AFM analysis of the extracellular polymeric substances (EPS) released during bacterial attachment on polymeric surfaces

Extracellular polymeric substances (EPS) secreted by bacteria have a key role in adhesion and aggregation of bacterial cells on solid surfaces. In the present study, atomic force microscopy (AFM) has been used to study the adhesion propensity of bacterial strain St. guttiformis, and the ultrastructure and distribution of the EPS materials, on hydrophobic poly(tert-butylmethacrylate) (PtBMA) and hydrophilic polystyrene maleic acid (PSMA) surfaces. The results showed that bacterial attachment to the PSMA surface over incubation periods of 24-72 h was insignificant, whereas there was a strong propensity for the bacterial cells to attach to the PtBMA surface, forming multi-layered biofilms. For the PSMA surface, planktonic EPS adsorbed onto the polymeric surface and formed a continuous surface layer. For the PtBMA surface, non-contact mode imaging revealed that capsular EPS on the cell surface exhibited granular structures with the lateral dimensions of 30-50 nm and the vertical roughness of 7-10 nm. Lateral force imaging showed inter-connected elongated features which had lower frictional property compared to the surrounding EPS matrix, suggesting possible segregation of hydrophobic fractions of the EPS materials. The planktonic EPS adsorbed onto the PtBMA surface also showed similar nanometer-scale granular structures and could form stacks up to 150 nm in height. However, lateral force imaging did not show frictional differences, as in the case of capsular EPS. This is attributed to possible differences in the composition of the two EPS materials, and/or greater deformation of the planktonic EPS in the contact imaging mode which may obscure the fine surface features.