The nanomechanical properties of gram-negative bacteria ( Shewanella putrefaciens ) were investigated in situ in aqueous solutions at two pH values, specifically, 4 and 10, by atomic force microscopy (AFM). For both pH values, the approach force curves exhibited subsequent nonlinear and linear regimens that were related to the progressive indentation of the AFM tip in the bacterial cell wall, including a priori polymeric fringe (nonlinear part), while the linear part was ascribed to compression of the plasma membrane. These results indicate the dynamic of surface ultrastructure in response to changes in pH, leading to variations in nanomechanical properties, such as the Young's modulus and the bacterial spring constant.

Hydrophobic interactions may be fundamental in the cohesiveness and other properties of bacterial aggregates called flocs. Exopolymeric substances (EPS) form a matrix in which bacteria and other particles are embedded. However, knowledge about EPS hydrophobic properties is rather scanty. In this study, EPS extracted from activated sludges from different origins were fractionated, exploiting their property of sorbability by non-ionic resins and the formation of precipitates under-acidic environment. In the light of the results obtained with the XAD-8 resin, at least 7% of the dissolved carbon and 12% of the proteins may be considered as hydrophobic. No carbohydrates were found in the hydrophobic fraction. The preciprtable fraction at pH 2 on the other hand contained 64% carbohydrates and 76% proteins. This suggests that EPS are heteropolymers principally made up of carbohydrates and proteins.