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المستخلص
Staphylococcus aureus (S. aureus) is one of the oldest known and most important of bacterial pathogens, causing a wide range of superficial and deep infections.
S. aureus is a Gram-positive, non-sporing, non-motile coccus, and except for rare strains, non capsulated. It is approximately 0.8 to1 µm in diameter, which forms grape-like clusters indicative of its ability to divide in more than one plane. In broth it appears in small groups, pairs, singles and short chains.(1, 2)
It is capable of both aerobic and anaerobic growth, but growth is more abundant under aerobic conditions. It has optimum temperature of growth of 30- 37oC .The optimum pH for growth is 7.0-7.5.(3)
S. aureus grows on ordinary nutrient media. Colonies of S. aureus on nutrient agar are usually large, 6-8mm in diameter, smooth, entire, slightly raised and opaque. Most strains produce golden yellow colonies due to pigment production. (1) On blood agar, colonies have the same appearance but may be surrounded by a zone of β-haemolysis. On MacConkey agar, colonies are pink and are small to medium in size. (2)
Selective media for S. aureus are used for isolation in samples that contain predominance of other types of bacteria. Staphylococci can tolerate high salt concentrations. Mannitol salt agar contains 1% mannitol, 7.5% NaCl and phenol red as an indicator. Most strains of S. aureus ferment mannitol and form colonies surrounded by yellow zones due to acid production. (2)
S. aureus is usually identified by a small battery of tests such as, catalase test which is an important test in distinguishing streptococci (catalase-negative) from staphylococci, and coagulase test which is essential to distinguish S. aureus from coagulase-negative staphylococci (CoNS). (4, 5)
Other biochemical reactions that distinguish S. aureus from other CoNS as well as micrococci include heat stable nuclease which is produced by 99% of strains. It ferments glucose, maltose, lactose and sucrose. S. aureus is also oxidase and indole negative, produces acetoin, and reduces nitrate to nitrite