Deformation of Lactococcus lactis surface in atomic force microscopy study

Abstract

The surface of the bacterium Lactococcus lactis was investigated under water by contact mode atomic force microscopy (AFM) while varying the imaging parameters (imaging force, scanning velocity, scanning direction). Height images were three-dimensionally (3-D) reconstructed using GOCAD© software; this revealed grooves oriented along the scanning direction. Although the grooves were created by the scanning probe, they were not due to a single passage of the probe; the periodicity of the grooves was indeed always three to four times larger than the scanning line periodicity (ratio between the image size and the number of scanning lines). Upon repeated imaging at low force, the grooves were re-created each time with the same morphology; groove depth was increased at higher imaging force. The groove formation is tentatively explained by a deformation of the surface, due to compression or to adhesion, and its slow relaxation. While the grooves reveal a perturbation of the cell surface by the AFM probe, they provide pertinent information about the nanomechanical properties of the cell surface.