In general, bacteria may appear as either round (cocci) or rod-shaped (bacilli). Either shape may be gram-positive or gram-negative. A mixture of gram-positive and gram-negative bacteria can occur in the same field. Clostridium is shown as an example of gram-positive bacilli (rods) in the top image. Clostridium species may decolorize easily and appear to be gram-negative, as seen with some of the pink-staining organisms. The lower image is Enterococcus faecalis, appearing as gram-positive cocci.
