Isn’t it cool how when we get sick or injured, our bodies go to work to fix what’s wrong and help us feel better? If you were to develop a fever, there’s a good reason for that; it’s our body’s way of fighting off an infection caused by a bacteria or virus. Essentially, fevers not only activate our immune systems, but the rise in temperature is a method of killing the bacteria (since it’s hard for bacteria to multiply at a temperature above 98.6 degrees). In a process called hemostasis (or blood clotting, as it is more familiarly known), our bodies are able to stop wounds and cuts from bleeding by sending platelets to the injured site, usually a blood vessel. These platelets attach to this damaged area (the vessel tissue) and temporarily stop the bleeding, like a cork in a bottle. Then, a substance called fibrin joins the platelets. Together, they form a more stable clot. Finally, this clot transforms into the initial vessel tissue, and the wound is fully healed!
It turns out that many other organisms, besides humans, also have self-healing abilities and ways to protect themselves, including microbes. A bacterium called Bacillus subtilis is rod shaped, motile (meaning that it can move around), gram-positive (meaning it has very thick cell walls), and extensively used in scientific studies because of their ability to withstand harsh conditions like UV light exposure and high temperatures. Though B. subtilis can contaminate food, it is by no means a dangerous pathogen. It resides in soil, as well as the gastrointestinal tract of humans and ruminants. In order to protect itself from harm, it develops something called a biofilm.
According to the National Library of Medicine, a biofilm is a cluster of surface-associated microbial cells, enclosed in an extracellular polymeric substance matrix. In other words, biofilms are groups of bacteria clumped together in a matrix that protects them from things like antibiotics (which are designed to kill bacteria). In fact, the reason why antibiotics sometimes don’t work on us when we are sick is because of biofilms! According to the Biomedical Central Journal, around 80% of recurring and chronic microbial infections in our bodies are the result of biofilms; they’re clearly very strong!
Now, you may be wondering—what happens if a biofilm becomes damaged? Can it repair itself, or do the cells just die off? In 2021, Wang et al. researched this same exact question and discovered that biofilms are able to not only self-heal, but continue to grow afterwards!
Using a strain of Bacillus subtilis called MTC871, Wang et al. kept it in an incubator at 37°C for four hours. After diluting the culture and giving it time to grow, biofilm formation was induced, and different biofilm structures were present in each colony of the B. subtilis strain after ten hours. In order to determine whether the biofilm would repair itself, Wang et al. made small cuts in it to cause some damage. The researchers then tracked the growth using a microscope and a camera to capture images every ten minutes and detect the emitted light, respectively.
The results of this experiment were mindblowing. Wang et al. first made a rectangular cut on the 40-hour-old biofilm in a radial direction (from the center to the edge). They discovered that after 36 hours, the edge of the biofilm had partially healed (which is shown in Figure 2a), keeping its circular shape.
Figure 2a: Four images of self-healing at four moments. Healing time is measured in 12 hour increments.
Healing rate was also measured; researchers found that the healing rate slowly increased from the center of the biofilm to the edge of the biofilm, as shown in Figure 2b. This means that the biofilm grew faster at the edge than at the middle; this edge is now called the new healing edge.
Figure 2b: The rate of self-healing. Points a and k are areas that were cut on the biofilm. Points A-E are areas on the biofilm healing edge. Points A and E represent new healing edge points at 49 hours. Points B and D represent new healing edge points at 55 hours. Point C is the merged healing edge point at 61 hours.
Figure 2c emphasizes this further; the healing rate increased around the new wound healing in comparison to the initial wound healing. They also discovered that the later the new wound healing grew, the higher the healing rate was, meaning that the biofilm prioritized which areas needed healing the most.
Figure 2c: Healing rates of points a-k versus time. The biofilm healing time is measured in 5 hour increments. The biofilm healing rate is measured at 50 micrometers per hour.
Figure 2d shows that the maximum healing rate is nearly three times higher than the biofilm growth rate! B. subtilis is healing its biofilm way faster than growing it!
Figure 2d: The growth rate of the biofilm during healing. Time (h) is measured in five hour increments. Biofilm growth rate is measured at 10 micrometers per hour.
Wang et al. also compared the growth rate of the original biofilm edge with the growth rate of the new healing edge from the cut. Figure 2e demonstrates this; the growth rate of point A decreased 9 hours after being cut, which lines up closely to the growth rate of the original biofilm edge. In contrast, the growth rate of point B was not as constant after 15 hours; the growth rate continued to increase and decrease at various intervals.
Figure 2e: Growth rate of points A-E versus time.
In Figure 2f, P1 and P2 show the contours of the 40-hour biofilm with the initial cut, as well as the 55-hour-old biofilm after 15 hours of healing, respectively. Q1 and Q2 in P3 represent selected newly healing edges.
Figure 2f: P1 and P2 are contours of the 40 hour biofilm. Q1 and Q2 in P3 represent selected newly healing edges.
Wang et al. found that the growth rate at point 1 was the largest, which is showcased in Figure 2g. Essentially, this means that the biofilm’s expansion would slow down, and would actually prefer to grow vertically after point 1 (shown in Figure 2f), until the two cut sections finally merge. The growth rate of point B is the lowest, shown in the second point. After merging, the growth rate increases to point 3, and finally decreases because there aren’t enough nutrients left to sustain it.
Figure 2g: Demonstrates the average gray value of Q1 and Q2, which is positively proportional to the thickness.
So, to conclude, microbes can protect themselves from harm, just like humans can. Some microbes, like Bacillus subtilis, form biofilms in order to shield themselves from being damaged by things like antibiotics. Wang et al. discovered that after inducing biofilm formation in Bacillus subtilis and damaging its biofilm with cuts, it was essentially able to self-heal after a period of time. This is important because it can help researchers solve the problem of antibiotic resistance. This occurs when antibiotics are introduced to a population of bacteria, killing most of them—but a few resistant bacteria survive, reproduce, and the population then becomes resistant to the antibiotics. Since biofilms help bacteria protect itself, it adds to the problem of antibiotic resistance, so learning more about biofilms can shed light on ways we can combat this issue. For a further study, it would be interesting to know if biofilms of other microbes are able to heal themselves after being cut, or if this is only specific to B. subtilis. Next time you’re fighting off an illness or healing from an injury, just remember that microbes can do that, too!
About the Author:
Ari Henshaw ‘22 is a Psychology Major from Massachusetts. She works as a research assistant in the Schwartzer Lab, working with mice and looking at how diet and stress affect the brain. After Mount Holyoke, she hopes to work in the healthcare field and pursue a DNP. Outside of academics, she enjoys going on walks in good weather, cooking and baking for friends and family, as well as reading.
Donlan, R. M. (2002). Biofilms: Microbial Life on Surfaces. Emerging Infectious Diseases, 8(9), 881–890. https://doi.org/10.3201/eid0809.020063
Fever. (2022). Medlineplus.gov; National Library of Medicine. https://medlineplus.gov/fever.
Hemostasis: Stages and How the Process Stops Blood Flow. (2021). Cleveland Clinic. https://my.clevelandclinic.org/health/symptoms/21999-hemostasis
Sharma, D., Misba, L., & Khan, A. U. (2019). Antibiotics versus biofilm: an emerging battleground in microbial communities. Antimicrobial Resistance & Infection Control, 8(1). https://doi.org/10.1186/s13756-019-0533-3
Wang, X., Dong, F., Liu, J., Tan, Y., Hu, S., & Zhao, H. (2021). The self-healing of Bacillus subtilis biofilms. Archives of Microbiology, 203(9), 5635–5645. https://doi.org/10.1007/s00203-021-02542-w
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