Biofilm Formation


Biofilms are communities of bacterial cells that are encased in a self-produced extracellular matrix—sort of like a bunch of cells stuck in a gluey substance. Biofilms are important, both in medicine and in industry. Nuisance biofilms can form in pipes and in industrial equipment, while infectious biofilms are difficult for doctors to treat. Because they are encased in a matrix, biofilm cells are relatively shielded from antibiotics, the immune system, and other therapeutics; moreover, because some cells in the interior of a biofilm are not growing, they are especially resistant to antibiotics, making biofilm infections notoriously difficult to eradicate. Therefore, scientists are actively looking for other ways to combat biofilm infections. One of the ways we can do this is to gain a better understanding of how biofilms form—the signals and mechanisms that tell cells to settle down and secrete an extracellular matrix.

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In the Cabeen lab we study biofilm formation by P. aeruginosa, a opportunistic human pathogen that can infect many body sites but is particularly well-known for lung infections, particularly in individuals with cystic fibrosis (which is a genetic disorder). Because of its medical importance, much is known about how biofilms are signaled and constructed in P. aeruginosa. Upstream signals induce the formation of a bacterial second-messenger compound known as cyclic-di-GMP (cdG), a cyclic dinucleotide molecule that can be thought of as a sort of "internal office memo" in the bacterial cell. High levels of cdG in the cell generally encourage biofilm formation, yet we still have much to learn about all the proteins that control cdG levels or respond to cdG to govern biofilm formation.

Therefore, we have taken a screening approach that differs from previous screens in P. aeruginosa. We use colony morphology as a way to assess biofilm formation: wrinkled colonies indicate biofilm formation, whereas smooth and flat colonies indicate that no biofilm has formed (Figure 2). In this way, we can easily assess by visual inspection whether a mutant strain has increased (more wrinkled) or decreased (less wrinkled) biofilm relative to a parental (non-mutant) strain. This approach has uncovered several genes with previously unappreciated roles in biofilm formation.

We are currently continuing with the screen to uncover even more genes with roles in biofilm formation while following up on the genes we have already discovered. We are asking:

How do proteins that reduce biofilm formation fit in with known signaling pathways?

How do proteins that increase biofilm formation fit in with known signaling pathways?

How do our candidate proteins affect cdG signaling?

Do our candidate proteins respond to or interact with cdG?

What other proteins interact with our candidate proteins?

It's easy to get started on this topic in our lab with very little experience, as the visual screen is easy to learn and master. Meanwhile, our current candidate genes are ripe for further characterization. An exciting aspect of this project is that we follow the genes where they lead us—we may have to learn several different techniques and approaches to get the answers we need. Students who are interested in these questions should contact us and visit the lab.