Negative staining and transmission electron microscopy of bacterial surface structures

Negative staining is an essential and versatile staining technique in transmission electron microscopy that can be employed for visualizing bacterial cell morphology, size, and surface architecture at high resolution. Bacteria are usually transferred by passive electrostatic adsorption from suspensions in physiological saline onto suitable hydrophilic support films on electron microscopic grids. There they are contrasted, or “stained,” by heavy metal ions in solution such as tungsten, uranyl, molybdate, or vanadate compounds. Here, I describe how to visualize the interaction between the bacterial M1 protein and complement factors C1q and C3 on the surface of group A streptococcus by negative staining with uranyl formate on carbon support films. The methodology should be generally applicable to the study of a large number of other bacteria-protein interactions.