How a Biofilm’s Strange Shape Emerges From Cellular Geometry

Summary

In colloids as in biofilms, these interactions are governed by two opposing forces. One is repulsive: Two particles or two cells can’t physically occupy the same space at the same time. The other force is attractive. Cells are covered with sticky proteins that can fasten two cells together, much like the medium binding a colloid. If the repulsive force is stronger, cells don’t aggregate. But if the attractive force is stronger, it can spark the initial formation of a biofilm. What differentiates colloids from biofilms is growth, which a biofilm must balance between the horizontal and the vertical. It’s not unlike urban sprawl. Both Houston, Texas, and Queens, New York, house populations of around 2.3 million people, but they have completely different urban geometry. Around Houston, cheap and abundant land lets residents spread out, with mostly horizontal growth. Queens, on the other hand, is hemmed in by water and surrounding municipalities, so residents build more vertically. That’s how 2.3 million residents occupy 640 square miles in Houston and 109 square miles in Queens. Just like Texans and New Yorkers, biofilms weigh the trade-offs of horizonal versus vertical growth. “A single cell can only be in one place at any moment in time,” Yunker said. “The more that cells at the edge of a biofilm grow up, the less they can grow out.” This results in a kind of cellular geometry, which Yunker found plays an outsize role in a biofilm’s overall fitness, measured by its ability to expand and take in nutrients. “When we look at an animal, geometry is the first thing that comes to mind,” said Ming Guo, a mechanical engineer and biophysicist at the Massachusetts Institute of Technology who was not involved in the research. “Underneath that is the detail — the genetics, epigenetics, protein profile and cell types — that together define the entire biology.” Yunker decided to focus on what happens at a biofilm’s leading edge — the outer perimeter where cells most actively grow, ...