Cocci (in singular coccus) are spherical-shaped bacteria, while bacilli and spirilli are rod-shaped and spiral-shaped bacteria, respectively (in general, bacteria can be divided into these three categories by shape). <em>Staphylococcus aureus</em> forms irregular clusters of cocci (i.e., grape-like cocci) when viewed through a microscope. Some Gram-positive cocci include, for example, <em>Staphylococcus aureus</em> and <em>Streptococcus pneumoniae</em>. These species are well known to be bacterial pathogens capable of infecting humans.
