While just over half of humans live in urban areas, 99.9 percent of bacteria opt for urban dwelling in the form of biofilms. Biofilms are clusters of billions of bacteria — often joined by protozoa, algae, fungi, products of corrosion, and debris — existing within an extracellular matrix made of protein and sugar. They can cling to almost any surface, from the inside of a catheter to the surface of our teeth.
Biofilms are advantageous to bacteria, who adapt based on where in the biofilm “city” they’re located. Both the physical shape of the biofilm and the matrix structure itself protect the bacteria living toward the center from attack by antibiotics and predators, while those dwelling in the outer neighborhoods are more vulnerable. Those safer central cells might serve as genetic reservoirs or amass food reserves, while the outer dwellers might beat back attacks. And while to some extent the inhabitants are still competitive against each other, there is also cooperative living within the biofilm.
Scientists are now working to decode the rules that govern the ways biofilms form and evolve. The idea is that just as an urban planner can use their understanding of regulations and principles of engineering to make a city more or less inhabitable, a bioengineer or microbiologist can use the rules that govern biofilms to render any given location more or less hospitable for colonies of bacteria. This is a new way of considering bacteria, because bacteria growing in a culture in a lab act very differently than the complex urban center presented by the biofilm.
Microbiologist Katharine Coyte’s recent work with microfluidic chips explores how the number of bacteria and the flow of water can shape and limit the growth of biofilms. This work joins a growing body of literature that demonstrates the way physical rather than chemical forces — the stickiness of the extracellular matrix and hydrodynamic shear, for example — push the microbes to form complex collectives. This is a departure from the work of earlier biofilm researchers, who were much more focused on the chemical environments of the colonies.