Think of this: It is a Saturday afternoon and you get home after a long day of running errands and plop down on the sofa wanting to relax for a few minutes. You reach into your pocket for your phone only to realize that it isn’t there. You check your other pocket and soon you are patting down your legs like a TSA agent for a phone that isn’t there. You try to retrace your steps and think of where you last had it but your mind goes blank. However, you do not panic and instead log into the Find My Phone app on another device and soon you see a glowing pin representing your phone on the screen. What if I told you that there is a bacterium that can perform the same function? Just like the Find My Phone app, it can detect where its prey resides.
This bacteria is none other than Myxococcus xanthus (M. xanthus), a Gram-negative soil microbe. As a predatory bacterium, it feeds primarily on other bacteria, fungi, and archaea by causing these cells to lyse, i.e. undergo cell death by bursting. M. xanthus cells are especially known for their social behavior through their formation of biofilms. These are matrices of extracellular structures formed by the cells to successfully adhere in a colony. To put into simpler terms, they form a sort of net over the cells to keep them together. These biofilms are essential to their survival in the environment, for communication between the M. xanthus cells and finally for detecting their prey (which will be discussed in detail later in this post).
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| Figure 1: Myxococcus xanthus predatory life cycle showing cellular development and multicellular behavior |
Another unique feature of M. xanthus is its intricate multicellular life cycle which enables the bacteria to hunt its prey and survive in its environment. Figure 1 above gives a simple view of the cell cycle and shows its two responses to whether prey bacteria is present or not. At the beginning of the cycle, the cells start out as inactive vegetative rod-shaped cells (a). From there, the cells can display either two behaviours: Scouting (b), which involves the movement of cells, at a low cell density, through the formation of extracellular polysaccharides, or branching (c) which, at high cell density, allows cells to move much faster as a group. From here the group of bacteria have two responses depending on whether prey is present or not. If there is no prey detected by M. xanthus, then the microbe will undergo certain processes to eventually make spores (h) and conserve its numbers. In the presence of prey, the microbe will force the prey cells to undergo cell-lysis (d).
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| The resemblance is uncanny! |
So how is M. xanthus similar to a location tracking app? When you used the app to track your missing phone, it detected signals emitted by your phone to pinpoint its presence and then location, allowing you to know if it was close by, perhaps in the kitchen, or far away, maybe in the store you left an hour ago. In the same way, the M. xanthus bacteria detects signals emitted by prey bacteria. These signals are known as quorum signals, specifically acyl homoserine lactones (AHL) which are released through intercellular communication between prey cells. It is the detection of these quorum signals that determine the response of M. xanthus, i.e. whether or not it should conserve its population by making spores, or to feed.
In a research study by Daniel Lloyd and David Whitworth exploring the responses of M. xanthus on quorum signals, they found that AHLs enhance the motility of M. xanthus cells. Figure 2 below shows the results of an experiment conducted where they exposed colonies of M. xanthus to four different types of AHLs in addition to a control when performing colony expansion assays. Results from that experiment showed that the presence of AHLs did in fact increase the distance swarmed by the bacteria after a time frame of 48 hours. This is seen especially in the experiments using the media DCY, DCY/10 and TM in all four AHLs. They also found that the effect on stimulation of the M. xanthus cells was due to the kind of AHLs being used in the experiment and was not at all affected by the type of medium in which the experiment was taking place. This shows that AHLs have an excitatory effect on the bacteria, where its detection prepares the bacteria for predatory behaviors. In addition, the graph shows that medium type had no significant effect on the motility of M. xanthus colony expansion.
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| Figure 2: Graph depicting the motility rates of swarming M. xanthus in response to different AHLs and lawns of prey. Colonies of M. xanthus were placed on agar plates with the following AHLs: DCY (nutrient-rich medium), DCY/10 (reduced nutrient-rich medium), TM (nutrient free medium) Ec (lawn of E. coli) and Bs (lawn of B. megaterium). After 48 hours, M. xanthus colony expansion was measured. |
They also observed that the presence of AHLs not only increased the motility of M. xanthus cells but also affected its very life cycle! They investigated the effect of AHLs on the production of myxospores and found that AHLs shifted the balance between the number of M. xanthus spores and vegetative cells. This experiment is best explained in Figure 3 below. The x-axis shows the hours of development after vegetative cells were spotted onto nutrient free medium while the y-axis shows the number of spores formed after being exposed to different concentrations of AHLs. Compared to the control (an environment free from AHLs) spore formation was much slower, as the concentration of AHLs increased. This difference is seen best at the 72nd hour where the control shows a significantly greater number of spores than the other AHLs. The greatest effect was seen in C8-AHL and C10-AHL, which are the purple and light blue lines in the graph respectively. After the full 120 minutes, cells in these two conditions only produced about ⅓ of the spores formed by the control. These results are profound because they show that AHLs incite a predatory response from the M. xanthus cells where from the fruiting body stage of the cycle, fewer cells stay in the dormant myxospore stage and more progress to the vegetative stage which is a state prepped for predation either through scouting or branching as we learned earlier on.
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Figure 3: Graph showing how sporulation was affected after exposure to AHLs of different concentrations (C4-AHL, C6-AHL, C8-AHL and C10-AHL). Cells were spotted onto nutrient-free medium and show that each concentration produced a significantly fewer number of spores than the control which was without AHLs. |
The research that these scientists are undertaking is important to understanding the microbial ecology and the predator-prey relationships that exist. By responding to the quorum signals secreted by prey, we understand that predator-prey relationships may be even more complex and could affect how we use bacteria to target and fight disease-causing bacteria in the human body. Future studies are suggested to shift beyond M. xanthus behavior and rather look more intricately into the genetics of the bacteria. Here scientists can explain whether the expression of certain genes result in the response to AHLs where the inhibition of these genes can make M. xanthus immune to AHLs, thus affecting predation.
Whatever the case may be, Myxococcus xanthus is an incredible predatory bacteria whose multicellular network allows it to thrive in different environments. It makes use of unique technology which detects the presence of AHL quorum signals given off by surrounding prey microbes. So the next time you misplace your phone and use your handy dandy app, remember that this technology is not new to this powerful bacteria.
References:
Lloyd, Daniel G, and David E Whitworth. “The Myxobacterium Myxococcus xanthus Can Sense and Respond to the Quorum Signals Secreted by Potential Prey Organisms.” Frontiers in microbiology vol. 8 439. 14 Mar. 2017, doi:10.3389/fmicb.2017.00439.
Keane R, Berleman J. “The Predatory Life Cycle of Myxococcus xanthus.” Microbiology 162(1):1-11 doi:10.1099/mic.0.000208.
About the Author:
Denise Aninakwah '20 is a Neuroscience and Behavior major at Mount Holyoke. Her passion for accessible health care is why she wishes to pursue optometry after graduating and provide eye care to disadvantaged communities. Outside of academics, she is the Co-Chair of the African and Caribbean Students' Association on campus and also enjoys spending quality time with her family and friends.
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