PROBING BACTERIAL ADHESION USING FORCE SPECTROSCOPY - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 288

0.40 nN for F adh between BSA and PAO1. The higher adhesion of the mutant

strain was believed to be due to absence of B-band saccharides and the
shorter A-band unit on strain AK1401, which allowed for the lipid A and core
region to be more exposed than in the parent strain. The lipid A and core
region have strong afinity for BSA because of hydrogen bonding. We did not
ind that electrostatic or steric interactions were dominant in controlling P.

aeruginosa interactions with BSA. This work also demonstrated that afinity

of a bacterium for a protein coating depends on molecular properties of the
bacterial surface molecules. In other words, it may be very dificult to design
surfaces that are resistant to all types of bacterial colonization, because subtle
differences in bacterial surface molecules control whether or not they will ind
a particular surface attractive. This inding suggests that a biomaterial should
have more than one type of functionality, perhaps incorporating multiple
mechanisms of preventing bacterial adhesion on a given biomaterial.

13.4 MICROBE MICROBE INTERACTIONS

In the natural environment, bacteria are much more commonly found
associated with a solid surface, rather than in free-loating form. AFM has

been very useful in measuring the forces the bacteria experience during
the initial adhesion process, which is an established strategy for preventing
bioilm formation. However, bacterial interactions with an inert surface are
different than their interaction with another bacterium, or another type of
biological cell.
As an example, we studied the adhesive interactions between a single
cell of

48

that was immobilized to an AFM cantilever, and either
silicone rubber or silicone coated with a

C. parapsilosis

P. aeruginosa bioilm.

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Using a C.

parapsilosis cell probe, the interaction with a silicone substrate was adhesive,

with forces of 2.3 ± 0.25 nN in the approach portion of the force cycle. However,
when the

bioilm, the
attractive force from the approach curve decreased to 2.0 ± 0.40 nN. We also
saw an unusual repulsive force (2.0 nN) in the AFM approach curve at longer
distances of ~75 nm ( Fig. 13.4 ) . This repulsion may be attributed to steric
and electrostatic interactions between the two microbial polymer brushes.
The attractive forces are too large (and occur at distances too long) to be
van der Waals or electrostatic interactions. We think that EPS molecules from
C. parapsilosis

C. parapsilosis probe was contacted with a P. aeruginosa

were forming speciic adhesive interactions with functional
groups on the silicone rubber. Although we did not model such forces, they
likely are hydrogen bonding interactions. The forces were slightly decreased
when the

C. parapsilosis contacted the P. aeruginosa (bacterial) bioilm,