Probing microbial cell surface charges by atomic force microscopy

Most microorganisms possess a negative surface charge under physiological conditions due to the presence of anionic carboxyl and phosphate groups. Cell surface charge plays an important role in controlling cell adhesion and aggregation phenomena, as well as antigen-antibody, cell-virus, cell-drug, and cell-ions interactions. We have used atomic force microscopy (AFM) with chemically functionalized probes to investigate the surface charges of yeast cells. Force-distance curves and adhesion maps recorded with probes terminated with ionizable carboxyl groups were strongly influenced by pH: while no adhesion was measured at neutral/alkaline pH, multiple adhesion forces were recorded at pH : less than or equal to 5. Three pieces of evidence indicated that these changes were related to differences in the ionization state of the cell surface functional groups. First, the adhesion force vs pH curve was correlated with microelectrophoresis data, the pH of the largest adhesion force corresponding to the cell isoelectric point, i.e., pH 4. Second, treating the cells with Cu(II) ions caused a reversal of the cell surface charge at neutral pH and promoted the adhesion toward the negatively charged probe. Third, control experiments using nonionizable hydroxyl-terminated probes indicated that the changes in adhesion forces were not simply due to the titration of the probe surface charges. This study shows that AFM with chemically modified probes is a valuable approach in microbiology and biophysics for probing the local electrostatic properties of microbial cell surfaces.