Directly Measuring the Adhesive and Elastic Properties of Bacteria using a Surface Force Apparatus

Abstract

Bacterial adhesion is the first step of biofilm formation that plays various roles in the environment and the human body. Examples of undesirable roles of biofilm formation include metal rust, sewage sludge and bacteria-related diseases. Desirable roles are biofiltration and bioremediation.For a decade, Atomic Force Microscopy (AFM) has been the primary tool used to study the adhesion and elastic properties of individual bacteria. In this work we show it is possible to use a Surface Forces Apparatus (SFA) to measure elastic and adhesive properties of small collections of surface bound bacteria. The measurements are conducted with incomplete, patterned bacterial films and we have developed a protocol to image the contact area with AFM after the experiment. Using the SFA, we measured the force profile between a P. Aeruginosa PAO1 film and a bare mica surface. We repeated the measurement in the same contact position for up to ten days to determine the effect of desiccation on the film material properties, and then moved to the new contact area to measure the film thickness and elastic properties. A large shrinkage of the bacterial film thickness was measured during the first few days due to the bacterial film desiccation and rearrangement. The proportion of shrinkage depends on factors such as the bacterial film coverage, roughness, temperature and relative humidity. Thickness compressibility was estimated from the force curves. As a force approximation, the stress at the center of the contact (σ) and the area of the contact were estimated by applying the Hertz model. Since the film is incomplete the calculated area in contact was reduced by a factor estimated from the optical image of the contact zone. Adhesiveness was measured in receding force profiles. Maximum adhesive force was detected in the first day, due to the high capillary force, decreased by the bacterial film desiccation and increased again due to the conditioning film.