The Complex Colonization Process of Hawaiian Bobtail Squid by Bioluminescent Bacteria

Category Science

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In a study published in eLife, Penn State University researchers have illuminated the intricate manner in which bioluminescent bacteria and the Hawaiian bobtail squid cooperate to form a mutually beneficial relationship. This involves bacteria colonizing the squid, while the glow of the bacteria allegedly helps provide camouflage for the squid from predators. The complex coordination of cellular signalling during colonization is important for understanding how bacteria colonize their hosts more generally.

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Bioluminescent bacteria and the Hawaiian bobtail squid have formed a longstanding mutually beneficial relationship. The intricate manner in which the bacteria coordinate their behavior to colonize the squid—utilizing cellular signaling and environmental cues—has been illuminated in a new study led by researchers at Penn State.

In the study, which was published in the journal eLife, the researchers illustrate a mechanism that is likely widespread in a broad array of bacteria. Understanding this coordination of cellular signaling will be important for understanding how bacteria colonize their hosts more generally.

Vibrio fischeri is a type of bacteria that is commonly found in many marine environments throughout the world

"The bacteria we study, known as Vibrio fischeri, is associated with many different marine hosts, but its association with the Hawaiian bobtail squid is the best characterized," said Tim Miyashiro, associate professor of biochemistry and molecular biology in the Penn State Eberly College of Science and the research team’s leader.

Squids possess a specialized light organ located within their mantle’s underside, which is occupied by the bacteria. The bacteria’s glow purportedly helps camouflage the squid from potential predators. In exchange, the bacteria receive nutrients from the squid to facilitate their growth. Interestingly, squids are not born with the bacteria within their light organs. Bacteria from the environment must colonize the light organ after the squid hatch.

The bacteria typically colonize the squid after the squid hatch, suggesting that the bacteria needs light to find its host

"Aspects of bacterial behavior in the light organ have been characterized," said Miyashiro, "but the cellular mechanisms that allow the bacteria to colonize the squid in the first place are still poorly understood, so we set out to investigate how the bacteria initiates colonization." .

Within the light organ, bacterial behavior is coordinated through "quorum sensing." This involves bacteria releasing signaling molecules which increase in concentration as the bacterial population grows denser. When a sufficient number of bacteria are present—reaching a quorum—a signaling pathway activates, triggering bioluminescence production and suppressing their mobility.

Colonization of the squid requires the bacteria to bypass the quorum sensing pathway which normally lead to luminescence and a lower motility

However, before colonizing the light organ, bacteria form large cell aggregates. If the quorum sensing pathway were activated at this stage, they might not be motile enough to move into the light organ.

"So, the question is ‘how do the bacteria avoid the quorum sensing pathway when they form these large aggregates outside of the squid and instead initiate behavior that promotes colonization?’" said Miyashiro. "What we saw was that the aggregation pathway activates the production of a small RNA molecule that is normally repressed by quorum sensing. Therefore, when the signaling pathway that leads to aggregation is activated outside the squid, the RNA molecule is expressed, which enables the cells to bypass quorum sensing to remain motile and dark." .

The small RNA molecule Qrr1 is the main component of the quorum sensing pathway and is responsible for suppressing the luminescence and promoting mobility of the bacteria

This small RNA, named Qrr1, is part of the quorum sensing pathway. It suppresses the bacteria’s bioluminescence production and promotes mobility until a quorum is reached. When a quorum is reached, Qrr1 expression is subsequently deactivated.

"Qrr1 has also been shown to be important for promoting colonization," said Miyashiro. "Yeast cells that lack Qrr1 are increasingly resistant to being colonized by Vibrio. Therefore, the coordinated regulation of the pathway that leads to colonizing the squid—and the environment itself—is also influencing this small RNA that is responsible for promoting colonization, illustrating the intricate dynamics of the colonization process." .

The study was the first to characterize the bacteria's colonization behaviour outside of the host

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