Bacterial interaction forces with surfaces
evaluated in a parallel plate flow chamber and using AFM

Henk J. Busscher
Department of  Biomedical Engineering
University Medical Center Groningen and University of Groningen
Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands.

Microbial adhesion and subsequent growth to biomaterials implant surfaces in the human body are the 
initial events leading to a biomaterials infection. Despite all preventive and therapeutic measures, 
biomaterials infection is the main cause of implant failure.  This lecture focuses on the forces that 
mediate microorganism adhesion to biomaterials surfaces. Such adhesion can be mediated by a 
variety of forces, including Lifshitz-Van der Waals, electrostatic and acid-base forces. A parallel plate 
flow chamber is ideally suited for adhesion studies because of the well-defined mass transport 
conditions, i.e. convective-diffusion. Interaction forces can be inferred from experiments using different 
shear rates, as well as by passing air-bubbles through the system, causing microbial detachment by 
the passing liquid-air interface.  Shear-off and residence time-dependent desorption experiments in a 
parallel plate flow chamber have indicated that microbial interaction forces become stronger over time, 
even in the absence of growth.  This so-called bond strengthening can also be monitored by 
measuring retract force-distance curves as a function of the time during which contact has been 
allowed between a microbial AFM probe and a substratum surface. Various examples of bond 
strengthening measured using AFM will be given.  Poisson analyses of the minor adhesion force 
peaks in retract force-distance curves demonstrate that bond strengthening is mainly due to the 
progressive involvement of acid-base interaction forces. Lifshitz-Van der Waals forces on non-
conducting substratum surfaces turn out to be repulsive in this analysis, presumably because the 
forced nature of AFM contact decreases the distance between the interacting surfaces to beyond the 
interaction minimum. On conducting substrata, Poisson analysis indicates attractive Lifshitz-Van der 
Waals next to acid-base attraction, which we attribute to the development of a mirror charge in a 
conducting substrate from the negatively charged microorganism.  The values of microbial interaction 
forces derived from flow chamber experiments and AFM, however, are orders of magnitude apart, 
which might be due to the fact that the AFM contact is more heavily enforced upon the system than 
contact during convective-diffusion in a flow chamber.