This slide shows the large size of the rods in comparison to mammalian cells.

Although the color reproduction is not good on this slide it serves to demonstrate the spores inside the vegetative bacterial cells. The spores do not stain due to their impervious chemical structure and therefore appear as clear spots inside the stained vegetative cell.

A spore stain will stain the spore one color (usually red) and the vegetative cells another color (blue or green depending on the counter-stain). Differentiation of Bacillus sp. from Clostridium sp. is very easy because Bacillus only grows aerobically and Clostridium only grows anaerobically. Only one species of Bacillus is pathogenic and that is B. anthracis. Bacillus is most frequently encountered as a contaminant and must be recognized as such.