic principles that underpin the essence of symbiosis and, more specifically, how one symbiosis, the squid-vibrio association, provides insight into the persistent microbial colonization of animal epithelial surfaces.

Figure 1. The adult host Euprymna scolopes, the Hawaiian bobtail squid.

The human hand at the right offers an idea of the animal size.

Figure 2. The recruitment of the symbiont Vibrio fischeri from the environment.

Upper, left: the nascent light organ (black) can be seen through the body wall (white arrow) of the living juvenile animal; right: a diagram of a ventral view reveals the position of the light organ in the center of the mantle cavity. During ventilation, water that is rich with environmental bacteria passes over the organ (blue arrows). Middle, confocal microscopy of the juvenile organ reveals complex ciliated fields on either side that shed copious amounts of mucus. V. fischeri aggregates above pores on the surface (dashed circles). Lower, left: a dense field of cilia with a central pore, where V. fischeri cells will enter host tissues; right, aggregated V. fischeri cells entering host tissues. RFP, red fluorescent protein.

Figure 3. Colonization by the symbiont.

Left, one half of the light organ illustrating the path of V. fischeri into the host tissues. Middle, confocal micrograph of green fluorescent protein (GFP)-labeled V. fischeri cells in a host crypt. Right, a transmission electron micrograph of V. fischeri associating with dense microvilli along the apical surfaces of host epithelia.