Atomic force microscopy and theoretical considerations of surface properties and turgor pressures of bacteria

The properties of viable bacteria were investigated with the Atomic Force Microscope (AFM). By depositing bacteria on aluminum oxide filters, the adhesion of Si₃N₄ tips to the surfaces of Gram-negative bacterial strains possessing different lipopolysacharides (LPS) (i.e. Pseudomonas aeruginosa PAO1 and its isogenic mutants) was investigated without the use of surface modifying or bonding agents to adhere cells to the filter. Our measurements suggest that adhesion forces for Si₃N₄ to these bacteria were below our detection limit of 50-100 pN. Turgor pressures were also investigated for a spherical Gram-positive bacterium (Enterococcus hirae) as well as the rod-shaped Gram-negative P. aeruginosa. A simple relationship between bacterial indentation depth and turgor pressure for the spherical bacterium was first derived and gave a turgor pressure for E. hirae in deionized water of 4-6×10⁵ Pa. This is the first such measurement for a spherical Gram-positive bacterium. AFM deformations of the cell envelope of P. aeruginosa gave turgor pressures in the range 0.1-0.2×10⁵ Pa in growth medium and 1.5-4×10⁵ Pa in distilled water. These pressure ranges compared well with previously published values derived by other means for Gram-negative rods. The imaging of bacteria under growth medium was only possible on aluminum-oxide filters. It is proposed that the 20 nm diameter pores of these filters might facilitate the attachment of bacteria. A Monte-Carlo study was carried out which showed that bacterial adhesion will be both encouraged and stronger if hydrogen bonding takes place between LPS O-sidechains and the inside surface of the filter's pores.