Study Suggests Bacteria’s Internal Dialog Controls External Messaging

Bacteria may have been following the old adage “think before you speak” for millions of years.  

Bacteria communicate in an incredibly complex chemical language, making and sending chemical messages to neighboring bacteria. Those conversations can be about deciding whether there’s enough of them to launch an attack against the host or if it’s time to lay low and keep eating, dividing and growing the community.  

New research from Carnegie Mellon University scientists reveals that individual bacterial cells coordinate their messaging internally before sending it out to their neighbors. And, as an individual bacterial cell is orchestrating how it is "talking," it is still "hearing" what the rest of the nearby bacterial community is saying.

“It can still respond to signals from other cells, but it's controlling how it's transferring information to the world. It's very elegant, very intricate,” said N. Luisa Hiller, an associate professor of biological sciences.

The findings, which were published in Cell Reports, may have important implications for strategies to develop antibacterial drugs and vaccines.

Cell-to-cell communication systems are conserved across many species of bacteria, including Streptococcus pneumoniae, which was the focus of the new research. Also known as pneumococcus, the bacteria are commonly found in the respiratory tracts of healthy people, but if they disseminate to other tissues, they can cause serious disease. Pneumococcal infections lead to more than a million annual deaths worldwide, especially in young children and the elderly.  

Pneumococcus typically grows in colonies with each bacterium housing a complex network of interacting signaling systems. These systems send out a variety of molecules to communicate with its neighbors. The research team at Carnegie Mellon has uncovered a link shared by at least two systems, indicating that they coordinate their messaging before sending it out to the environment to cause a population-level change. In this case, colonization and disease.

“The interesting part is that this coordination is only detected when the bacteria have a host, like a cell culture line of lung cells,” said Karina Mueller Brown, a Carnegie Mellon alumna who is a postdoctoral research scholar at the University of Pittsburgh School of Medicine. “The host is required to observe this mechanism.”

Mueller Brown used the tools of molecular biology to study multiple signaling pathways in parallel. In doing so, she identified a common thread that connects two key signaling pathways, both of which contribute to S. pneumoniae’s ability to successfully set up shop in a host and cause disease.

Mueller Brown discovered that two key molecules — PptAB, which transports peptides across the bacterial cell membrane, and Eep, an enzyme that breaks down peptides — work together to activate one signaling pathway and inhibit another. The result is both pathways coming on at the same time. And it only happens in the presence of host cells.

“We don't know exactly what this coordination does, but we do know that these molecules affect large groups of genes. The logical assumption is that it's deciding what genes are turned on and in what order. And at the end of the day, we know it affects virulence,” Hiller said.

Because the PptAB and Eep molecules are conserved across species and related streptococci, the researchers said it's reasonable to suppose that this mechanism may occur in other pathogens. And because these systems influence how pneumococcus switches from a benign bacterium to a pathogen, understanding the molecular details of how they are coordinated can identify potential drug targets to halt that switch.

The discovery is the first to show that the coordination happens before the bacterial cell sends the message out into the environment.

When Mueller Brown found this connection between these two systems, it came as a surprise to Hiller, Mueller’s Brown Ph.D. advisor.

“When Karina showed me the data that this system, which we know activates these molecules, actually inhibits this other molecule, I was skeptical. There was no reason to suppose that molecule A would connect with system B,” Hiller said.

But as Mueller Brown continued investigating, it became clear that the two pathways were intertwined.

“It was very rewarding to work on this project because, as we moved forward, nothing was as we expected. But now that we have the big picture — this coordination — it makes sense that it works in such a fashion,” Mueller Brown said.

Along with Hiller and Mueller Brown, the following people from Carnegie Mellon contributed to the research: Rory Eutsey, laboratory manager; Derek Wang, a summer pre-college program student; and Amanda Vallon, who graduated with a master’s degree in biomedical engineering and a bachelor’s degree in biological sciences. Additional support came from Ozcan Gazioglu and Hasan Yesilkaya from the University of Leicester and Jason W. Rosch from St. Jude Children’s Research Hospital. Funding for the research includes support from the National Institutes of Health (R01 AI139077) and Carnegie Mellon’s Glen de Vries fellowship.