The team of physicists, mathematicians, chemists, and biologists examined the formation of biofilms in Bacillus subtilis, a type of rod-shaped bacteria often found in soil. Their focus on this particular species was led by Roberto Kolter, Professor of Microbiology and Immunobiology at Harvard Medical School, an expert on biofilms and the genomics of B. subtilis.
"This project establishes a link between the phenotype, the physically observable traits of biofilm growth, and the genetic underpinning that allows spreading to happen in B. subtilis," notes co-principal investigator Michael Brenner, the Glover Professor of Applied Mathematics and Applied Physics at SEAS.
The researchers had speculated about a possible connection between the biofilm's quest for nutrition and the process of spreading. Because biofilms absorb nutrients through their exposed surface area, they can only swell vertically to a certain point before the surface-area-to-volume ratio makes it impossible to adequately nourish every cell. At this point, the biofilm must begin to spread outward so that the surface area increases along with the number of cells.
The ECM, a complex mesh of proteins, sugars, and other components outside of the individual cells, holds the key to one aspect of this movement: it apparently increases osmotic pressure within the biofilm.
In response to the increased pressure, the biofilm immediately absorbs water from its surroundings, causing the entire mass to swell upward. The final change in the shape of the biofilm is due to a combination of this swelling and the horizontal spreading that follows.
Seminara and Brenner created a mathematical model that mirrored many of the team's physical observations. The model supported the experimental observations; by considering the relationship between s
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