Our immune systems consist of a complex network of cells, chemicals, and proteins that protect us from illness and foreign invaders. One of the most important parts of our immune system is the ability of white blood cells to rapidly multiply upon encountering a threat, such as a bacterial infection. Too much of this natural immune response can lead to autoimmune disorders, while too little can lead to chronic illness or sepsis.

To better understand the mechanisms underlying this immune response, researchers at Tokyo University of Science, in collaboration with the RIKEN Center for Sustainable Resource Science and the University of Tokyo, studied a species of bacteria that naturally oscillates in population size—a behavior similar to that of white blood cells in our circulatory system.

The researchers found that the bacterial population was able to maintain a stable oscillating equilibrium without the need for complex regulatory mechanisms. They used mathematical models to demonstrate how the bacteria were able to achieve this through a balance between positive feedback loops and negative feedback loops.

This research provides a valuable insight into how our bodies are able to maintain homeostasis and prevent population explosions of immune cells. It also has implications for understanding and treating cancer, which is characterized by the uncontrolled growth of cells. By gaining a better understanding of how our bodies naturally regulate cell growth, researchers may be able to develop more effective treatments for cancer and other diseases.

The research team, led by Professor Hiroaki Kashiwagi of Tokyo University of Science, focused on a species of bacteria called *Caulobacter crescentus*. This bacterium naturally oscillates in population size, with the number of cells increasing and decreasing over time in a predictable pattern.

The researchers used mathematical models to investigate the mechanisms underlying these population oscillations. They found that the bacteria were able to achieve this through a balance between positive feedback loops and negative feedback loops.

Positive feedback loops occur when an increase in a certain variable leads to a further increase in that variable. In the case of *C. crescentus*, the growth of the bacterial population leads to an increase in the production of a hormone called quorum sensing factor. This hormone then stimulates further growth of the bacterial population.

Negative feedback loops occur when an increase in a certain variable leads to a decrease in that variable. In the case of *C. crescentus*, the growth of the bacterial population leads to an increase in the production of a protein called Hfq. This protein then inhibits the production of quorum sensing factor, which in turn slows down the growth of the bacterial population.

The researchers found that the balance between these positive and negative feedback loops allowed the bacterial population to maintain a stable oscillating equilibrium without the need for complex regulatory mechanisms.

This research provides a valuable insight into how our bodies are able to maintain homeostasis and prevent population explosions of immune cells. It also has implications for understanding and treating cancer, which is characterized by the uncontrolled growth of cells. By gaining a better understanding of how our bodies naturally regulate cell growth, researchers may be able to develop more effective treatments for cancer and other diseases.

The research team's findings were published in the journal *Physical Biology*.