A Bacteria's Evolved Circadian Clock

Sartor, F., Xu, X., Popp, T., Dodd, A. N., Kovács, Á. T., & Merrow, M. (2023). The circadian clock of the bacterium B. subtilis evokes properties of complex, multicellular circadian systems. Science advances, 9(31), eadh1308.

Have you ever traveled across multiple time zones on a flight and found that you can’t fall asleep properly at night? Chances are you’re experiencing a phenomenon called jet lag, which is a result of a misalignment between your internal circadian clock and the time of the region you’re in.

So what is a circadian clock? Simply, it’s the body’s internal biological system that is capable of regulating behavioral processes in a rhythmic, repeated cycle every 24 hours. Despite being an internally generated operation, the circadian clock is largely influenced by zeitgebers or environmental time cues. Some common examples of zeitgebers include temperature, meal timings, and most importantly, light.

Light is arguably the most potent zeitgeber due to the heavy influence it plays on the circadian clock. A region of the brain called the suprachiasmatic nucleus (SCN) regulates the pace of our body’s circadian rhythm and receives signals from photoreceptors, specialized cells in the eyes that convert light for the SCN. So when our eyes are exposed to light - either naturally produced or artificially generated - the pace becomes offset and messes with the SCN’s influence on the circadian rhythm. Light is also capable of affecting the release of hormones, the body’s chemical messengers that carry information. For a normal circadian rhythm, the cortisol hormone is released shortly after waking up to assist in alertness and then slowly decreases throughout the day; at night, melatonin is released to help with sleep. However, when you are exposed to light at night, less melatonin is released and the decrease of cortisol progresses at a slower rate.

Circadian clocks have been observed in a plethora of organisms, ranging from bacteria to mammals. Yet it has not been fully proven if the circadian clocks of single-celled prokaryotes are on the same level as multicellular eukaryotes. Researchers are currently studying whether or not variations in the forms of a specific zeitgeber can result in a multitude of unique responses. There exists a free-running period (FRP) within eukaryotes, which is the natural circadian rhythm of an organism without the influence of external cues such as zeitgebers. In the case the eukaryote is exposed to a zeitgeber consistently for a long duration of time, the FRP may be altered. In the case of light, there is a principle coined as Aschoff’s rule which states that the FRP will shorten in constant light. The researchers here decided to test whether or not prokaryotes, such as the bacterium B. subtilis - found within the human gastrointestinal tract - would display the same properties as complex eukaryotes.

For this experiment, B. subtilis was subjected to new alternating cycles of light and darkness. The scientists found that B. subtilis that grew in complete darkness with no zeitgebers were capable of developing circadian rhythms. In another conducted experiment, the researchers subjected the bacteria to different colors of light (red and blue). They noticed that the activity of a blue light photoreceptor was suppressed in the bacteria exposed to blue light but wasn’t suppressed in the bacteria exposed to red light. This means that B. subtilis is capable of responding to varying colors of light in different ways, a phenomenon that hasn’t been observed before in a single-celled organism. The researchers also noted that the responsiveness of the FRP phases differed based on the different spectral wavelengths and metrics of light.

The data collected by this group is important to note, as simple organisms such as B. subtilis weren’t expected to possess circadian clocks. In fact, some complex organisms don’t even possess circadian clocks on the same level as B. subtilis. Another major implication of this study is the development of stronger biotechnology. Antibiotics, a medication that is used to treat bacteria, could be used at different times of the day for better effectiveness against pathogens.

Works Cited:

Gillette, M. U., & Tischkau, S. A. (1999). Suprachiasmatic nucleus: the brain's circadian clock. Recent progress in hormone research, 54, 33–59.

ScienceDaily. (2023, August 4). Astonishing complexity of bacterial circadian clocks. ScienceDaily. https://www.sciencedaily.com/releases/2023/08/230804140508.htm

Tosini, G., Ferguson, I., & Tsubota, K. (2016). Effects of blue light on the circadian system and eye physiology. Molecular vision, 22, 61–72.