Bacterial Adhesion and Interaction with Biomaterial Surfaces - Biointerfaces: Where Material Meets Biology - page 386

for visualizing the high resolution of living bacterial cell surface and char-
acterizing their surface properties. 176 M ´ ndez-Vilas et al. characterized two
strains of S. epidermidis with different slime production by AFM and revealed
the nanostructure of cell surfaces and the adhesion correlated to the slime-
production zone on surfaces. 177 Dufr ˆ ne and colleagues reviewed the ap-
plication of AFM techniques in exploring the microbial surfaces such as
analysis of cell surface proteins and detection of individual cell surface re-
ceptors. 178,179 The development and application of AFM greatly contribute to
understanding of

d n 3 r 4 n g | 3

the structure-function relationship of bacterial cell

surfaces.
Apart from visualization of high resolution images of bacterial cell surface
and biomaterial surface, an important feature of AFM is the ability to directly
measure the interaction forces between bacterial cells and material surface
at the nano-newton scale. Force measurements are made by recording the
deflection of the cantilever measured as a function of the relative probe
motion as it approaches and then retracts, or separates, away from the
surface, called as force-distance curve. 174 A force-distance curve records the
variation of interaction forces as the bacteria approaches and then separates
from the sample surface. Physicochemical forces such as van der Walls and
electrostatic forces can be measured from the approaching region of force
curve, while the adhesion force or binding strength is measured from the
deflection of cantilever during separation. Furthermore, the slope of re-
pulsive section of the approaching curve also provides the information about
nano-mechanical properties of bacterial cell surface. 180 In force measure-
ment, bacteria are immobilized either on a substrate 169 or on an AFM
probe 181 so that it is possible to measure the cell-material interactions and
map the adhesion properties of cell surface by force volume image mode.
The immobilization of bacterial cells to AFM probe also makes it possible to
measure the interaction forces of cell-cell which is important for biofilm
development. 182 The typical values of adhesion forces of cell-material or
cell-cell vary in a wide range from pico-newtons to nano-newtons, even in
tens of nano-newtons, depending on cell, material surface, environment,
and operation conditions. The larger adhesion force, the greater is the ten-
dency towards bacterial adhesion. Therefore, measurement of adhesion
forces on single bacterium cell surface, cell-cell interactions, and cell-
substratum can lead to better understanding of the mechanism of bacterial
adhesion and biofilm formation on biomaterial surfaces.
AFM probing the interaction forces of microbial cells requires them to be
physically or chemically attached to a substrate surface or onto AFM probe.
Among them, the immobilization of cells on an AFM probe is a challenge for
experiments. A protocol for the immobilization of bacterial cells on an AFM
probe, which was reported in the literature, 156,181 is described in Appendix
A.2. Kang and Elimelech 183 also reported a similar method to attach the live
single cell onto polydopamine-coated tipless AFM probe.
Molecular recognition is an important event in immunological reactions
and microbial infection. Using biologically modified AFM tips, it is possible

.