Writing about coral in the Journal Science, paleontologist Dr. George D. Stanley noted:
Symbiosis is the most relevant and enduring biological theme in the history of our planet.
If you aren’t familiar with the term, “symbiosis” refers to organisms of different species living together. There are three general forms:
(1) Parasitic symbiosis, in which one organism benefits and the other is harmed
(2) Commensal symbiosis, in which one organism might benefit but neither is harmed
(3) Mutualistic symbiosis, in which all organisms in the relationship benefit
I have written extensively on mutualistic symbiosis (see here, here, here, here, here, here, here, here, here, and here, for example). Not only does it fascinate me, but it was also the major scientific issue that led me away from atheism. When one sees the amazing mutualistic relationships that exist all over nature, it becomes clear that these organisms were designed to work together.
Bacteria tend to develop a lot of mutualistic relationships. Indeed, you would not be nearly as healthy as you are if it weren’t for the many mutualistic bacteria that live in and on your body. And while it is widely-known that you can find mutualistic bacteria in many parts of a mammal’s body, it was thought that you would never find them living in the eye for any extended period of time. That’s because mammal eyes contain an enzyme called lysosyme, which kills bacteria. However, new research indicates that at least one species of bacterium, Corynebacterium mastitidis, makes its home in at least some mammal eyes.
The researchers were studying mice and found that some had the bacterium in their eyes, while others did not. Now, of course, it could be that the bacterium had found its way to the eyes and had not been killed yet, so the researchers added the bacterium to the eyes of some of the mice that didn’t originally have it. Even after several weeks, the bacterium was alive and well on the eyes. To make sure this wasn’t a result of bad immunity, they added other species of bacteria to the eyes, and all those were killed by the mice’s immune systems.
Interestingly enough, when mice without the bacterium in their eyes were put in the same cages as mice that had it in their eyes, the bacterium was not transferred. However, when the mice that had the bacterium in their eyes had pups, the pups all had it. This led the researchers to conclude that the bacterium doesn’t “infect” the eyes (at least not easily), but it is passed down from one generation to the next.
But what is the bacterium doing there? To find out, the researchers treated some of the mice that had the bacterium with an antibiotic that kills it. They then infected those mice as well as mice that still had the bacterium in their eyes with a fungus that infects eyes. They found that the mice without the bacterium got the infection, but the mice that still had the bacterium in their eyes did not. Thus, the bacterium is protecting the eyes of the mice from at least some forms of infection.
Amazingly, the bacterium seems to do this by triggering an immune response from the mice. While this immune response destroys other organisms (like other bacteria and the fungus used in the experiment), it doesn’t kill the bacterium! The researchers don’t know how it survives, but they confirmed that it does. Here is what the authors state:
Our findings indicate that true commensalism with benefit to the host can exist at the ocular surface and uncover the importance of the local gamma-delta-T cell response in this process. We suggest that tuning of the local immune response by commensals may be necessary to maintain immune homeostasis in the ocular mucosa and may play a broad role in diseases of the ocular surface.
The “gamma-delta-T cell response” is the immune response that kills infecting organisms but not the bacterium that triggers it.
Now notice that the authors are careful to call the relationship between this bacterium and the mice “commensalism” and not “mutualism.” There are probably three reasons for this. First, while the mouse clearly benefits in at least some situations, it is not certain that the bacterium does. However, I would say that since the bacterium has a place to live (and presumably food to eat), it is probably benefiting. Second, this bacterium is found on the skin of many mammals and is thought to neither harm nor benefit the mammals. Thus, it is commonly called a commensal bacterium. Finally, the researchers can’t rule out the possibility that the bacterium becomes harmful in specific situations.
Since I don’t have to be as careful on my blog as the researchers do in the scientific literature, I will go ahead and suggest that this is another wonderful example of the abundant mutualistic symbioses that exist throughout God’s creation. As I stated in a previous post, there is at least some evidence that nature was designed to operate this way, and the negative relationships we see among organisms now are a result of the curse under which all creation groans. (Romans 8:20-22)