Changes in surface roughness and topography on the macroscopic scale are known to affect bacterial attachment and retention. Little quantitative information is available as to how changes in surface topography on the micron and submicron scale affect the strength of bacterial attachment to substrata. A novel method is described using the atomic force microscope where a varying shear/lateral force (in nanonewtons) is used to detach individual bacterial cells from various substrata of different surface topographies. Lateral changes of 0.1 mum in the surface topography are sufficient to affect the strength of bacterial attachment. An increase in applied force from 4 to 8 nN was necessary to move bacteria retained in surface defects of approximately 1 mum wide and 0.2 mum deep compared with cells attached on smooth surfaces.