Surprising beauty found in bacterial cultures

In a paper published in a recent issue of eLife, a team of researchers at UC San Diego's BioCircuits Institute (BCI) and Department of Physics, led by Research Scientist and BCI Associate Director Lev Tsimring, reports that when non-motile E. coli (Escherichia coli) are placed on an agar surface together with motile A. baylyi (Acinetobacter baylyi), the E. coli "catch a wave" at the front of expanding A. baylyi colony.

The agar provided food for the bacteria and also a surface on which E. coli couldn't easily move (making it non-motile). A. baylyi, on the other hand, can crawl readily across the agar using microscopic legs called pili. Thus, a droplet of pure E. coli would barely spread over a 24-hour period, while a droplet of pure A. baylyi would cover the entire area of the petri dish.

Yet when the E. coli and A. baylyi were mixed together in the initial droplet, both strains flourished and spread across the whole area as the non-motile E. coli hitched a ride on the highly mobile A. baylyi. However, what most surprised researchers were intricate flower-like patterns that emerged in the growing colony over a 24-hour period.

"We were actually mixing these two bacterial species for another project, but one morning I found a mysterious flower-like pattern in a petri dish where a day earlier I placed a droplet of the mixture. The beauty of the pattern struck me, and I began to wonder how bacterial cells could interact with each other to become artists," said Liyang Xiong, Ph.D. '19, who was a graduate student in the Physics Department and is the lead author of the study.

To uncover how the flower patterns were formed, Xiong et al. developed mathematical models that took into account the different physical properties of the two strains, primarily the differences in their growth rate, motility, and effective friction against the agar surface. The theoretical and computational analysis showed that the pattern formation originates at the expanding boundary of the colony, which becomes unstable due to drag exerted by the E. coli that accumulate there.

In areas where there is less E. coli accumulation, there is also less friction, allowing the boundaries to push out faster. In the areas where there is more E. coli accumulation and more friction, the boundaries stagnate. This is what creates the "petals" of the flower.

Further analysis suggests this type of pattern is expected to form when motile bacteria are mixed with a non-motile strain that has a sufficiently higher growth rate and/or effective surface friction, which could have important implications in studying growing biofilms.

Biofilms are communities of microorganisms -- including bacteria and fungi -- that adhere to each other and to surfaces, creating strong matrices that are difficult to break down. Common examples include dental plaque and pond scum. They also grow in medical devices such as pacemakers and catheters. Learning how non-motile bacteria can "stick" to motile bacteria may provide insight into how biofilms are formed and how they can be eliminated.

"Bacterial pattern formation has been an active area of research in the last few decades," said Lev Tsimring, "However, the majority of laboratory studies and theoretical models were focused on the dynamics of single-strain colonies. Most bacteria in natural habitats live in multi-strain communities, and researchers are finally beginning to look for mechanisms controlling their co-habitation. While a number of biochemical mechanisms of inter-species communication and cooperation have been identified, we found that surprising complexity may result from purely physical interaction mechanisms."