Background
Staphylococcal infections are usually caused by the organism Staphylococcus aureus. However, the incidence of infections due to Staphylococcus epidermidis and other coagulase-negative staphylococci has been steadily increasing in recent years. This article focuses on S aureus but also discusses infections caused by coagulase-negative staphylococci when important differences exist.
Pathophysiology
S aureus is a gram-positive coccus that is both catalase- and coagulase-positive. Colonies are golden and strongly hemolytic on blood agar. They produce a range of toxins, including alpha-toxin, beta-toxin, gamma-toxin, delta-toxin, exfoliatin, enterotoxins, Panton-Valentine leukocidin (PVL), and toxic shock syndrome toxin–1 (TSST-1). The enterotoxins and TSST-1 are associated with toxic shock syndrome. PVL is associated with necrotic skin[1] and lung infections and has been shown to be a major virulence factor for pneumonia[2] and osteomyelitis.[3] Coagulase-negative staphylococci, particularly S epidermidis, produce an exopolysaccharide (slime) that promotes foreign-body adherence and resistance to phagocytosis.
Nienaber et al have demonstrated that methicillin-susceptible S aureus isolates causing endocarditis are more likely to be from a specific clonal cluster (CC30) and to possess specific virulence genes as compared to MSSA isolates from the same regions causing soft tissue infection. Isolates from patients with endocarditis were more likely to possess genes for 3 different adhesins and 5 different enterotoxins. The gene for PVL was found in the minority of both groups.[4]
In a study of 42 Staphylococcus lugdunensis isolates, most isolates were able to form at least a weak biofilm, but the amount of biofilm formed by isolates was heterogeneous with poor correlation between clinical severity of disease and degree of biofilm formation.[5]
Epidemiology
Frequency
United States
Up to 80% of people are eventually colonized with S aureus. Most are colonized only intermittently; 20-30% are persistently colonized. Colonization rates in health care workers, persons with diabetes, and patients on dialysis are higher than in the general population. The anterior nares are the predominant site of colonization in adults; carriage here has been associated with the development of bacteremia.[6] Other potential sites of colonization include the throat,[7] axilla, rectum, and perineum.
International
S aureus infection occurs worldwide. Pyomyositis due to S aureus is more prevalent in the tropics.
Mortality/Morbidity
Mortality due to staphylococcal infections varies widely. Untreated S aureus bacteremia carries a mortality rate that exceeds 80%. The mortality rate of staphylococcal toxic shock syndrome is 3-5%. Infections due to coagulase-negative staphylococci usually carry a very low mortality rate. Because these infections are commonly associated with prosthetic devices, the most serious complication is the need to remove the involved prosthesis, although prosthetic valve endocarditis may lead to death.
Thrombocytopenia is associated with increased mortality among patients with S aureus bacteremia.[8]
Race
Staphylococcal infections have no reported racial predilection.
Sex
The vaginal carriage rate of staphylococcal species is approximately 10% in premenopausal women. The rate is even higher during menses.
Age
Staphylococcal species colonize many neonates on the skin, perineum, umbilical stump, and GI tract. The staphylococcal colonization rate in adults is approximately 40% at any given time.
The mortality rate of S aureus bacteremia in elderly persons is markedly increased.[9]
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