Thou art a boil,
A plague sore, an embossed carbuncle
In my corrupted blood.
—Shakespeare: King Lear
Members of the genus Staphylococcus (staphylococci) are gram-positive cocci that tend to be arranged in grape-like clusters (Figure 24–1). They are catalase positive and the major species, Staphylococcus aureus is also coagulase positive. Infections produced by S aureus are typified by acute, aggressive, locally destructive purulent lesions. The most familiar of these is the common boil, a painful lump in the skin that has a necrotic center and fibrous reactive shell. Infections in organs other than the skin such as the lung, kidney, or bone are also focal and destructive, but have greater potential for extension within the organ and beyond to the blood and other organs. Such infections typically produce high fever and systemic toxicity and may be fatal in only a few days. The major virulence factors for these effects are surface attachment proteins, fibrinogen-binding proteins, and a pore-forming exotoxin. A subgroup (less than 10%) of S aureus infections has manifestations produced by secreted toxins in addition to those associated with the primary infection. Symptoms include diarrhea, rash, skin desquamation, and multiorgan effects as in staphylococcal toxic shock syndrome (TSS). Superantigen toxins are involved in these diseases. Ingestion of preformed staphylococcal enterotoxin causes a form of food poisoning in which vomiting begins in only a few hours. Staphylococcus epidermidis and other non-aureus species produce less aggressive disease typically associated with biofilm-mediated attachment to medical devices such as indwelling catheters and biomedical implants like heart valves and artificial joints.
Staphylococcus aureus. Gram stain showing the gram-positive cocci in clusters resembling bunches of grapes (arrows) and neutrophils (arrowhead). (Image contributed by Professor Shirley Lowe, University of California, San Francisco School of Medicine, with permission.)
STAPHYLOCOCCI: GROUP CHARACTERISTICS
Although staphylococci have a marked tendency to form clusters (from the Greek staphyle, bunch of grapes), some single cells, pairs, and short chains are also seen. Staphylococci have a typical gram-positive cell wall structure. Like all medically important cocci, they are nonflagellate, nonmotile, and non–spore-forming. Staphylococci grow aerobically but are facultatively anaerobic. In contrast to streptococci, staphylococci produce catalase. More than one dozen species of staphylococci colonize humans; of these, S aureus is by far the most virulent. The ability of S aureus to form coagulase separates it from other, less virulent species (Table 24–1). It is common to lump the other species together as coagulase-negative staphylococci (CoNS).
TABLE 24–1Features of Human Staphylococci ||Download (.pdf) TABLE 24–1 Features of Human Staphylococci
|SPECIES ||COAGULASE ||α-TOXIN ||SAgs ||HABITAT ||BIOFILM ||BOILS ||UTIa ||DEEP INFECTIONS |
|Staphylococcus aureus ||+ ||+ ||+ ||Anterior nares, perineum ||+ ||+ ||– ||Pneumonia, osteomyelitis, abscesses, TSS |
|S epidermidis ||– ||– ||– ||Anterior ...|