In situ high-resolution atomic force microscope imaging of biological surfaces was performed on cells with relatively rigid surfaces (e.g., bacteria). The surface of Lactobacillus helveticus (a rod-shaped bacterium) was investigated before and after exposure to LiCl, a denaturant. Image details were stable both at variant force loads and under different scan directions. From images of the oblique lattice structure (i.e., S layer of L. helveticus), it was estimated that the lateral resolution of the images was up to 2 nm. This resolution can be explained by assuming that there is an apex with a curvature of radius of similar to 10 nm near the end of the tip. Modelling of this geometry indicates that such a tip configuration is particularly suitable for in situ high-resolution imaging of relatively soft objects covered by a rigid shell (membrane).