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 c ell surface softness of a fibrillated oral streptococcal strain Streptococc us salivarius HE and of a nonfibrillated strain S. salivarius HBC12 is pres ented, and the data interpretation is validated by comparison with results from independent techniques. Upon approach of the fibrillated strain in wat er, the AFM tip experienced a long-range repulsion force, starting at simil ar to 100 nm, attributed to the compression of the soft layer of fibrils pr esent at the cell surface. In 0.1 M KCl, repulsion was only experienced whe n the tip was closer than similar to 10 nm, reflecting a stiffer cell surfa ce due to collapse of the fibrillar mass. Force-distance curves indicated t hat the nonfibrillated strain, probed both in water and in 0.1 M KCl, was m uch 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 w ater being the only specimen that showed characteristic topographical featu res attributable to fibrils, These results are in excellent agreement with previous indirect measurements of cell surface softness by dynamic light sc attering and particulate microelectrophoresis and demonstrate the potential of AFM to directly probe the softness of microbial cell surfaces.