Myxobacteria: The Commune of Germs

Huh? "Commune" and "germs" in the same sentence?

What are myxobacteria?

Myxobacteria are a group of soil-dwelling bacteria that display surprisingly complex social behaviors.

A well studied example, Myxococcus xanthus,

Photo of a Myxococcus xanthus "mound" viewed through a scanning electron microscope (SEM), from SciencePhoto.com

can serve as a model for how microorganisms like bacteria can coordinate and cooperate with each other, rather than existing as isolated individual colonies. Myxobacteria often live and move in organized "groups", called swarms. These swarms are comprised of thousands of bacterial cells, where they can act in a collective manner, doing things not typically possible for an individual bacterium or colony. It gets a bit crazy, but these swarms can travel as a pack (in a "mound" formation, seen to the right ) and act as predatory organisms, "hunting" together for nutrients and resources. Just wait, we'll get into it!

Wait... How can they move in a giant clump together like that?

The way myxobacteria move is a highly coordinated process, using two different, but complementary

movement systems: adventurous ["A"] motility and social ["S"] motility. Adventurous motility allows individual bacterium to explore their environment independently, while social motility allows for groups of cells to move together.

Graphic taken from Myxobacteria: Moving, Killing, Feeding, and Surviving Together; Figure 2 within the article. Adventurous [A] motility is shown in the top section, and social [S] motility is shown on the bottom section. For reference, "ECM" is extracellular matrix, "FA" is focal adhesion, and "pili" is a hair-like protein structures found on some bacteria that can help with adhesion and possibly gene/plasmid transfer.

Unlike many other kinds of bacteria, myxobacteria don't rely on flagella for movement. Instead, they glide across surfaces by secreting a "slime" (comprised from the extracellular matrix [ECM] ) and using type IV pili. Moving in this manner lets cells maintain close physical contact with each other and the surfaces they're on, along with following shared pathways and responding to environmental cues collectively.

So back to this "predatory behavior"...

One of the most interesting features observed in myxobacteria is their ability to engage in cooperative predation. Rather than consuming nutrients individually, these little buggers work together to locate and break down microorganisms deemed prey, such as other bacteria and fungi.

Wonderful graphic sourced from this research paper on the predation-behaviors of myxobacteria. Myxobacteria apparently also have the ability to discriminate between "kin" and "nonkin" myxobacteria, and they will actively kill intruders! Not very hospitable behavior. "FB" is referencing the fruiting bodies the bacteria create during periods of starvation.

As the swarm approaches the target, the bacteria "grow" or move over the target, "reversing" their movement to allow myxobacteria cells to be in contact with the target cells. Once contact is established, the myxobacteria start secreting a variety of enzymes, secondary metabolites, and antimicrobial compounds to kill and lyse the prey cells. After the "wolf-pack"-style killing, the myxobacteria feast.

Communication and coordination

Before we get into how these guys communicate with each other, we need to discuss "A-signals" and "C-signals". The "C-signal" is a chemical signal that is cell-to-cell-contact-dependent and drives multicellular development of the myxobacteria. The "A-signal" is also a chemical signal, but it aids in generating an extracellular substance that is part of fruiting body formation.

Fruiting bodies of Myxococcus xanthus hunting, lysing, and absorbing an E. coli colony. Great photo taken from Kennesaw State University.

Okay! Since that's sorted, we can discuss how they communicate. The complex behaviors that myxobacteria display is made possible through a sophisticated system of communication and gene regulation. To "talk" with each other, the bacteria secrete and exchange chemical signals that coordinate group [S] movement, cell aggregation, predation cues, and developmental processes.

Key pathways, such as those involving the aformentioned A-signals and C-signals, regulate how cells respond to changes in their environment and to each other. These signaling networks influence the expression of a large portion of the genome, particularly during developmental transitions (like creating fruiting bodies). As a result, the myxobacteria can coordinate their activities across very large populations, effectively allowing for a cell community to develop.

Getting through the rough times...together :)

When environmental conditions become unfavorable (like starving -- not fun), myxobacteria go through an incredibly cool transformation. Cells aggregate into multicellular structures, known as fruiting bodies, where they differentiate into specialized cell types! Some cells develop into “myxospores”, which are resistant forms capable of surviving harsh conditions, while others take on supportive roles or undergo programmed cell death, known as apoptosis.

Fruiting bodies from various myxobacteria species. Image sourced from the publication Assessment of Evolutionary Relationships for Prioritization of Myxobacteria for Natural Product Discovery.

Why these guys matter, what they can and could do, and why we should care

Myxococcus xanthus fruiting bodies. Image source.

The study of myxobacteria provides extremely valuable insights into core biological processes, such as cooperation, communication, and development. They show that while we tend to think of microorganisms such as bacteria as “simple”, these “simple” organisms are capable of highly coordinated and complex cooperative behaviors. Through group movement, collective feeding strategies, and organized development, they blur the line between unicellular and multicellular life. Their ability to coordinate complex group behaviors could also help researchers better understand how multicellular life may have evolved from single-celled ancestors.

0.5mm Fruiting bodies on a decaying coastal Oak in Marin County. Composite of 77 individual images with a 5x lens. Image credit: Alison Pollack.

Myxobacteria also produce a wide range of

bioactive compounds (such as those secondary metabolites we talked about earlier). Many of these compounds have potential applications in medicine and biotechnology! By studying these bacteria, scientists can gain a deeper appreciation for the complexity of microbial life and its broader significance in natural ecosystems.