Vibrio fischeri: The bacterial Light Sabers of the Ocean
Updated: Jul 13, 2021
I'd like you all to meet the bobtail squid. This cute little guy, has formed a bond ,or has become a symbiont, with some microbial friends, particularly the prokaryote, or bacteria, Vibrio fischeri. Like a Jedi's light saber, V. fischeri helps protect it's host. The Force is strong between V. fischeri and the bobtail squids....find out more below!
Bioluminescence Of Vibrio fischeri
From looking at the light emitted by stars and imagining new worlds, to the holiday lights that bring us closer together and remind us of warmth and compassion, to the celebratory boom of fireworks on the Fourth of July, light is a symbol of hope, compassion, imagination, and survival.
We are highly dependent on light, much of which is driven by electricity, but can also be produced by living organisms, in a process we call bioluminescence.
Today, we are going to talk about a very specific bioluminescence dependent symbiotic relationship; the relationship between the Hawaiian bobtail squid and Vibrio fischeri.
Unlike our relationship with light, the Hawaiian bobtail squid doesn’t use light as a way to be seen, but as a way to camouflage. This squid is not the only one using microbes to evade predators; several other squid species and some fish, including the Japanese bobtail squid and the Monocentris japonicus, foster this Vibrio fischeri relationship to produce light.
The definition of symbiosis is a relationship where both partners benefit. It’s one of the most interesting relationships within the microbial world and is seen in all environments. From humans, to plants, to our oceans, microbes have established positive relationships with their macro-neighbors.
Each Symbiotic relationship is unique, but there are also a lot of similarities between them; think of it like a government. Across the world each government is different but fundamentally a government is in place to do three main things, (i) to protect its citizens, (ii) to provide resources and (iii) invest in the future of those citizens and the government's success.
In return, the citizens are responsible to obey the laws set by the government and support the government, often through a system like taxes. Now everyone has a love hate relationship with the government for one reason or another, things don’t always work out for the best interest of all citizens and this is also true for our microbial symbionts.
In the microbial world the government is the host and the citizens are the microbes. The host will try to sanction who can and cannot enter as well as give incentives like nutrients to attract certain kinds of microbes. Some microbes have learned clever ways to beat the blockade that the host puts up and sneak into the host.
The host protects and provides a safe place for the microbes to reside, while the microbes provide nutrients or other services back to the host.
In this symbiotic example, the Hawaiian bobtail squid is the government and the Vibrio fischeri are the citizens; they are allowed to enter the country and are given a place to live, food, and protection. Vibrio fischeri start colonizing the baby squids as soon as they hatch and nestle down into nutrient-dense crypts. They communicate with the squids through the type IV pili. Once the squid gets this signal, the light organ will begin to develop.
So these two organisms, the squid and V. fischeri are truly connected. The squid will not fully develop without its microbial symbionts. When they’ve settled into their new home, V. fischeri will produce a soft blue glow, which researchers believe is to help disguise the squid, allowing them to evade the sight of other foes.
The Dark Side Of Symbiosis
This is a pretty cushy gig, a house constantly full of food and all you have to do is produce a bit of light, not bad for a microbe in the deep dark sea. As you can imagine, competition is rather high to get a squatter spot within the squid. The symbiosis between the Hawaiian bobtail squid and Vibrio fischeri has been captivating the minds of scientists for decades, but it’s not all hugs and kisses between these partners. Sometimes you have to get aggressive to secure a cushy gig like this.
The squid’s light organ is rather small and each entry point (crypts) only emits 1-2 cells per crypt. To overcome this obstacle, some Vibrio fischeri employ the use of toxins to out compete other microbes. The toxins are administered through a harpoon like mechanism called a type VI secretion system (T6SS).
T6SS is found in approximately 25% of all Gram negative bacteria, but historically humans have only associated a T6SS with pathogens. For instance, in the related species V. cholerae, the causal agent of cholera, the T6SS system can be used to eliminate natural microbes in your gut, reducing competition for V. cholerae and exacerbating inflammation, promoting the onset of the disease. The fact that it exists in a symbiotic organism is very intriguing and shows nothing is truly entirely evil nor good, but merely aiming for survival.
Turning On The Lights
So once the squid hatches, and V. fischeri battles its way to a new home, where does this light come from and how is it turned on? It’s all due to quorum sensing, a bacterial communication network. One Vibrio fischeri can not produce bio-luminescence, but when the population gets big enough, they start producing communication molecules, called auto inducers, telling the others, “hey I’m over here”. At the core of this light inducing group behavior is the enzyme luciferase which reacts with other elements in the environment to form light.
With just 1-2 cells admitted into the crypt at a time how do V. fisheri reach a high enough population to have quorum? Vibrio are rapid reproducers. Once inside the crypt they can reproduce remarkably fast all while releasing their auto inducers (the communication molecule). When they have enough members they can now reach a quorum, a legal term meaning the minimum number of members needed to make a meeting valid.
At this point, they collectively change their behavior, which in bacteria is done by activating certain genes, and act in unity to form bioluminescence. United they stand but divided they fall.
The genus, Vibrio, is so fascinating! This is just one snippet of the Vibrio story, a story which we will explore more in the future. Some Vibrio are pathogenic to humans, causing disease like cholera and gastroenteritis while others, like V. fischeri, form symbiotic relationships and produce a beautiful and captivating glow of bio-luminescence.