In this procedure, heat-killed bacteria are exposed to the purple dye crystal violet and iodine. This combination forms a dye complex in the bacterial cell wall. Treatment of the stained bacteria with a decolorizer like ethanol will wash away the dye complex from some bacteria but not others. Bacteria that lose the dye complex can be counterstained with the red dye saffranin so that they appear red. The basis of the Gram reaction lies within the structure of the cell wall, described below.
Bacteria also come in many different shapes.
Individual cells may also be arranged in pairs or clusters or chains. Thus, may morphologies are possible and these can be useful for the identification of bacterial genera. The ability of a bacterium to cause disease is known as its virulence. Factors involved in determining virulence potential are discussed here . In terms of the medical aspects of bacterial structure , we are most interested in those features that interact with the host.