Physical properties of native bacterial biofilm cells measured by atomic force microscopy*

Atomic force microscopy offers a way to probe physical properties of bacteria that are adhered to a surface. We study early stage biofilms that natively adhere to a glass surface, without artificial fixation methods. We present images and force curves from five different bacteria, consisting of two gram positive and three gram negative strains, as well as both smooth and rough gram negative strains. The linear portion of the approach curve reveals the gram positive strains are stiffer than the gram negative strains. The non-linear portion of the approach curve, determined by the initial interaction between the tip and cell, differentiates the smooth and rough strains. Fixation of free-swimming planktonic cells by NHS and EDC dramatically changed the measured properties. These results can be understood from the structure of the cells.