Bioluminescent bacteria coordinate signaling to colonize the squid’s light

Bioluminescent bacteria and the Hawaiian bobtail squid have formed a mutually beneficial relationship for a long time. How bacteria coordinate their behavior to colonize squid—via cellular signaling and cues from the environment—is detailed in a new study led by Penn State researchers.

The paper describing the study is available online in the journal eLife. The researchers also demonstrated that the mechanisms they describe are likely widespread in a broad array of bacteria and understanding the coordination of this cellular signaling will be important for understanding how bacteria colonize their hosts more generally.

“The bacterium we studied, known as Vibrio fischeri, is associated with many different marine hosts, but its relationship to the Hawaiian bobtail squid is the most well-characterized,” said Tim Miyashiro, professor of biochemistry and molecular biology at Penn State Eberly College. Science and research team leader.

Squids have a special light organ tucked into the underside of their mantle which is occupied by bacteria. The bacterial glow is believed to help camouflage the squid when viewed by potential predators from below. The bacteria, in turn, get nutrients from the squid to support their growth. Squids, 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 bacterial behavior in light organs have been characterized,” says Miyashiro, “but the cellular mechanisms that allow bacteria to colonize squid are poorly understood, so we set out to investigate how bacteria initiate colonization. .”

Within the light organ, the behavior of bacteria is coordinated via “quorum sensing”. The bacteria release signaling molecules that increase in concentration as the bacterial population grows and becomes denser. When enough bacteria are present—when a quorum is reached—signaling pathways are activated in such a way that the bacteria begin to produce bioluminescence and their ability to move is suppressed. Prior to colonizing the light organs, bacteria also form large cell aggregates, but if the quorum sensing pathway is activated, they may not be motile enough to move to the light organs.

“So the question is ‘how do bacteria avoid the quorum sensing pathway when they form these large aggregates outside the squid and instead initiate behaviors that promote colonization?’” said Miyashiro. “What we saw is that the aggregation pathway activates the production of small RNA molecules that are normally suppressed by quorum sensing. Therefore, when the signaling pathways leading to aggregation are activated outside the squid, RNA molecules are expressed, which allows cells bypassing quorum sensing to remain mobile and dark.”

The small RNA – called Qrr1 – is part of a quorum sensing pathway that suppresses the ability of bacteria to produce bioluminescence and increases motility until a quorum is reached. When the quorum is reached, the Qrr1 expression is then closed.

“Qrr1 has also been shown to be important for promoting colonization,” said Miyashiro. “You might expect that Qrr1 would be as suppressed during aggregation as it is during quorum sensing, but that is not the case. So, we conducted a number of experiments aimed at characterizing the molecular control of Qrr1 expression during aggregation.”

The researchers demonstrated that Qrr1 can be activated by transcription factors—proteins that control when and where genes are turned on in cells—which also control genes involved in aggregation. The transcription factor—a protein called SypG—is similar to that used to regulate Qrr1 by the quorum sensing pathway. This similarity allows SypG to promote Qrr1 expression in aggregates during colonization and ensures Qrr1 is not expressed once in light organs to allow bioluminescence.

“The complex regulatory architecture that controls Qrr1 expression enables it to play these two important roles and helps coordinate the behavioral shift from colonization to bioluminescence,” said Miyashiro. “When we looked across the bacterial family that includes V. fischeri, we saw very similar structures which suggested to us that this type of coordination may be important for many symbiotic bacteria.”

In addition to Miyashiro, the research team at Penn State includes Ericka D. Surrett, graduate student in the biochemistry, microbiology, and molecular biology (BMMB) program; Kirsten R. Guckes, postdoctoral scholar in Miyashiro’s lab; Shyan Cousins, and undergraduate students; Terry B. Ruskoski, BMMB graduate student; Andrew G. Cecere, research technologist in Miyashiro’s lab; and C. Denise Okafor, assistant professor of biochemistry and molecular biology and chemistry. The research team also included Denise A. Ludvik and Mark J. Mandel at the University of Wisconsin-Madison.

This work was supported by the US National Institute of General Medical Sciences, Howard Hughes Medical Institute Gilliam Fellowship, and National Institute of Allergy and Infectious Diseases Fellowship. Miyashiro is a member of the One Heath Microbiome Center at Penn State and the Penn State Huck Institutes for the Life Sciences.