Not only humans, animals and plants have an internal clock - bacteria also follow a complex day-night rhythm. This is shown by a study using the soil bacterium Bacillus subtilis. Even in complete darkness, the genes of the bacterium are read differently depending on the time of day. Further insights into the biological clock in bacteria could help to use antibiotics in a more targeted manner in the future, to better promote beneficial bacteria in our microbiome and to adapt production in biotechnology to the natural cycles of the bacteria.
Our internal clock controls our day-night rhythm. It determines when we feel tired and influences the daily cycles of metabolism, hormones and the immune system. Animals, plants and fungi also have such an inner clock. Even cyanobacteria, which carry out photosynthesis and are dependent on light for this, have been shown to have an internal clock. Only a few years ago, researchers found in other bacteria in the laboratory that their gene expression apparently follows daily rhythms. How exactly the internal clock of the bacteria works, however, was previously unclear.
Artificial day-night rhythm in the laboratory
A team led by Francesca Sartor from the Ludwig Maximilians University in Munich has now taken a closer look at the day-night rhythm of the soil bacterium Bacillus subtilis. They used both laboratory strains of the bacterium and two wild forms that came from soil samples from Slovenia and Denmark and had not previously been examined for any internal clock. In order to make the gene expression of the bacteria visible, the researchers inserted a gene for the enzyme luciferase into the bacterial genome. If the corresponding gene was read, a greenish glow appeared.
Sartor and her team put the bacteria's internal clock to the test by cultivating them under different conditions. Sometimes they had a normal day-night rhythm with twelve hours of light and twelve hours of darkness, sometimes they were kept in permanent darkness for days, sometimes the researchers shortened the alternation of light and darkness to phases of just six hours each.
Similarities to complex organisms
"Even in complete darkness, we determined 24-hour rhythms in the bacteria, both in the laboratory strains and in the wild isolates," reports the research team. Proteins that react to light were produced according to a daily rhythm - although the bacteria did not receive any external stimuli in the dark. In the laboratory strain, this gene activity, which is controlled by the time of day, decreased significantly after about three days in the dark, but in the wild strains it was still detectable after five days in the dark. "This suggests that the circadian rhythms and their ability to adjust to light cycles is a general property of B. subtilis and is not limited to a single laboratory derivative," the team writes.
If the researchers exposed the bacteria to a shortened day-night rhythm, the gene expression largely adapted to this new rhythm and even maintained it for a few days after the end of the treatment. Similar so-called after-effects have already been reported in rodents whose internal clocks were disrupted in a similar way. "It is amazing that a single-celled organism with such a small genome has a circadian clock with some properties reminiscent of clocks in more complex organisms," says co-author Antony Dodd of the John Innes Center in Norwich.
Applications in medicine and biotechnology
Further insights into the biological clock in bacteria could be used in numerous areas relevant to humans. "We could use knowledge about the clock to make improvements in the medical field or to increase the sustainability of food production or biotechnology," says Sartor's colleague Martha Merrow. For example, it would be conceivable to administer antibiotics at specific times when the bacteria are particularly susceptible to them. We could also promote our own microbiome better if we knew about its inner clock.
Sartor and her team consider Bacillus subtilis to be a suitable model organism for further research into circadian rhythms in bacteria. In future studies, they want to find out, among other things, what influence the multicellular organization, for example in a biofilm made up of a large number of individual bacteria, has on the internal clock of the bacteria involved.
Source: Francesca Sartor (Ludwig-Maximilians-Universität Munich) et al., Science Advances, doi: 10.1126/sciadv.adh1308