Evolution-in-Action

Experimental Evolution of Biofilms with Pseudomonas fluorescens


Meet Pseudomonas fluorescens

Pseudomonas fluorescens is a harmless strain of bacteria that can be found naturally in the environment. The particular strain of bacteria that we use in our Evolution-in-Action Program, SBW25, was isolated from the root of a sugar beet plant.

Like most Pseudomonads, it is capable of forming a thick exopolysaccharide layer called biofilm. In addition to forming biofilm, P. fluorescens SBW25 is also capable of adapting quickly, thus allowing it to diversify. 

 

The Biofilm Life Cycle

During a normal biofilm lifecycle, planktonic cells in the media adhere to a surface, mature and then disperse to allow for continuation of the cycle. 

 
 

The Bead Transfer Model

We have developed a model that mimics the normal biofilm lifecycle, which is shown below. Through the serial transfer of 7 mm polystyrene beads, planktonic cells are required to adhere to the bead, mature and then disperse onto an oppositely marked bead.

With this model, we can study how bacteria evolve within biofilms

 

Introduction of a bead both localizes interactions among colonizing cells and preserves any beneficial mutants that arise during growth

Serial bead transfers are carried out over the course of seven days, during which the biofilm will become diverse. Identification of these adaptive mutants is accomplished through the spread-plating of each culture, which is then screened for varying colony morphologies that are associated with mutations that have led to biofilm formation.

 

The expected visual results can be seen on the right, where a biofilm coated test tube and agar plate with varying colony morphology were generated by high school students that conducted Evolution-in-Action in their classroom.