Staphylococcus is a Gram-positive spherical bacterium occurring in microscopic clusters and resembling grapes. Culture of
the nose and skin of normal humans invariably yields staphylococci. There are two pigmented colony types: Staphylococcus
aureus (yellow) and Staphylococcus albus (white). The latter species is more recently named Staphylococcus epidermidis. Although there are more than 20 species of Staphylococcus described, only Staphylococcus aureus and Staphylococcus epidermidis are significant in human disease. S. aureus colonizes mainly the nasal passages, but it may be found regularly in most other anatomical locations. S. epidermidis is an inhabitant of the skin.
“Staph aureus” (its medical nickname) forms a large yellow colony when cultured on a nutrient rich medium while S. epidermidis forms a smaller white colony. S. aureus is often hemolytic on blood agar while S. epidermidis is non-hemolytic. All Staphylococci are “facultative anaerobes,” meaning that they grow by aerobic respiration or by fermentation that yields principally lactic acid. The are catalase-positive and oxidase-negative. Staphylococci are perfectly spherical cells, one micrometer in diameter. They grow in clusters because staphylococci divide in two planes. The configuration of the cocci helps to distinguish staphylococci from streptococci, which are slightly oblong cells that grow in chains (because they divide in one plane only).
S. aureus should always be considered a potential pathogen dangerous to humans while most strains of S. epidermidis are nonpathogenic and may even play a protective role in their host as normal flora. However, S. epidermidis may become a pathogen in the hospital environment. Staph aureus causes a variety of suppurative (pus-forming) infections and toxic conditions in humans. It causes superficial skin lesions such as boils, styes and furunculosis as well as more serious infections such as pneumonia, mastitis, phlebitis, meningitis and urinary tract infections. It can also cause deep-seated infections such as osteomyelitis and endocarditis. S. aureus is a major cause of nosocomial (hospital acquired) infection of surgical wounds and infections associated with indwelling medical devices such an catheters into the bladder. S. aureu causes food poisoning by releasing enterotoxins into food, and toxic shock syndrome by release of powerful antigens into the blood.
If exposed to antibiotics, but not eradicated, bacteria can mutate into forms which are able to resist antibiotics. This has happened as a result of the overuse and misuse of antibiotics by doctors and patients of the years. A particularly dangerous form of staphylococcus which has developed from this process is the so-called “methacillin resistant staph aureus” or MRSA. This is one of the “superbugs” which has developed resistance to some, and in some cases almost all, antibiotics.