Fig 1. Gram positive cocci.
Fig 2. 70,000X magnification electron micrograph of an ultra-thin section of Streptococcus pyogenes. Image source: Dr. Vincent A. Fischetti, Rockefeller University.
Fig 3. Colonies of Staphylococcus aureusd.
Coccus is a bacterium with a rounded or spherical shape (Fig 1). Cocci may be true spheres (e.g., staphylococci), helmet-shaped (e.g., pneumococci), or kidney-shaped (e.g., Neisseriae). They may occur alone, in pairs, or in groups. If found in pairs they are called diplococci, threes are a triad, etc. Other group types include: tetracoccus (groups of four), streptococcus (chains), sarcina (cubes of eight), and staphylococcus (irregular clusters). Diseases caused by cocci include pneumonia, tonsillitis, bacterial heart disease, meningitis, sepsis, and various skin diseases.
Diplococci are spherical or ovoid, nonmotile, Gram-positive (see Gram's stain) bacteria, so named because they occur in pairs. Pneumonia, meningitis, and gonorrhea are caused by types of diplococcus bacteria. Formerly a bacterial genus Diplococcus was recognized, but is not used any more.
Streptococcus is a genus of spherical or oval bacteria that grow in pairs or bead-like chains (Fig 2). Although some species of streptococci are harmless, others are responsible for many common infections, including strep throat, tonsillitis, scarlet fever, impetigo, cellulitis, erysipelas, and puerperal fevers. A related organism is a common cause of pneumonia and one type may cause endocarditis in damaged heart valves.
Staphylococcus is a genus of spherical bacteria that grow in grape-like clusters and are found on the skin and mucous membranes of human beings and other animals (Fig 3). Pathogenic staphylococci are responsible for numerous skin, soft tissue, and bone infections, less often causing sepsis, a cavitating pneumonia, bacterial endocarditis, and enterocolitis. Boils, carbuncles, impetigo, and osteomyelitis are commonly due to staphylococci. Treatment usually requires drainage of pus from abscesses, and antibiotics, although some strains have become highly drug-resistant.