Direct observation of the cell-wall remodeling in adhering Staphylococcus aureus 27217: an AFM study supported by SEM and TEM

TL;DR

This study uses AFM, SEM, and TEM to observe the dynamic cell-wall remodeling of Staphylococcus aureus during early biofilm formation, revealing structural and mechanical changes in subpopulations over 24 hours.

Contribution

It provides the first detailed nanoscale visualization of cell-wall changes and subpopulation dynamics during initial biofilm development in S. aureus.

Findings

Identification of 'bald' and 'hairy' subpopulations with distinct structures and stiffness

Observation of herringbone surface pattern and its detachment over time

Detection of extracellular polymeric substances contributing to biofilm formation

Abstract

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.