Critical shear stresses of Pseudomonas aeruginosa biofilms from dental unit waterlines studied using microfluidics and additional magnesium ions

Abstract

A microfluidic approach was used to study the effect of shear stress on biofilms from a dental unit waterline (DUWL)-isolated P. aeruginosa strain, PPF-1. During the application of relevant shear stress levels to DUWLs, the response of the PPF-1 biofilm was observed and compared to that of a well-studied clinical P. aeruginosa strain, PAO1. The response measurements were repeated for biofilms exposed to additional Mg2+ ions. Optical density maps were transformed into pseudo three-dimensional representations of the complex biofilm structures, and computational fluid dynamic simulations were used to determine the critical shear stresses for biofilm sloughing. In the absence of Mg2+, PPF-1 biofilms showed weaker attachment than PAO1 biofilms and highly intertwined slough/regrowth cycles occurring within the shear stress range of 1.42 ± 0.32 and 0.95 ± 0.27 Pa. This suggests that in a low ionic environment, the PPF-1 strain produces ejected biofilm material nearly continuously, which can result in increased downstream colonization of engineered flow systems. Introducing Mg2+ into the PPF-1 biofilm culture increased mechanical stability, which resulted in elevated tolerances to shear stresses up to a critical value of 5.43 ± 1.52 Pa, which was similar to the critical shear stress value of 4.23 ± 1.22 Pa for the PAO1 strain. Moreover, the enhanced Mg2+ concentrations seemed to place the PPF-1 biofilm into a viscoplastic mechanical state, which resulted in signature responses to critical shear stresses, such as catastrophic sloughing involving abrupt tearing that produced clean edges at the fracture boundary, indicating that the biofilm had become brittle.