Micro-cantilever method for measuring the tensile strength of biofilms and microbial flocs

Cohesive strength is an important factor in determining the structure and function of biofilm systems, and cohesive strength plays a key role in our ability to remove or control biofilms in engineered systems. A micro-mechanical device has been developed to directly measure the tensile strength of biofilms and other microbial aggregates. An important feature of this method is the combination of a direct measurement of force with particle separations that occur at a scale comparable to that observed for biofilm systems. The force required to separate an aggregate is determined directly from the deflection of cantilevered glass micropipettes with a 20-40-μm diameter. Combined with an estimate of the cross-sectional area of the aggregate at the point of separation this measurement indicates the cohesive strength of the aggregate. Samples of return activated sludge (RAS) and a Pseudomonas aeruginosa biofilm were tested using the device. The measured cohesive strengths of the RAS flocs ranged from 419 to 206,400 N/m², while many of the flocs exceeded the range of measurement of the device. Fragments of P. aeruginosa biofilm had cohesive strengths ranging from 395 to 15,640 N/m², with a median value of 3020 N/m². The median equivalent diameters of the particles detached from the aggregates were 32 μm for RAS and 30 μm for the P. aeruginosa biofilms.