Development and testing of a novel microcantilever technique for measuring the cohesive strength of intact biofilms.

Cohesive strength is an important parameter for understanding and modeling the mechanics of biomass detachment from bacterial biofilms. It is challenging to measure the mechanical properties of biofilms, however, because biofilms may desiccate when removed from liquid medium and they are inherently fragile. Poppele and Hozalski (Poppele and Hozalski, 2003, J Microb Methods 55:607-615) presented a microcantilever method for measuring the tensile strength of detached biofilm fragments while submersed in liquid medium. Here we present a modification of the microcantilever method to quantify the strength of intact bacterial biofilms. Initial testing was performed on Pseudomonas aeruginosa biofilms and on Staphylococcus epidermidis biofilms grown in rotating disk reactors. The cohesive strength values were highly variable (i.e., coefficients of variation ranging from 71% to 143%) and ranged from 59 to 18,900 Pa for the P. aeruginosa biofilms and from 61 to 5,840 Pa for the S. epidermidis biofilms. The biofilms also appeared to be isotropic as strength did not vary with angle of testing relative to the direction of applied shear. Strength testing using both the intact and fragment methods was performed on five samples of P. aeruginosa biofilms, and the strength populations were not from the same distribution in three cases. Equivalent diameters for the fragments detached from biofilms during strength testing ranged from 5 to 500 microm, which is within the range of size of biofilm fragments observed in the effluents of lab-scale and full-scale bioreactors. The microcantilever is a simple yet powerful tool for measuring the cohesive strength of intact biofilms at a relevant scale.