Characterizing Pilus-Mediated Adhesion of Biofilm-Forming <i>E. coli</i> to Chemically Diverse Surfaces Using Atomic Force
Microscopy
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Abstract
Biofilms are complex communities
of microorganisms living together
at an interface. Because biofilms are often associated with contamination
and infection, it is critical to understand how bacterial cells adhere
to surfaces in the early stages of biofilm formation. Even harmless
commensal <i>Escherichia coli</i> naturally forms biofilms
in the human digestive tract by adhering to epithelial cells, a trait
that presents major concerns in the case of pathogenic <i>E.
coli</i> strains. The laboratory strain <i>E. coli</i> ZK1056 provides an intriguing model system for pathogenic <i>E. coli</i> strains because it forms biofilms robustly on a
wide range of surfaces.<i>E. coli</i> ZK1056 cells spontaneously
form living biofilms on polylysine-coated AFM cantilevers, allowing
us to measure quantitatively by AFM the adhesion between native biofilm
cells and substrates of our choice. We use these biofilm-covered cantilevers
to probe <i>E. coli</i> ZK1056 adhesion to five substrates
with distinct and well-characterized surface chemistries, including
fluorinated, amine-terminated, and PEG-like monolayers, as well as
unmodified silicon wafer and mica. Notably, after only 0–10
s of contact time, the biofilms adhere strongly to fluorinated and
amine-terminated monolayers as well as to mica and weakly to “antifouling”
PEG monolayers, despite the wide variation in hydrophobicity and charge
of these substrates. In each case the AFM retraction curves display
distinct adhesion profiles in terms of both force and distance, highlighting
the cells’ ability to adapt their adhesive properties to disparate
surfaces. Specific inhibition of the pilus protein FimH by a nonhydrolyzable
mannose analogue leads to diminished adhesion in all cases, demonstrating
the critical role of type I pili in adhesion by this strain to surfaces
bearing widely different functional groups. The strong and adaptable
binding of FimH to diverse surfaces has unexpected implications for
the design of antifouling surfaces and antiadhesion therapies