The staphylococci are Gram-positive spherical cells, usually arranged in grapelike irregular clusters. They grow readily on many types of media and are active metabolically, fermenting carbohydrates and producing pigments that vary from white to deep yellow. Some are members of the normal microbiota of the skin and mucous membranes of humans; others cause suppuration, abscess formation, a variety of pyogenic infections, and even fatal septicemia. The pathogenic staphylococci often hemolyze blood, coagulate plasma, and produce a variety of extracellular enzymes and toxins. The most common type of food poisoning is caused by a heat-stable staphylococcal enterotoxin. Staphylococci rapidly develop resistance to many antimicrobial agents, which consequently presents difficult therapeutic problems.
The genus Staphylococcus has at least 45 species. The four most frequently encountered species of clinical importance are Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus lugdunensis, and Staphylococcus saprophyticus. S. aureus is coagulase positive, which differentiates it from the other species. S. aureus is a major pathogen for humans. Almost every person will have some type of S. aureus infection during a lifetime, ranging in severity from food poisoning or minor skin infections to severe life-threatening infections. The coagulase-negative staphylococci (CoNS) are normal human microbiota and sometimes cause infection, often associated with implanted devices, such as joint prostheses, shunts, and intravascular catheters, especially in very young, old, and immunocompromised patients. Approximately 75% of these infections caused by CoNS are caused by S. epidermidis; infections caused by S. lugdunensis, Staphylococcus warneri, Staphylococcus hominis, and other species are less common. S. saprophyticus is a relatively common cause of urinary tract infections in young women, although it rarely causes infections in hospitalized patients. Other species are important in veterinary medicine.
Morphology and Identification
Staphylococci are spherical cells about 1 µm in diameter arranged in irregular clusters (Figure 13-1). Single cocci, pairs, tetrads, and chains are also seen in liquid cultures. Young cocci stain strongly Gram-positive; on aging, many cells become Gram-negative. Staphylococci are nonmotile and do not form spores. Under the influence of drugs such as penicillin, staphylococci are lysed.
Gram-stain of Staphylococcus aureus showing Gram-positive cocci in pairs, tetrads, and clusters. Original magnification ×1000. (Courtesy of L. Ching.)
Micrococcus species often resemble staphylococci. They are found free living in the environment and form regular packets of four (tetrads) or eight cocci. Their colonies can be yellow, red, or orange. Micrococci are rarely associated with disease.
Staphylococci grow readily on most bacteriologic media under aerobic or microaerophilic conditions. They grow most rapidly at 37°C but form pigment best at room temperature (20–25°C). Colonies on solid media are round, smooth, raised, and glistening (Figure 13-2). S. aureus usually forms ...