"The role of mechanical shear in the development of biofilms of bacteria"

Coffee and cookies will be served at 3:45pm.

Abstract: Biofilms are communities of interacting bacteria and other single-celled organisms that are attached to each other by a matrix of polymer and protein that is produced by the constituent bacteria. Biofilms are an attractive model system for multicellularity, because they show characteristics found in higher organisms, such as spatial organization with differentiated gene expression and cell function, in a reductionist system that is much more easily manipulated than are higher organisms. Biofilms are also a substantial practical problem, since they cause chronic, often-intractable infections that can last up to decades, and they can foul important built structures, such as ships and systems for water treatment and distribution. Biofilms have physical properties that are not possessed by non-biofilm collections of bacteria - the most prominent physical distinctives of biofilms are their spatial structure and their mechanical cohesion and adhesion. These are widely thought to strongly impact biological characteristics of biofilms, such as antibiotic resistance and evasion of clearance by the immune system, but very few specifics are known. We work to understand these connections. Here I present one specific case as an example of our larger body of work:

Biofilms initiate when bacteria attach to a surface. Attachment to a surface results in increased levels of an internal chemical signal that controls the expression of many bacterial genes to begin the change to a biofilm state. What specific cue is sensed by bacteria and results in increased signaling levels has not been known. We have recently shown that bacteria sense mechanical shear as a cue for surface attachment. Shear can result from motility of bacteria on the surface and from the flow of fluid over surface-attached bacteria. We use a combination of genetic manipulation and fluid flow to vary the shear experienced by bacteria and show that variable shear changes the level of internal, biofilm-triggering chemical signal. We use a few-parameter model to describe signal levels as a function of shear and the feedback loop between signaling and the strength of surface attachment, and find that this model describes our data well when biologically-reasonable values are used for biological parameters. Thus, our work has shown a physical basis for an important biological process.