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  • Alex Cloherty

Microbial Buddies: Part I

On this blog, you've already heard a lot about how bacteria and viruses can benefit humans. For instance, we need our microbial buddies for productive digestion. However, we're not the only eukaryotes (complex cells with a nucleus - i.e. not bacteria, archaea, or viruses) who make friends with our microbes! Today we start a three-part series about other complex organisms that also work together with bacteria or viruses.

Today, in Part I of the Microbial Buddies series, we'll explore bioluminescence, the generation of light by living creatures. To do this, we will use a pretty cool model organism: The Hawaiian bobtail squid, scientifically known as Euprymna scolopes. To study phenomena like bioluminescence, scientists often choose one animal in particular (the model organism) to work with, like the famous lab rat. Often this choice is as much due to the fact that the organism is easy to grow and cheap, as well as the fact that it has the trait that scientists are looking to study. That's why rats, worms, and houseflies are so popular in labs. However, in the research fields of both bioluminescence and symbioses, the Hawaiian bobtail squid (I'll call it E. scolopes from here on) has proved to be a pretty intriguing model organism.

Check out this cool dude!

E. scolopes has a unique and intimate relationship with one particular type of bacteria: Vibrio fischeri. These bacteria colonize (i.e. grow in) the light organs of E. scolopes over the course of each day. When the bacterial population reaches a sufficiently large size, the bacteria start to glow. The process of bacterial communication that triggers this living light bulb is another mind-blowing topic that we'll return to in a future Microbial Mondays post.

The light produced by the bacteria is due to a reaction that is carried out by one particular bacterial protein: the aptly named luciferase enzyme. Luciferase breaks down molecules that store a great deal of energy, and releases this energy as light. This light benefits the squid in a couple of different ways. Firstly, the squid is a night hunter that feeds in shallow waters, so the eerie blue illumination due to the bacteria helps it blend in with the moonlight: camouflage! In addition, researchers recently found that the bioluminescence produced by the bacteria has a second function in squid: it helps control the squid's circadian rhythm (the wake/sleep cycle)! The blue light wakes up the squid in the same way that the morning light wakes up us diurnal (awake in the daytime) mammals.

The intimate relationship between squid and bacteria begins early in the life of E. scolopes. When a squid is first born, it doesn't actually have a light organ yet: only a little hole that will let V. fischeri in. Once the bacteria starts to infiltrate this hole, V. fischeri will send messages to the cells of the squid, which triggers the formation of the light organ. The bacteria actually help direct the growth of this whole organ in E. scolopes!

Later in life, the squid also directs the growth of its resident bacterial population. When there are too many bacteria in a small enclosed space, like a light organ, they start to run out of nutrients and die. The squid makes sure this doesn't happen. Every morning, before squid bedtime, E. scolopes squeezes out almost all of the bacteria from its light organ, releasing it back into the sea. Over the course of the day, while the squid is sleeping, the small amount of bacteria remaining in the light organ begin to grow. By the time the sun sets again, the bacteria have reached the necessary population size to start glowing again. A blue light starts to illuminate the squid, simultaneously providing it with camouflage and signaling to the brain that it's squid-morning. How beautiful is that?!

If you'd like to learn more about bioluminescence, I recently discovered this excellent website and blog written by fellow microbiologists. They also have some fantastic movies of V. fischeri and E. scolopes in action!

The research that uncovered this exceptional relationship between E. scolopes and V. fischeri doesn't just stop at these two organisms. It also gives clues about new, yet unknown, ways in which bacteria may influence us humans. For instance, as the scientists who discovered the role of V. fischeri in the circadian rhythm of E. scolopes noted, "resident microbial partners could similarly influence well-documented daily rhythms in other systems, such as the mammalian gut." In the vernacular, they mean that human gut bacteria might influence our brains, specifically our wake/sleep cycle, too! As a side note, sentences like this in scientific papers should always be read in a really excited, almost yelling, tone - that's how scientists feel when we write them.

That's all for Part I of the Microbial Buddies series. Next week, we'll continue with Part II, about the role of bacteria in plant nutrition!

Until next week - sleep soundly as the squid!

- Alex

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