General characteristics of Staphylococci

· Bacteriology

Staphylococci are gram-positive cocci arranged in cluster (grape-like fashion). The genus name, Staphylococcus, is derived from the Greek term staphle, meaning “bunches of grapes.” On gram stained smears, they appear as gram positive spherical cells (0.5 to 1.5 μm) arranged singly, in pairs, and in clusters. Although the Staphylococci can be identified on the basis of microscopy, sometimes microscopy alone cannot differentiate Staphylococci from other gram-positive cocci.  Therefore, Catalase test should be done to differentiate Staphylococci from other gram-positive cocci. Staphylococi are catalase positive.  Staphylococci are members of the newly formed family Staphylococcaceae.

They resemble some members of the family Micrococcaceae such as the genus Micrococcus. Micrococci are catalase-producing, coagulase-negative, gram-positive cocci found in the environment and as members of the indigenous skin flora. They are often recovered with Staphylococci and can be differentiated easily on the basis of tests like modified oxidase test, anaerobic acid production from glucose, resistance to bacitracin (0.04 units) and lysostaphin test.  Some micrococci have a tendency to produce a yellow pigment. Other gram-positive cocci that are occasionally recovered with Staphylococci include Rothia mucilaginosa, Aerococcus, and Alloiococcus otitis (recovered from human middle ear fluid).

Staphylococci are nonmotile, non–spore-forming, and aerobic or facultatively anaerobic except for S. saccharolyticus, which is an obligate anaerobe. Colonies produced after 18 to 24 hours of incubation are medium sized (4 to 8 mm) and appear cream-colored, white or rarely light gold, and “buttery-looking.”Rare strains of Staphylococci are fastidious requiring carbon dioxide, hemin, or menadione for growth. These so-called small colony variants grow on media containing blood, forming colonies about 1/10th the size of wild-type strains after at least 48 hours of incubation. Some species are β-hemolytic. Staphylococci are common isolates in the clinical laboratory and are responsible for several suppurative types of infections. These organisms are normal inhabitants of the skin and mucous membranes of humans and other animals.

Staphylococcus species are initially differentiated by the coagulase test; a positive test is a clot formed in plasma due to production of enzyme coagulase. Coagulase-producing (coagulase-positive) staphylococci are Staphylococcus aureus, Staphylococcus intermedius, Staphylococcus delphini, Staphylococcus lutrae, and some strains of Staphylococcus hyicus. Isolates such as Staphylococcus lugdunensis and Staphylococcus schleiferi also can be occasionally mistaken for coagulase-positive Staphylococci because of the presence of clumping factor. Clumping factor causes bacterial cells to agglutinate in plasma. With the exception of Staphylococcus aureus, these are often animal-associated species and are less frequently isolated from humans.

Staphylococci that do not produce coagulase are referred to as CoNS (Coagulase negative Staphylococci). Most clinically significant and commonly recovered species in this group are Staphylococcus epidermidis and Staphylococcus saprophyticus. Staphylococcus haemolyticus and Staphylococcus lugdunensis are occasional isolates and can be significant pathogens. Staphylococcus epidermidis has been known to cause various health care-acquired or nosocomial infections, whereas Staphylococcus saprophyticus is associated mainly with urinary tract infections (UTIs), predominately in adolescent girls and young women. Staphylococcus haemolyticus is a CoNS occasionally recovered in wounds, septicemia, UTIs, and native valve infections. Staphylococcus lugdunensis can occasionally be confused with Staphylococcus aureus, if slide coagulase method is used solely for identification. Staphylococcus lugdunensis can be aggressive in its ability to be infective and has been associated with catheter related bacteremia and endocarditis. Because of reporting criteria, Staphylococcus lugdunensis may need to be identified to the species level to provide the correct treatment options when reporting antimicrobial susceptibilities.

There are more than 40 recognized species of CoNS. Most of these species have been isolated from humans, usually from the skin and mucous membranes. Certain species are found in very specific sites, such as the head (S. capitis) or ear (S. auricularis). Others have been isolated from animals and animal products.

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