There are approximately the same number of bacterial cells in our bodies as human cells (Sender et al, 2016). These microorganisms help us with many of our own functions and, through billions of years of co-evolution, we have formed a symbiotic relationship. We provide a suitable environment for the bacteria and, in return, they help to process nutrients and many other activities in our bodies. The bacteria undergo constant evolution to further improve their fitness (their ability to grow and reproduce). This has caused a great strain on our healthcare system, as this evolution has led to the spread of harmful things, like antibiotic resistance.
Evolution can happen through random mutations in DNA. Small mistakes can spread to millions of daughter cells through reproduction if the mutations confer better or neutral fitness changes. Another, more concerning way to spread these changes is the transfer of DNA between cells, and sometimes viruses (bacteriophages). We can see remnants of these transfers when we sequence bacterial genomes and find segments of DNA viruses in them.
In a recent study by Song et al (2019), a strain of bacteria was found that actually co-opted the virus that it was infected with to differentiate between cells that had the phage versus cells that did not. The presence of the phage inside the bacteria improved its fitness by eliminating other cells that did not have the phage, potentially conserving resources for itself (Song et al, 2019). The virus benefits by ensuring that only cells that contain its own DNA can continue to reproduce.
Understanding these types of symbiotic relationships can help us engineer strategies for the prevention of future outbreaks. Although these experiments were strictly performed in a lab, it is important to know just how bacteria can adapt and spread potentially harmful pathogenic features, such as antimicrobial resistance. Bacteriophages are already known as a method of spreading genetic material. Although the microorganisms are strictly acting to improve their own fitness, this can have consequences on our own health as they become more resilient, and potentially more dangerous when they infect humans. Researchers have shown that they are capable of developing new tools and, in response, we must also be prepared for when we see these in our hospital clinics.
References
Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacteria cells in the body. PLoS biology, 14(8), e1002533.
Song, S., Guo, Y., Kim, J. S., Wang, X., & Wood, T. K. (2019). Phages Mediate Bacterial Self-Recognition. Cell reports, 27(3), 737-749.
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