Bacteria form complex structures like those seen in animals

Bacterial biofilms, slimy collections of microbes, can develop concentric rings containing cells with different biological features

By Alex Wilkins

Bacterial biofilms contain a level of structural organisation that we thought was unique to plants and animals.

Biofilms, slimy clumps of microorganisms like bacteria and fungi, were long thought to be biologically simple, with no more than a primitive level of structural organisation. This contrasts with many multicellular organisms, including animals, in which cells can grow into different forms at different times and places during the body’s development to produce complex and varied biological structures.

Now, Gürol Süel at the University of California, San Diego, and his colleagues have discovered that bacterial biofilms are less simple than we had thought. The researchers found that the biofilms form ring-like structures as they grow and consume the nutrients in their environment. As the nutrient supply diminishes, certain cells essentially become frozen in time in terms of the way they function, as a wave of nutrient depletion washes over them. This is known as a “clock and wavefront”, and has previously been seen only in animals and plants.

Süel and his colleagues made the discovery during an experiment to explore the response of a Bacillus subtilis biofilm to being starved of vital nitrogen. This typically causes bacterial cells to change and become more resilient in an adaptation called sporulation.

But rather than all the cells in the biofilm adapting in the same way, the researchers could demonstrate that stress-mitigating genes produced by the biofilm caused only some cells to adapt, creating concentric rings through the roughly circular biofilm. This tree ring-like structure is consistent with a “clock and wavefront” mechanism (see picture, above).

“If we just think of as globs of bacterial cells, even if they’re from one species, we’re mistaken,” says Süel. “They’re highly organised, and they’re organised in a very non-trivial way. This organisation seems to be reminiscent of what vertebrates and plants did during development, so there must be a connection there.”

Though the research was focused only on observing the patterns, Süel proposes that the patterning could be the biofilm diversifying its resilient cells to try to increase its chances of survival.

While biofilms have been shown to be more complicated in recent years, being capable of forms of memory and long-distance communication, the discovery of complex structures could challenge the assumed divide between simple, unicellular organisms and complex, multicellular ones.

“That debate will be rekindled by this study,” says Tanmay Bharat at the University of Oxford. “From an evolutionary cell biology perspective, it would be interesting to study where the differences lie. What defines a true multicellular organism?”

Biofilms are also responsible for a wide array of natural phenomena, both good and bad. They can be used in water filtration and to prevent corrosion, but they are also the leading cause of clinical infections, as well as being highly corrosive in some scenarios. Understanding the true underlying structure of these bacterial films could change the ways in which they are used and mitigated.

“You can’t just assume that one approach or one chemical agent is sufficient to do the job, because the biofilm is a complex community,” says Süel.

Journal reference: Cell, DOI: 10.1016/j.cell.2021.12.001