  We present a study about AFM imaging of living, moving or self-immobilized,
bacteria in their genuine physiological liquid medium. No external
immobilization protocol, neither chemical nor mechanical, was needed. For the
first time, the native gliding movements of Gram-negative Nostoc cyanobacteria
upon the surface, at speeds up to 900microns/h, were studied by AFM. This was
possible thanks to an improved combination of a gentle sample preparation
process and an AFM procedure based on fast and complete force-distance curves
made at every pixel, drastically reducing lateral forces. No limitation in
spatial resolution or imaging rate was detected. Gram-positive and non-motile
Rhodococcus wratislaviensis bacteria were studied as well. From the approach
curves, Young modulus and turgor pressure were measured for both strains at
different gliding speeds and are ranging from 20 to 105MPa and 40 to 310kPa
depending on the bacterium and the gliding speed. For Nostoc, spatially limited
zones with higher values of stiffness were observed. The related spatial period
is much higher than the mean length of Nostoc nodules. This was explained by an
inhomogeneous mechanical activation of nodules in the Nostoc. We also observed
the presence of a soft extra cellular matrix (ECM) around the Nostoc bacterium.
Both strains left a track of polymeric slime with variable thicknesses. For
Rhodococcus, it is equal to few hundreds of nm, likely to promote its adhesion
to the sample. While gliding, the Nostoc secretes a slime layer the thickness
of which is in the nanometer range and increases with the gliding speed. These
results open a large window on new studies of both dynamical phenomena of
practical and fundamental interests such as the formation of biofilms and
dynamic properties of bacteria in real physiological conditions.
