Direct probing by atomic force microscopy of the cell surface softness of a fibrillated and nonfibrillated oral streptococcal strain

In this paper, direct measurement by atomic force microscopy (AFM) of the cell surface softness of a fibrillated oral streptococcal strain Streptococcus salivarius HE and of a nonfibrillated strain S. salivarius HBC12 is presented, and the data interpretation is validated by comparison with results from independent techniques. Upon approach of the fibrillated strain in water, the AFM tip experienced a long-range repulsion force, starting at similar to 100 nm, attributed to the compression of the soft layer of fibrils present at the cell surface. In 0.1 M KCl, repulsion was only experienced when the tip was closer than similar to 10 nm, reflecting a stiffer cell surface due to collapse of the fibrillar mass. Force-distance curves indicated that the nonfibrillated strain, probed both in water and in 0.1 M KCl, was much stiffer than the fibrillated strain in water, and a repulsion force was experienced by the tip at close approach only (20 nm in water and 10 nm in 0.1 M KCl). Differences in cell surface softness were further supported by differences in cell surface morphology, the fibrillated strain imaged in water being the only specimen that showed characteristic topographical features attributable to fibrils, These results are in excellent agreement with previous indirect measurements of cell surface softness by dynamic light scattering and particulate microelectrophoresis and demonstrate the potential of AFM to directly probe the softness of microbial cell surfaces.