Staphylococci are ubiquitous in our environment and in the normal flora of our bodies. They are particularly numerous on skin and in the upper respiratory tract, including the anterior nares and pharyngeal surfaces. Some are also associated with human infectious diseases.
Staphylococci are gram-positive cocci, characteristically arranged in irregular clusters like grapes (see colorplate 1). They are hardy, facultatively anaerobic organisms that grow well on most nutrient media. There are three principal clinically important species: Staphylococcus epider-midis, Staphylococcus saprophyticus, and Staphylococcus aureus. S. epidemidis, as its name implies, is the most frequent inhabitant of human surface tissues, including skin and mucous membranes. It is not usually pathogenic, but it may cause serious infections if it has an unusual opportunity to enter past surface barriers, for example, in cardiac surgery patients or those with indwelling intravenous catheters. S. saprophyticus has been implicated in acute urinary tract infections in young women approximately 16 to 25 years of age. It has not been found among the normal flora and is not yet known to cause other types of infection. It is included in this exercise for completeness. Like S. epidermidis, S. aureus is often found among the normal flora of healthy persons, but in contrast, most staphylococcal disease is caused by strains of this species.
S. aureus strains produce a number of toxins and enzymes that can exert harmful effects on the cells of the infected host. Their hemolysins can destroy red blood cells. The enzyme coagulase coagulates plasma, but its exact role in staphylococcal infection is not yet known. Leukocidin is a staphylococcal toxin that destroys leukocytes. Hyaluronidase is an enzyme that acts on a substrate that is a structural component of connective tissue. Its activity in a local area of infection breaks down the tissue and permits the staphylococci to penetrate more deeply; hence, it is called "spreading factor." (Some streptococci also produce hyaluronidase.) Staphylokinase can dissolve fibrin clots, thus enhancing the invasiveness of organisms that would otherwise be walled off by the body's fibrinous reactions. An enterotoxin is elaborated by some strains of S. aureus. If these are multiplying in contaminated food, the enterotoxin they produce can be responsible for severe gastroenteritis or staphylococcal food poisoning. Some strains produce toxic shock syndrome (TSS) by elaborating a toxin referred to as TSST-1. This disease is seen primarily in menstruating women who use highly absorbent tampons. S. aureus colonizing the vaginal tract multiplies there and releases TSST-1, causing a variety of symptoms including shock and a rash. TSS has also been documented in children, men, and nonmenstruating women who have a focus of infection at nongenital sites. Strains of S. epidermidis and S. saprophyticus do not produce these toxic substances.
Common skin infections caused by S. aureus include pimples, furuncles (boils), carbuncles, and impetigo. Serious systemic (deep tissue) infections that result from S. aureus invasion include pneumonia, pyelonephritis, osteomyelitis, meningitis, and endocarditis. In addition to pneumonia, S. aureus may also produce infections of the sinuses (sinusitis) and middle ear (otitis media).
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