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Staphylococcus aureus and Staphylococcal Disease (page 1)
(This chapter has 6 pages)
© Kenneth Todar, PhD
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.
Table 1. Important phenotypic characteristics of Staphylococcus aureus
Gram-positive, cluster-forming coccus
nonmotile, nonsporeforming facultative anaerobe
fermentation of glucose produces mainly lactic acid
ferments mannitol (distinguishes from S. epidermidis)
golden yellow colony on agar
normal flora of humans found on nasal passages, skin and mucous
pathogen of humans, causes a wide range of suppurative infections,
as well as food poisoning and toxic shock syndrome