  The anchoring biofilm layer is expected to exhibit a different response to
environmental stresses than for portions in the bulk, due to the protection
from other strata and the proximity to the attachment surface. The effect of
hydrodynamic stress on surface-adhered biofilm layers was tested using a
specially designed microfluidic bio flow cell with an embedded three-electrode
detection system. In situ electrochemical impedance spectroscopy (EIS)
measurements of biocapacitance and bioresistance of Pseudomonas sp. biofilms
were conducted during the growth phase and under different shear flow
conditions with verification by other surface sensitive techniques.
Distinctive, but reversible changes to the amount of biofilm and its structure
at the attachment surface were observed during the application of elevated
shear stress. In contrast, regular microscopy revealed permanent distortion to
the biofilm bulk, in the form of streamers and ripples. Following the
application of extreme shear stresses, complete removal of significant portions
of biofilm outer layers occurred, but this did not change the measured quantity
of biofilm at the electrode attachment surface. The structure of the remaining
biofilm, however, appeared to be modified and susceptible to further changes
following application of shear stress directly to the unprotected biofilm
layers at the attachment surface.
