Staphylococcus aureus and Staphylococcal Disease
The Staphylococci
Staphylococci (staph) are Gram-positive spherical bacteria that occur in microscopic clusters resembling grapes. Bacteriological culture of the nose and skin of normal humans invariably yields staphylococci. In 1884, Rosenbach described the two pigmented colony types of staphylococci and proposed the appropriate nomenclature: Staphylococcus aureus (yellow) and Staphylococcus albus (white). The latter species is now named Staphylococcus epidermidis. Although more than 20 species of Staphylococcus are described in Bergey's Manual (2001), only Staphylococcus aureus and Staphylococcus epidermidis are significant in their interactions with humans. S. aureus colonizes mainly the nasal passages, but it may be found regularly in most other anatomical locales, including the skin, oral cavity and gastrointestinal tract. S epidermidis is an inhabitant of the skin.
Taxonomically, the genus Staphylococcus is in the Bacterial family Staphylococcaceae, which includes three lesser known genera, Gamella, Macrococcus and Salinicoccus. The best-known of its nearby phylogenetic relatives are the members of the genus Bacillus in the family Bacillaceae, which is on the same level as the family Staphylococcaceae. The Listeriaceae are also a nearby family.
Staphylococcus aureus forms a fairly large yellow colony on rich medium; S. epidermidis has a relatively small white colony. S. aureus is often hemolytic on blood agar; S. epidermidis is non hemolytic. Staphylococci are facultative anaerobes that grow by aerobic respiration or by fermentation that yields principally lactic acid. The bacteria are catalase-positive and oxidase-negative. S. aureus can grow at a temperature range of 15 to 45 degrees and at NaCl concentrations as high as 15 percent. Nearly all strains of S. aureus produce the enzyme coagulase: nearly all strains of S. epidermidis lack this enzyme. S. aureus should always be considered a potential pathogen; most strains of S. epidermidis are nonpathogenic and may even play a protective role in humans as normal flora. Staphylococcus epidermidis may be a pathogen in the hospital environment. Staphylococci are perfectly spherical cells about 1 micrometer in diameter. The staphylococci grow in clusters because the cells divide successively in three perpendicular planes with the sister cells remaining attached to one another following each successive division. Since the exact point of attachment of sister cells may not be within the divisional plane, and the cells may change position slightly while remaining attached, the result is formation of an irregular cluster of cells.
The shape and configuration of the Gram-positive cocci helps to distinguish staphylococci from streptococci. Streptococci are slightly oblong cells that usually grow in chains because they divide in one plane only, similar to a bacillus. Without a microscope, the catalase test is important in distinguishing streptococci (catalase-negative) from staphylococci, which are vigorous catalase-producers. The test is performed by adding 3% hydrogen peroxide to a colony on an agar plate or slant. Catalase-positive cultures produce O2 and bubble at once. The test should not be done on blood agar because blood itself contains catalase.
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