Circadian rhythms are common throughout nature, regulating processes in the body on a 24-hour cycle to help organisms cope with environmental changes from day to night. Despite bacteria making up about 15% of Earth’s living matter, there is very little known about how this process could affect them – or if it does at all. Previous research has shown bacteria that photosynthesize have biological clocks, as can be expected for organisms reliant on light. However, the circadian rhythms of bacteria that don’t photosynthesize have remained an unexplored mystery. A new paper published in Science Advances has broken new ground on this topic by studying Bacillus subtilis, a bacterium found in soil.
"We've found for the first time that non-photosynthetic bacteria can tell the time," said Professor Martha Merrow, lead author from the Ludwig Maximilans University in Munich in a statement. "They adapt their molecular workings to the time of day by reading the cycles in the light or in the temperature environment."
The bacterium B. subtilis was chosen as – although not under the strict conditions used to study circadian rhythms – it had been observed to have some rhythms over a 24 hour period. Although it lacks core proteins for biological clocks present in photosynthesizing bacteria, the bacterium does have blue and red light photoreceptors that could help it adapt to a 24 hour day. Blue light is known to have a role in circadian rhythms. B. subtilis also has genes encoding PAS domains, a structural pattern in proteins that acts as a molecular sensor, key for proteins involved in circadian clocks.
The researchers focused on two genes encoding proteins with PAS domains: KinC and YtvA. YtvA is a blue light receptor, and KinC is involved in regulating processes such as the formation of biofilms and spores. The authors note that spore formation is regulated by a biological clock in some species of fungi. Cultures of B. subtilis were exposed to cycles of zeitgebers – predictable environmental cues for circadian rhythms. Temperature and light were the zeitgebers used in this study.
It was found that the expression of YtvA and KinC had a circadian rhythm, changing with fluctuations in temperature and light. For example, YtvA expression increased in the dark and decreased in the light, and adjusted accordingly when the light/dark cycle was reversed. The researchers state that through experimenting with cycles of different lengths, it was established that B. subtilis did have a circadian system, rather than just responding to environmental changes.
Dr Antony Dodd, co-author of the study from the John Innes Centre, said “Now that we have established that bacteria can tell the time we need to find out the processes that cause these rhythms to occur and understand why having a rhythm provides bacteria with an advantage." This discovery could have wider implications. For example, it raises the questions of whether the time of day affects antibacterial treatment or the risk of infection from exposure to bacteria.