In a series of experiments, a team of biologists at the University of California, San Diego studied the structure of growing colonies comprised of two species of rod-shaped bacteria, Escherichia coli and Acinetobacter baylyi; the researchers were surprised to see that the highly-motile Acinetobacter baylyi accelerated the spread of the slow Escherichia coli and that the structure of the expanding two-species colony became highly heterogeneous and produced flower-like patterns.
Escherichia coli is a good swimmer, but has a hard time moving on solid surfaces. When a droplet of liquid containing these slow bacteria is placed in a Petri dish containing a jelly-like substance called agar, the droplet barely expands over a 24-hour period.
On the other hand, a droplet containing Acinetobacter baylyi expands rapidly on agar because these bacteria are able to crawl using microscopic legs called pili.
“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,” said Dr. Liyang Xiong, a researcher in the Department of Physics and the BioCircuits Institute at the University of California, San Diego.
“The beauty of the pattern struck me, and I began to wonder how bacterial cells could interact with each other to become artists.”
Dr. Xiong and colleagues set out to investigate how a colony containing both Escherichia coli and Acinetobacter baylyi developed on a solid surface.
The experiments showed that when a droplet of liquid containing both species was placed on agar, both species grew and spread rapidly, as if Escherichia coli hitchhiked on the highly motile cells of Acinetobacter baylyi.
Furthermore, the growing colony developed a complex flower-like shape.
The team then developed mathematical models that took into account the different physical properties of the two bacterial species, 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 Escherichia coli that accumulate there.
In areas where there is less Escherichia coli accumulation, there is also less friction, allowing the boundaries to push out faster.
In the areas where there is more Escherichia 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 species that has a sufficiently higher growth rate and/or effective surface friction, which could have important implications in studying growing biofilms.
“Bacterial pattern formation has been an active area of research in the last few decades,” said Dr. Lev Tsimring, a researcher in the BioCircuits Institute at the University of California, San Diego and the San Diego Center for Systems Biology.
“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.”
The team’s results were published in the journal eLife.
Liyang Xiong et al. 2020. Flower-like patterns in multi-species bacterial colonies. eLife 9: e48885; doi: 10.7554/eLife.48885