Spiral insights: Scientists observe mechanical waves in bacterial communities

A new study by researchers at the Chinese University of Hong Kong reports the occurrence of mechanical spiral waves in bacterial material.

Spiral waves are commonly found in man-made and natural systems (such as the heart). These arise from the interaction of neighboring elements, such as cardiomyocytes in the heart. These spiral waves can have varying effects, sometimes leading to life-threatening conditions such as heart fibrillation.

The new study was published in natural physics, exploring spiral waves in bacteria—something that has never been observed before. The researchers focused specifically on Pseudomonas aeruginosa. They are commonly found in soil and water and can also be found in hospitals.

This study is a continuation of their previous work, in which the authors studied long-range material transport through open fluid channels in bacterial communities.

“When we studied the development of bacterial tubes, we discovered the signature of density waves and were intrigued by this beautiful wave pattern,” study co-author Dr. Shiqi Liu told Phys.org.

Pirus motor

These spiral waves the researchers observed in bacteria are an emerging phenomenon. Emergent phenomena are an important aspect of complex systems in which the interaction of individual entities leads to unobservable phenomena.

This means we need to understand what is happening at the level of each entity, in this case Pseudomonas aeruginosa. These bacteria have pilus motors, which are key to spiral waves.

Pili motors are molecular motors that attach to pili, the tiny hair-like appendages on the surface of bacterial cells. These motors play important roles in various processes in bacteria, such as motility and surface attachment.

Dr. Yilin Wu, co-author of the study, explained: “The propagating spiral waves are caused by the coordinated activity of pilus motors, a grapnel-like motor organelle found in many bacterial species.”

The mechanical motion of pilus motors in many bacteria generates these spiral waves, which act like ripples on the bacterial surface.

Protein markers and coupled oscillators

To study spiral waves, researchers used experimental techniques and mathematical models.

The researchers relied on using fluorescent proteins as markers. They tracked the movements of single cells by labeling small groups of cells with these fluorescent proteins.

They then used microscopes to observe the behavior of single bacteria and groups of bacteria. The researchers also used these markers to track cell density to visualize the spatial distribution of cells within bacterial populations.

To further understand the role of pilus motility activity in spiral wave generation, the researchers treated bacterial populations with drugs known to affect pilus motility activity. By observing the effects of these treatments on wave dynamics, they could infer the importance of hair motors in wave formation.

Finally, the researchers developed a mathematical model based on coupled oscillators, in which the motion of one oscillator affects the others and vice versa. Mathematical models were built to simulate the behavior of bacterial populations and validate their experimental work.

Non-reciprocating interactions and large-scale coordination

The researchers found that the spiral waves are generated by the coordinated activity of pilus motors. They also observed that the wave was self-sustaining and stable, with its spiral core almost stationary.

This stability is a feature shared by certain types of electrical and chemical spiral waves in other living systems. However, the spiral waves observed in bacteria are different from other spiral waves.

Dr. Liu explained: “The spiral tension waves we found in bacterial populations are caused by cyclic mechanical processes at the single-cell level and are different from the spiral waves in most chemical/biological processes, where Spiral waves are in the form of oscillations.

“Moreover, spiral tension waves in bacterial populations arise spontaneously without external stimuli or inhomogeneities, whereas spiral waves in many other systems require stimuli or spatial inhomogeneities.”

In addition, the researchers also demonstrated the role of non-reciprocal interactions between bacterial cells in spiraling waves. They found that these interactions – which are asymmetric, meaning one cell’s influence on another is not mirror-image – are crucial for the stable formation of spiral waves.

Essentially, this means that these interactions can lead to some form of self-organization (or maintenance), giving rise to large-scale collective behavior or emergent phenomena, such as the propagation of spiral waves.

Biofilm and diffusion

These findings shed light on bacterial populations and behaviors, such as biofilm formation.

When bacteria adhere to surfaces, they produce extracellular polymeric substances (EPS). This material forms a structured community called a biofilm that embeds bacteria in an EPS matrix, protecting the bacteria from environmental stresses such as antibiotics and the host immune response.

This entire process is called biofilm formation and is critical for the survival of bacterial colonies. The flip side of this phenomenon—dispersion—is equally important.

When bacteria within a biofilm separate and spread to new locations, it is called dispersal. Diffusion can occur based on environmental factors, availability of nutrients, or as part of the bacterial life cycle.

This mechanism can help bacteria colonize new surfaces or host environments and can influence the spread of infectious diseases or the formation of microbial communities in various ecosystems.

The researchers believe that the Pirus motor could act not only as a mechanical actuator but also as a sensor. This means they can detect mechanical stimuli in the environment, allowing synchronized movement within bacterial populations.

“We believe that the coordination, or coupling, of pili activity enables bacterial populations to control large-scale tension and may influence their spread,” Dr. Wu explained.

Therefore, understanding spiral waves can help understand the behavior of bacterial species.

Furthermore, steady-state spiral waves have been found in many different systems. “Thus, pilus-driven wave patterns in bacterial material may provide a tractable mechanical analog for studying the origin and control of stable spiral waves in different living systems such as heart tissue,” explained Dr. Liu.

For future work, the researchers hope to study how to control spiral waves.

“This information may guide the control of stable spiral waves in other living systems. For example, control of spiral waves in cardiac tissue associated with life-threatening arrhythmias,” said Dr. Wu.

2024 Science