A recent study conducted by Penn State researchers has shed light on the fascinating interplay between bioluminescent bacteria and the Hawaiian bobtail squid. This mutually beneficial partnership involves intricate coordination of cellular signaling and environmental cues, as elucidated by the team.
Published in the journal eLife, the study reveals a mechanism likely prevalent among various bacterial species. Gaining insights into this cellular signaling coordination is crucial for comprehending the broader aspects of bacterial colonization in different hosts.
The Significance of Vibrio Fischeri
Lead researcher Tim Miyashiro, associate professor of biochemistry and molecular biology at Penn State’s Eberly College of Science, explains, “Our focus is on Vibrio fischeri, a bacteria that forms associations with numerous marine hosts, but its association with the Hawaiian bobtail squid is the most well-studied.”
The squid houses a specialized light organ beneath its mantle, which becomes colonized by these bacteria. The bacterial bioluminescence aids in the squid’s camouflage against predators, while the squid provides nutrients to support bacterial growth. Intriguingly, squid hatchlings are initially devoid of these bacteria in their light organs; colonization occurs when bacteria from the environment enter after hatching.
Unraveling Bacterial Colonization
While researchers have made progress in understanding bacterial behavior within the light organ, the specific cellular mechanisms underlying the initiation of colonization remain elusive. Therefore, the team embarked on investigating the bacterial strategies that initiate colonization.
Within the light organ, bacterial behavior is orchestrated through a process called “quorum sensing.” This involves the release of signaling molecules by bacteria, which accumulate as the bacterial population becomes denser. When a quorum is reached, a signaling pathway is activated, leading to bioluminescence production and inhibiting bacterial mobility.
The Role of Quorum Sensing
However, before colonizing the light organ, the bacteria form large cell aggregates. If the quorum sensing pathway were activated at this stage, the bacteria might not be mobile enough to move into the light organ.
Miyashiro highlights the key question, “How do the bacteria evade the quorum sensing pathway during aggregate formation outside the squid and instead initiate behavior that promotes colonization?” He further explains, “We observed that the aggregation pathway triggers the production of a small RNA molecule that is usually suppressed by quorum sensing. Consequently, when the signaling pathway leading to aggregation is activated outside the squid, the RNA molecule is expressed, allowing the cells to bypass quorum sensing, remain mobile, and remain non-luminescent.”
The Role of Qrr1
This small RNA molecule, known as Qrr1, is part of the quorum sensing pathway. It suppresses bioluminescence production and enhances bacterial mobility until a quorum is reached. Once a quorum is achieved, Qrr1 expression is subsequently deactivated.
Miyashiro remarks, “Qrr1 has also been found to play a crucial role in promoting colonization. One might assume that Qrr1 would be repressed during aggregation, just like in quorum sensing. However, that is not the case. Hence, we conducted several experiments to investigate the molecular control of Qrr1 expression during aggregation.”
Transcription Factors and Coordination
The researchers discovered that Qrr1 can be activated by a transcription factor called SypG, which regulates genes involved in aggregation. Remarkably, SypG shares similarities with the transcription factor responsible for regulating Qrr1 via the quorum sensing pathway. This resemblance enables SypG to promote Qrr1 expression during aggregate formation, facilitating colonization, and prevents Qrr1 expression within the light organ to allow for bioluminescence.
Miyashiro adds, “This intricate regulatory framework governing Qrr1 expression enables it to fulfill these two critical roles, effectively coordinating the transition from colonization to bioluminescence. Upon examining the broader bacterial family that includes V. fischeri, we observe similar structures that suggest the importance of this type of coordination in numerous symbiotic bacteria.”
Reference: “Two enhancer binding proteins activate σ54-dependent transcription of a quorum regulatory RNA in a bacterial symbiont” by Ericka D Surrett, Kirsten R Guckes, Shyan Cousins, Terry B Ruskoski, Andrew G Cecere, Denise A Ludvik, C Denise Okafor, Mark J Mandel and Tim I Miyashiro, 5 May 2023, eLife.
The research team at Penn State comprises Tim I. Miyashiro, Ericka D. Surrett, Kirsten R. Guckes, Shyan Cousins, Terry B. Ruskoski, Andrew G. Cecere, and C. Denise Okafor. The team also includes Denise A. Ludvik and Mark J. Mandel from the University of Wisconsin-Madison.
This study received support from the U.S. National Institute of General Medical Sciences, the Howard Hughes Medical Institute Gilliam Fellowship, and the National Institute of Allergy and Infectious Diseases Fellowship. Tim Miyashiro is affiliated with the One Health Microbiome Center at Penn State and the Penn State Huck Institutes for the Life Sciences.
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Frequently Asked Questions (FAQs) about bacterial symbiosis
What is the focus of the research study?
The research study focuses on the coordination of cellular signaling between bioluminescent bacteria and the Hawaiian bobtail squid in a mutually beneficial symbiotic relationship.
What is the role of Vibrio fischeri bacteria in this symbiotic relationship?
Vibrio fischeri bacteria are associated with various marine hosts, but their association with the Hawaiian bobtail squid is extensively studied. These bacteria colonize the squid’s light organ, providing bioluminescence for camouflage while receiving nutrients from the squid.
How do the bacteria initiate colonization of the squid’s light organ?
The bacteria utilize a process called quorum sensing to coordinate their behavior within the light organ. They release signaling molecules that increase in concentration as the bacterial population grows. Once a quorum is reached, a signaling pathway activates, leading to bioluminescence production and reduced mobility.
What happens before the bacteria colonize the light organ?
Before colonization, the bacteria form large cell aggregates. To avoid premature activation of the quorum sensing pathway, the aggregation pathway triggers the production of a small RNA molecule named Qrr1. This allows the bacteria to bypass quorum sensing, remain motile, and dark.
How is Qrr1 involved in bacterial colonization and bioluminescence production?
Qrr1 plays a dual role in the symbiotic relationship. During aggregation, Qrr1 expression is promoted by a transcription factor named SypG, allowing colonization. Once a quorum is reached, Qrr1 expression is deactivated, enabling bioluminescence production within the light organ.
What are the broader implications of this research?
Understanding the coordination of cellular signaling in bacterial symbiosis is crucial for comprehending how bacteria colonize hosts in general. The study suggests that similar regulatory mechanisms may be important for many symbiotic bacteria beyond the Hawaiian bobtail squid and Vibrio fischeri.
More about bacterial symbiosis
- eLife: Link to the study published in eLife
- Penn State Eberly College of Science: Official website of Penn State Eberly College of Science
- One Heath Microbiome Center at Penn State: Official website of One Heath Microbiome Center
- Penn State Huck Institutes for the Life Sciences: Official website of Penn State Huck Institutes