  Enterotoxigenic Escherichia coli (ETEC) are a major cause of diarrhea
worldwide, and infection of children in underdeveloped countries often leads to
high mortality rates. Isolated ETEC express a plethora of colonization factors
(fimbriae/pili), of which CFA/I and CFA/II that are assembled via the alternate
chaperone pathway (ACP), are amongst the most common. Fimbriae are filamentous
structures, whose shafts are primarily composed of helically arranged single
pilin-protein subunits, with a unique biomechanical capability allowing them to
unwind and rewind. A sustained ETEC infection, under adverse conditions of
dynamic shear forces, is primarily attributed to this biomechanical feature of
ETEC fimbriae. Recent understandings about the role of fimbriae as virulence
factors are pointing to an evolutionary adaptation of their structural and
biomechanical features. In this work, we investigated the biophysical
properties of CS2 fimbriae from the CFA/II group. Homology modelling its major
structural subunit CotA reveals structural clues and these are related to the
niche in which they are expressed. Using optical tweezers force spectroscopy we
found that CS2 fimbriae unwind at a constant force of 10 pN and have a corner
velocity of 1300 nm/s, i.e., the velocity at which the force required for
unwinding rises exponentially with increased speed. The biophysical properties
of CS2 fimbriae assessed in this work classify them into a low-force unwinding
group of fimbriae together with the CFA/I and CS20 fimbriae expressed by ETEC
strains. The three fimbriae are expressed by ETEC, colonize in similar gut
environments, and exhibit similar biophysical features, but differ in their
biogenesis. Our observation suggests that the environment has a strong impact
on the biophysical characteristics of fimbriae expressed by ETEC.
