Microscopic bacteria and wolf packs are not often used to compare the behaviors of one another because of their great size difference. What if we told you that there is an organism that can only be seen through a microscope that can act just like wolves? Modern-day wolves travel in packs for predation and without the wolf pack; alone, the wolves are more vulnerable and less effective in taking down their prey. This observation does not only prove to be true for these aggressive mammals from the woodlands but similarly, for an aggressive, microscopic bacterium named Myxococcus xanthus.
Image: Nick Young looking confused, surrounded by question marks with text that reads “When someone says that they can liken a microorganism to a wolfpack.” (source: makeameme.org)
Myxococcus xanthus is a gram-negative soil bacteria whose population thrives off of predatory behaviors amongst bacteria, archaea, and fungi from within its environment. Myxcococcus xanthus, abbreviated as M. xanthus, can cause other cells to self-destruct otherwise known as lysing to the scientific community. M. xanthus displays social behavior through their ability to produce biofilms, extracellular structures developed by cells to create a “connected” colony. The bacterium’s ability to form biofilms enables its ability to communicate across its colony but also assists with its social predatory behavior of preying in large packs similar to the previously mentioned behaviors of wolves in a wolfpack. Not only can M. xanthus be one of many predators in the predation of another microorganism, but it can also be the sole predator; this versatile microbe has the ability to cause cells to lyse.
In a research study conducted by Wenchao Zhang et al., the solitary predation of M. xanthus on Escherichia coli, commonly abbreviated as E. coli, was observed on a single-cell level. This research was conducted so that scientists can acquire a better understanding of how and if this microbe has the ability to successfully prey on other microorganisms individually. Scientists were also interested on why when in nature, its predatory go-to behavior is to prey in packs.
To start their research, their first experiment was to observe whether or not M. xanthus can display single predation while looking both at the prey and predator perspectives. In order to examine M. xanthus solitary predation on E. coli cells, they examined the morphology of E. coli cells (short rod) when it comes in contact with M. xanthus (long rod). In Figure 1A, researchers used live/dead fluorescence staining to distinguish the state of the E. coli cells: bright green meaning it’s in the live state and bright red means it’s in the dead state. When the M. xanthus and E. coli were placed into experimental cultures, the E. coli was marked with fluorescence while M. xanthus was not. The fluorescence of the E. coli changed from a bright green color to a red color, which shows that the E. coli have transitioned into a dead state when coming in contact with the M. xanthus cell. Figures 1B and 1C show the fluorescent intensities along the body axis of the intensities of the red and green fluorescence at different time stamps. The green intensity of the live E. coli cells (shown in the SYTO 9 panel) starts to turn red (shown in the PI panel), which shows that the cells are now in a dead state. In the PI panel, the dead E. coli cells show a disintegrated red intensity that spreads over the cell envelope, which implies cell leakage.
In Figure 1D, we see the stained DNA was leaked from when the M. xanthus cell came in contact with the E. coli cell which shows that the E. coli cells are lysed after coming into contact with a single M. xanthus, deeming solidarity predation successful. This experiment shows that M. xanthus has the potential to display single predation when coming in contact with live E. coli cells.
Now, it’s not common to see M. xanthus stray from the pack in reality. But that’s what makes this experiment so cool! We get to see something different and possibly discover something new. When given the task of being a lone wolf, M.xanthus has three different tactics to go in for the kill.
To pierce the membrane of the E. coli and cause its self-destruction so that it can absorb the nutrients, M. xanthus has to come into direct contact with its prey and can do so by the leading pole, non-pole, or lagging pole positions which are displayed in figure 5 of the Zhang et al. study. Leading pole is a head-on attack while nonpole can best be described as bumping into someone on a busy street. Lagging pole definitely gives off the imagery of someone bumping into you from behind in a car.
Images: (top gif) goat charging and hitting the back of two people on a motorcycle, (bottom gif) woman with a sign bumping into a man on the street making him spill his coffee on his shirt (source: giphy.com)
While these cells use their entire bodies to detect their prey, the way in which they contact them directly is in one of these 3 ways. Most (55%) of the wild type (WT) M. xanthus bacteria prefer to be direct and to the point with a leading pole contact, while the rest will take on the nonpole (30%) or lagging pole (15%) approach. When they tested the mutants (shown as the red and blue bars), the authors were able to conclude that ΔpilA and ΔaglZ cells show more confined moving patterns than WT which made those microbes favor the head-on method much more. In this experiment, the authors also wanted to see if the length of the cell had an effect on what contact method the microbes used but those variables seemed to have had little to no correlation.
As a part of the predatory behavior of M. xanthus, this bacterium has a pattern of doing one of two things after causing lysis in their prey. Post-killing, M. xanthus will either lyse its prey and leave it or choose to stay and continue reversing towards the depleted cell to consume its nutrients and intracellular components. Solitary predation by this microbe was found to not be as effective as predation in packs in terms of prey-cell consumption because of its pattern of not completely consuming its prey. The experiment in which they observed the M. xanthus’ go-to behavior when preying on E. coli is displayed in Figure 6.
Image: Gopher laying on a rock with its right upper limb up with text that reads: “Nooo! don’t leave me!!!!.” (source: Cheezburger.com)
Figure 6A depicts a cartoon of the predation process of M. xanthus; as shown, the microbe searches for its prey, attacks it and then chooses whether to stay or leave. Figure 6B is a bar graph displaying the patterns of the M. xanthus; during this experiment, scientists observed which option is preferable to the bacterium. M. xanthus has two surface motility systems; the first is called social motility (S motility) and the other is called adventurous motility (A motility) both enable the microbes’ ability to glide. Social motility is activated when PilA, a filamentous pilus, is in the environment and adventurous motility is enabled when its required protein, AglZ, is present. Figure 6B shows a graph that has separated M. xanthus into three independent strains. The bar labeled WT (wildtype) is the only group in which the bacterium strain has not been manipulated. Then, there are two bars labeled ΔpilA, the PilA filament component has been changed and in the bar labeled ΔaglZ, the required protein for gliding ability in adventurous motility was changed.
When comparing this experiment to data about the bacteria’s multicellular behavior researchers realize that the bacterium is more effective and efficient when preying in packs but it requires reciprocated energy from the prey. There is a much higher response when interacting with live prey as opposed to dead prey. Understanding the predatory behaviors of this bacterium can be advantageous to ecologists, environmentalists and many others in the scientific community. Antibiotics naturally secreted by M. xanthus are currently being studied for their potential use as an alternative for chemical pesticides by blocking pathogenic fungi found in plants. This newfound knowledge of M. xanthus could also be used for human medical and pharmaceutical purposes; the antibiotics produced by M. xanthus to kill their prey can be utilized as resources for medical discoveries.
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