Social butterfly: the extremely social lives of Myxococcus xanthus

Original Article:  Kaiser, D., & Warrick, H. (2014). Transmission of a signal that synchronizes cell movements in swarms of Myxococcus xanthus. Proceedings of the National Academy of Sciences, 13105-13110.

Post by: Liz Huang

When someone thinks about social networks and socializing; people think of Facebook, or going to a party, or perhaps interacting with coworkers at a job. We as a species socialize to communicate our needs and our desires, to build connections with others of our kind and to harness these social networks to survive as individuals. Socializing, social networks and communal behavior are all hallmarks of the human experience and intrinsic to so much of human nature that it is often difficult to realize that we are not the only species to experience this phenomenon.  Many different species —regardless of complexity— rely on their social lives to quite simply, survive. A particularly significant example of this type of socially complex life is a life form that is easily overlooked. A simple soil bacterium, Myxococcus xanthus which dwells in deep, moist soil rich in organic material, utilizes different social functions or perform different social functions in order to form fruiting bodies and swarms to adapt to environments and survive.

M. xanthus forming social networks in order to communicate and survive.

 Myxococcus xanthus bacteria utilize two primary social structures in its life; the fruiting body and the swarming body.When there is a low nutrient and food supply, thousands of Myxococcus xanthus bacteria form a dome-like structure called a fruiting body.

M. xanthus aggregates and form fruiting bodies when there are not enough nutrients.

The fruiting body is constructed via coordinated movements amongst the bacterium. In M. xanthus, a fruiting body is formed in which these bacterial cells aggregate into approximately 10⁵ cells that will grow into the fruiting body. These actions such as aggregation can be seen in this video, in which a M. xanthus forms into a fruiting body.

M. xanthus bacteria formed into a fruiting body

When construction is finished, the fruiting bodies become heat resistant spores that have the ability to re-germinate once environmental conditions become favorable again.

When nutrient and food supply is at an adequate level, the Myxococcus xanthus forms a structure called the swarming body.

M. xanthus bacteria swarming together.

This structure, in particular requires precise social communication to form. One comparison that can be made is that it is like a group of friends communicating via cell phone to meet up and do a group activity together.  In this structure, the bacteria coalesce in a coordinated formation — with sometimes up to thousands of cells — to move together, secrete enzymes to convert insoluble amino acids into soluble and predate on other microbes more efficiently. An example of this can be seen in this video, in which a M. xanthus bacterial swarm preys on E coli.  The swarming body is efficient in that it allows the Myxococcus xanthus bacteria to effectively utilize insoluble nutrients far better than a single bacterium.

A summary of how M. xanthus operates under different conditions.

The processes of swarming and forming fruiting bodies holds significance in observing not only Myxococcus xanthus, but other multicellular organisms as well.  Many multicellular organisms, such as humans form very complex structures with various levels of organization. The levels of organization range from the very simple —for example, cells— to the very complex — such as organisms. An example of this very complex, very organized structure would be the formation of a human fetus and the coordination of how its body parts are formed, what dictates what goes where and how cells communicate to divide and produce a viable human being.  In particular, the mechanisms as to how the highly organized structures form is extremely difficult to observe and moreover, understand and by understanding how one process occurs, we can apply that understanding to how other, larger processes occur.

When there is observation of the general development of this type of bacterium, testable rules that can be applied to this bacterium can also be applied elsewhere to larger and more complex multicellular organisms such as the morphogenetic development in animals (otherwise known as how an animal develops its organic shape). Ultimately the observation of the Myxococcus xanthus allow for a greater understanding of how complex structures in multicellular organism form and why and what causes them to form the way they do.

So now we must ask the question, how? How are these soil bacteria able to coordinate and adapt and communicate to form these structures?  This was the question that was asked by scientists Dale Kaiser and Hans Warrick in the journal “Transmission of a signal that synchronizes cell movements in swarms of Myxococcus xanthus” In this journal, scientists Kaiser and Warrick propose their evidence and reasoning that there is a signal that synchronizes the Myxococcus xanthus in swarming bodies.

Myxococcus xanthus swarms spread outwards as a result of how the cells interact with one another.  The only time these swarms stop or pause in any capacity is to reverse their direction —a behavior that is vital for the swarming capabilities of the cell.

Swarming cells are self propelled and there is no hierarchy in how they function. Each individual swarm cell functions as both a leader and follower and takes cues from the movements of the cells that surround it.  M. xanthus utilizes pili —which are hairlike appendages that are useful in behaviors like adhesion or motility—to pull in forward via extension, adhesion and retraction of the pili in a movement that is known as S-motility.

M. xanthus, here you can clearly see the pili that extend out of the bacterial cell.

 It has also been observed previously that M. xanthus also utilizes a type of movement called A-motility which relies on three different types of motors for propulsion. There is also evidence that several lipoproteins such as CgIB are essential for A-motility alongside the various motors and are localized to the outer surface of the cells. When there are cells that lack the vital lipoproteins, those cells can be rescued by mixing the mutant cells with wild-type cells, ultimately causing a normal A-motility in the mutant cells, though the cell itself still remains mutant.  This shows that the CgI lipoproteins —with the host proteins TraA and TraB present — can undergo a contact mediated lipoprotein transfer which means that the lipoproteins must physically contact in order to transfer. Through their observations of how these processes worked and operated, Kaiser and Worrick believe that it is the CgIB protein that ultimately forms the protein to protein contacts that are the signal that causes the construction of the rafts and mounds in the Myxococcus xanthus swarms.  The authors hope that through the evidence that this occurs, they can definitively assign the CgIB protein as being responsible for causing the social interaction and networking.

The authors utilized a process called time-lapse photomicroscopy to observe the processes of the swarming Myxococcus xanthus bacteria.  They observed two types of the M. xanthus bacteria; normal, wild-type and a mutant strand which lacked the CgIB lipoprotein. Both types were routinely grown on 1% algar.  In this observation, they focused on two multicellular elements of the bacteria during the swarming body process; single layered, planar rectangular rafts and round, multilayered mounds.

After observing the two types of Myxococcus xanthus, Kaiser and Warrick noticed that there was a synchronization of pacemakers —which is essentially the oscillator that causes the cell to reverse after a certain period of time —  in neighboring cells in the fifth layer of the mound, though there was a slight lapse in time.

This means that the cells “switching” on the fifth layer of the multicellular mound, was synchronized.

A view of the multicellular mounds

 There was also an observation of cells aligning and then reversing from one another, only to do the same with other proteins, indicating some form of cell-cell communication. Kaiser and Warrick saw that once the clusters of focal adhesion proteins (CgIB) aligned with other focal adhesion proteins on another cell, they stopped moving transiently in order to reverse and ultimately do the same with a new cell. This behavior then spread on to new cells and continued to do so throughout the mounds.

These signal changes are responsible for the changes in mound structure that occur in Myxococcus xanthus and along with the observations that were made simply about the formation of the structure, both the synchronization of pacemakers along with the alignment of focal proteins and the behavior resulting are indicative that there is a signal caused by CgIB that is responsible for the behaviors of M. xanthus swarms.

We can now better understand how the behaviors of the Myxococcus xanthus bacteria occur, and moreover, apply the knowledge that we have of this particular multicellular organisms to other multicellular organisms and their formation of complex structures via cell signaling and otherwise. Much like how human’s social life is pertinent for our survival, the sophisticated social life of this soil bacterium allows for M. xanthus bacteria to become more effective as a species.  Furthermore, the discoveries that have been made allows scientists to now test out what has been learned from this study in regards to cell signaling, behavior and contact signaling in processes that are far more familiar and relevant to humankind such as the growth and development of a human child in the womb — something extraordinary that social networks such as Facebook can most definitely not do. As a result of the discovery of a signal in this study by Kaiser and Warrick, one could also feasibly apply this knowledge to simulate certain behaviors that alter the expression of certain aspects of development in various multicellular organisms. I do wish, however that the scientists would go more in depth and explore not only swarming bodies, but fruiting bodies as well. The fruiting body is also a coordinated structure, and I wish that the scientists would explore that aspect of the Myxococcus xanthus bacterium and whether or not there are similar patterns, or a completely different process altogether. In order to get the full picture on cell signaling, even in one type of bacterial cell, I think it would be necessary to explore the entirety of the cell’s life, not just one specific portion of it. Through the understanding of how cells communicate, and more specifically the causation of the swarming body, we can now know what to look for when we are looking for in the causative agents of social behavior and organization of complex structures.

Sources:

Kaiser D, Warrick H. Myxococcus xanthus Swarms Are Driven by Growth and Regulated by a Pacemaker . Journal of Bacteriology. 2011;193(21):5898-5904. doi:10.1128/JB.00168-11.

Kaiser, D., & Warrick, H. (2014). Transmission of a signal that synchronizes cell movements in swarms of Myxococcus xanthus. Proceedings of the National Academy of Sciences, 13105-13110.