Vibrio fischeri and Squid Symbiosis: Bioluminescence Explained

Introduction to Vibrio fischeri and Bioluminescence

Vibrio species are gram-negative rods that are facultative anaerobes found mainly in aquatic environments. They are distinct from the Enterobacteriaceae in that they test positive for oxidase and possess polar flagella. Vibrio are a known cause of gastrointestinal diseases in humans; for example, Vibrio cholerae is the causative agent of cholera (Murray, 1998). However, the luminous bacterium Vibrio fischeri is a waterborne organism found either free-living or in a symbiotic, mutualistic relationship with squid.

Bioluminescence is the chemical production of light within the bodies of primarily marine fish and invertebrates (Sea and Sky, 2010). This chemical reaction occurs when the compound luciferin and oxygen are mixed together in the presence of the enzyme luciferase. The purpose of bioluminescence can be navigation, communication, or camouflage. At the molecular level, this reaction is governed by a well-described genetic system known as the Lux Operon.

Significance of the V. fischeri–Squid Symbiotic Model

The V. fischeri and squid symbiosis is of great historical significance for many reasons. It is a valuable model for examining bioluminescence, pheromone signaling, and symbiotic bacteria–animal interactions (Lyell, 2010). Pheromone signaling, such as quorum sensing, is an important form of bacterial communication. In quorum sensing, small molecules known as autoinducers are exchanged intercellularly (Breitbach, 2010). These molecular messages provide cell-density-dependent cues to bacteria, signaling when to turn on or off certain genes — including the genes responsible for bioluminescence.

The formation of a symbiotic state involves complex "communication" between the bacteria and host (Nyholm, 2008). Study of this relationship offers insights into each organism's well-being as well as the maintenance of a mutualistic relationship as opposed to a pathogenic one. The system has offered numerous opportunities to examine the problems presented by bacteria–host interactions and to understand how each organism overcomes these obstacles (Visick, 2009). The formation of a symbiotic state can help elucidate the healthy condition of organisms and the factors that are disrupted by pathogenic invasion (Chun, 2008). Study of symbiotic relationships can also yield insight into host–pathogen relationships and suggest potential medical targets for research (Breitbach, 2010).

This particular symbiotic system has the added advantage of being replicable in a laboratory setting, which enhances researchers' ability to manipulate and study the entire bacteria–host relationship rather than examining each organism separately in artificial environments. Additionally, since the symbiotic relationship of V. fischeri and squid is affected by environmental factors such as nutrient availability, temperature, and salinity, this system offers a unique perspective on how quickly an organism can adapt to changing environments. This will be especially important in the context of global warming, because the ability to adapt will determine whether an organism survives as its environment potentially changes very rapidly (Nyholm, 2008).

Colonization of the Squid Light Organ

V. fischeri are found free-living in aquatic environments — both freshwater and saltwater habitats, including lakes, rivers, and marine habitats (Nyholm, 2008). Squid draw in seawater through their mantle cavity, which exposes bacteria, including V. fischeri, to the light organ. Squid light organs are known to contain only one to three species of Vibrio or Photobacterium, illustrating the specificity of the squid–bacteria relationship, since other bacteria are constantly ventilated through the mantle and yet fail to colonize the light organ. Remarkably, during each ventilation by a juvenile squid, only one V. fischeri cell may be present in the mantle cavity and has less than one second before being expelled. Despite these great odds, V. fischeri consistently outcompetes other bacteria and successfully colonizes the light organ (Nyholm, 2008).

Although this is a mutually beneficial relationship, the squid maintains many bactericidal defenses within its light organ. First, V. fischeri aggregate on the surface of the light organ in a biofilm. They must then swim against a cilia-induced current through a narrow tube — a process in which motility is extremely important. The environment of this narrow tube contains antibacterial chemicals and toxins that V. fischeri has adapted to overcome. Once inside the light organ crypt, squid immune molecules similar to macrophages — known as hemocytes — protect the crypt space. Somehow, V. fischeri are recognized by hemocytes as "friendly" and are not encapsulated by them in adult squid (Nyholm, 2008).

On a daily basis, the squid expel the majority of V. fischeri from the crypt spaces, and the entire colonization process begins again. It is thought that this expelled population serves as the inoculum for juvenile squid, which are born without bacteria in the light organ. All of this occurs so the squid can exploit V. fischeri's bioluminescent quality: the squid light organ functions as a form of camouflage, preventing predators below from detecting the squid's shadow against moonlight from above (Nyholm, 2008). In return, V. fischeri gain a nutrient-rich environment in which to live.