Bioluminescent bacteria and the Hawaiian bobtail squid have formed a long-standing, mutually beneficial relationship. How bacteria coordinate their behavior to colonize the squid—through cellular signaling and cues from the environment—is detailed in a new study led by Penn State researchers.
A paper describing the study was published in the journal eLife. The researchers also show that the mechanism they describe is likely to be widespread in a wide range of bacteria and that understanding this coordination of cellular signaling is important for understanding how bacteria colonize their hosts in general.
“The bacterium we studied, known as Vibrio fischeri, associates with many different marine hosts, but its association with the Hawaiian bobtail squid is the best characterized,” said Tim Miyashiro, associate professor. in biochemistry and molecular biology at Penn State Eberly College of Science and is the leader of the research team.
Squid have a special light organ tucked under their mantle that is occupied by bacteria. The bacterial glow is thought to help the squid hide from potential predators from below. The bacteria, in turn, obtain nutrients from the squid to support their growth. Squid, however, are not born with bacteria in their light organs. Bacteria from the environment must enter the light organ after the squid hatches.
“Aspects of the bacteria’s behavior in the light organ have been identified,” says Miyashiro, “but the cellular mechanisms that allow the bacteria to colonize the squid in the first place are not well understood, so we began to investigate how bacteria start the colonization. .”
Within the light organ, bacterial behavior is coordinated through “quorum sensing.” The bacteria release signaling molecules that increase in concentration as the bacterial population grows and becomes more dense. If there are enough bacteria—if a quorum is reached—a signaling pathway is activated such that the bacteria begin to produce bioluminescence and their ability to move is inhibited. Before colonizing the light organ, bacteria also form large aggregates of cells, but when the quorum sensing pathway is activated they are not motile enough to migrate to the light organ.
“So, the question is ‘how do bacteria evade the quorum sensing pathway when they form these large aggregates outside of the squid and instead initiate colonization-promoting behavior?'” Miyashiro said. “What we see is that the aggregation pathway activates the production of a small RNA molecule that is normally suppressed by quorum sensing. Therefore, when the signaling pathway leading to aggregation is activated outside the squid, the RNA molecule is expressed, which can of cells to avoid quorum sensing to remain motile and dark.”
A small RNA—called Qrr1—is part of a quorum sensing pathway that inhibits the bacteria’s ability to produce bioluminescence and promotes motility until a quorum is reached. When a quorum is reached, Qrr1 expression is subsequently shut down.
“Qrr1 has also been shown to be important for promoting colonization,” Miyashiro said. “You might expect Qrr1 to be repressed during integration as during quorum sensing, but that is not the case. Therefore, we performed several experiments aimed at identifying the molecular control of Qrr1 expression in time of integration.”
The researchers showed that Qrr1 can be activated by a transcription factor—a protein that controls when and where genes are turned on in a cell—that also controls genes involved in aggregation. . The transcription factor—a protein called SypG—is the same one used to regulate Qrr1 through the quorum sensing pathway. bioluminescence.
“This complex regulatory architecture that controls Qrr1 expression allows it to play these two important roles and helps coordinate the behavioral transition from colonization to bioluminescence,” Miyashiro said. “When we look at the family of bacteria that includes V. fischeri, we see very similar structures that suggest to us that this type of coordination is likely to be important for many symbiotic bacteria.”
Ericka D Surrett et al, Two enhancer binding proteins activate σ54-dependent transcription of a quorum regulatory RNA in a bacterial symbiont, eLife (2023). DOI: 10.7554/eLife.78544
Provided by Pennsylvania State University
Citation: Bioluminescent bacteria coordinate signaling in squid light organ colonization (2023, July 17) retrieved on July 17, 2023 from https://phys.org/news/2023-07-bioluminescent-bacteria-colonize -squid.html
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