Two actinobacterial G protein systems control multicellular mycelial growth and morphology in Streptomyces venezuelae

Bacteria are often imagined as solitary wanderers, single cells drifting through their environment and dividing on their own. Yet many bacteria live as vast, cooperative communities, forming intricate networks that resemble the roots of a plant or the threads of a spider's web. Among these are the streptomycetes, a group of soil-dwelling bacteria that grow as long, branching filaments called hyphae. These filaments weave together to create a multicellular mycelium, a living mesh that explores the soil for nutrients. For this network to function, the individual filaments must coordinate their growth. They need to know where to extend, when to branch, and crucially, how to avoid bumping into one another in a way that creates a tangled, inefficient mess. This coordination requires a sophisticated internal communication system, a way for the cells to sense their neighbors and adjust their shape and direction accordingly.

In the soil, where food is often scarce and unevenly distributed, the ability to organize a multicellular network efficiently is a matter of survival. If the filaments grow haphazardly, they might crowd each other out, wasting energy and failing to cover the ground effectively. Scientists have long known that bacteria use chemical signals to talk to one another, but the specific mechanisms that allow a network of filaments to maintain a clean, organized structure while growing over a surface have remained a mystery. Recent research has turned its attention to a unique set of genetic tools found in these bacteria, known as G protein systems. These are molecular machines that act like switches, turning genes on or off in response to the environment. In the specific bacterium Streptomyces venezuelae, there are eight different versions of these systems, but until now, no one knew exactly what most of them were doing or how they helped the bacteria build their complex, multicellular homes.

A team of researchers at Lund University in Sweden set out to solve this puzzle by systematically removing each of these eight genetic systems, one by one, to see what happened to the bacteria. They grew the modified bacteria on different types of food, ranging from rich, nutrient-dense gels to very poor, minimal surfaces that mimic the harsh conditions of the soil. What they found was that two of these systems, working together, act as the architects of the bacterial network. When the researchers removed both of these specific systems, the bacteria lost their ability to organize themselves. Instead of spreading out in a neat, even layer where each filament has its own space, the mutant bacteria began to clump together. Their filaments stuck to one another, forming thick, rope-like bundles that looked like a tangled ball of yarn rather than a delicate web.

This clumping behavior was not just a matter of the bacteria sticking together by accident. The researchers observed that the mutant filaments grew in straight, rigid lines, whereas the wild-type bacteria grew in gentle, winding curves. The wild-type bacteria seemed to actively avoid their neighbors, growing into the empty spaces between other filaments to cover the surface evenly. This "self-avoidance" allowed them to explore the available food more efficiently. The mutants, lacking the two key genetic systems, lost this ability. They grew straight and stuck to whatever was nearby, including other mutant filaments and even the wild-type filaments they were grown alongside. The researchers tested whether the bacteria were releasing invisible chemicals into the air or water that caused this sticking, but they found no evidence of such signals. Instead, the problem appeared to be intrinsic to the mutant cells themselves; their surfaces had changed, making them sticky and prone to clumping.

By looking at the genes that were active in the mutant bacteria, the scientists discovered why this was happening. The mutants had turned up the volume on a large group of genes responsible for building the cell wall and producing sugary coatings on the outside of the cell. This overproduction of surface materials likely made the filaments sticky, causing them to adhere to one another. However, the story did not end with just the microscopic clumping. The researchers also noticed that the mutants behaved differently on a larger scale. While the wild-type bacteria would spread out in a thin, bald-looking film across the surface of the dish—a behavior known as exploratory growth—the mutants failed to do this. Instead, they formed dense, fuzzy colonies that looked like they were trying to grow upward into the air, even when the conditions were not right for such growth.

To understand why the mutants were failing to explore, the researchers looked for bacteria that had spontaneously fixed the problem. They found that some of the mutant bacteria had acquired new mutations that allowed them to spread out again, almost like the wild type. When they analyzed the DNA of these "suppressor" bacteria, they found that the fix often involved changes to a different set of genes that control a chemical signal known as cyclic-di-GMP. This signal is a master regulator that tells the bacteria when to stop growing and start making spores. The researchers concluded that the original mutants were growing too fast and maturing too early. Because they were rushing to form spores, they never took the time to spread out and explore the surface properly. The two genetic systems they had removed were acting as a brake, keeping the bacteria in a vegetative, growing state long enough to build a proper network.

The study revealed that these two genetic systems have two distinct jobs that are surprisingly separate. One job is to control the physical shape of the filaments and prevent them from sticking together, ensuring the network remains open and organized. The other job is to keep the bacteria from maturing too quickly, allowing them to explore their environment fully. The researchers showed that they could fix the exploration problem by changing the maturation signals, but this did not fix the clumping problem. The filaments still stuck together even when the bacteria were allowed to grow longer. This means that the way the bacteria build their shape and the way they decide when to grow or mature are controlled by different pathways, even though both are managed by the same pair of genetic systems.

This work provides a clear picture of how a simple bacterium manages the complex task of living as a multicellular community. It shows that the bacteria do not just grow randomly; they use specific genetic tools to sense their environment, adjust their shape, and coordinate with their neighbors. Without these tools, the network collapses into a tangled mess, and the colony fails to explore its surroundings effectively. The findings suggest that the ability to maintain a clean, organized structure is just as important for bacterial survival as the ability to find food. By understanding these mechanisms, scientists can begin to see how these ancient organisms have evolved to thrive in the complex, competitive world of the soil, building intricate cities of filaments that are far more organized than anyone previously realized.