  We took benefit from Atomic Force Microscopy (AFM) in the force spectroscopy
mode to describe the time evolution-over 24h-of the surface nanotopography and
mechanical properties of the strain Staphylococcus aureus 27217 from bacterial
adhesion to the first stage of biofilm genesis. In addition, Transmission
Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) experiments
allowed identifying two types of self-adhering subpopulations (the so-called
'bald' and 'hairy' cells) and revealed changes in their relative populations
with the bacterial culture age and the protocol of preparation. We indeed
observed a dramatic evanescing of the 'hairy' subpopulation for samples that
underwent centrifugation and resuspension processes. When examined by AFM, the
'hairy' cell surface resembled to a herringbone structure characterized by
upper structural units with lateral dimensions of ~70 nm and a high Young's
modulus value (~ 2.3 MPa), a mean depth of the trough between them of ~15nm and
a resulting roughness of ~5nm. By contrast, the 'bald' cells appeared much
softer (~ 0.35 MPa) with a roughness one order of magnitude lower. We observed
too the gradual detachment of the herringbone patterns from the 'hairy'
bacterial envelope of cell harvested from a 16h old culture and their
progressive accumulation between the bacteria in the form of globular clusters.
The secretion of a soft extracellular polymeric substance was also identified
that, in addition to the globular clusters, may contribute to the initiation of
the biofilm spatial organization.
