by Dr. Jaclynn Moskow
Staphylococcus aureus (S. aureus) is a facultative anaerobic, gram-positive coccus. S. aureus is part of the normal flora of the body, found in the skin, upper respiratory tract, gut, and genitourinary tract – and most commonly in the anterior nares. Twenty percent of individuals are persistent nasal carriers of S. aureus, and an additional thirty percent are intermittent carriers (1).
Under certain conditions, S. aureus can be pathogenic, causing a variety of infections, including skin conditions, pneumonia, gastroenteritis, endocarditis, osteomyelitis, septic arthritis, meningitis, bacteremia, and sepsis. Individuals at increased risk include patients with diabetes, cancer, HIV/AIDS, and other conditions that compromise the immune system. Intravenous drug users may introduce the bacteria into various tissues and/or the bloodstream. Hospitalization is in itself a risk factor for S. aureus infection.
S. aureus can cause a diverse array of skin infections, including folliculitis, impetigo, furuncles, carbuncles, cellulitis, and abscesses. S. aureus is the most common cause of skin infection in individuals with eczema, and many presumed cases of “eczema” are, in fact, inflammatory reactions to colonization by S. aureus (2).
S. aureus is the most common agent of surgical site infections (3), and a common cause of infection in burn patients. Animal bites, including bites from dogs and cats, can also lead to S. aureus skin infections.
Staphylococcal scalded skin syndrome, also known as “Ritter’s disease”, is caused by exotoxin-producing strains of S. aureus – and is characterized by diffuse erythematous cellulitis followed by extensive skin exfoliation (4). Fever is common, and patients are most often neonates, children, immunocompromised individuals, and individuals with severe renal disease. It is thought that the latter are at an increased risk due to a decreased ability to excrete the exotoxins in urine (5). Healthy adults rarely develop the syndrome, as a result of having antibodies to the exotoxins. Staphylococcal scalded skin syndrome is intraepidermal. Necrosis of the full epidermal layer may also occur as a result of S. aureus infection and is known as toxic epidermal necrolysis – a more severe form of the disease.
Various topical and systemic antibiotics can be used to treat S. aureus skin infections including beta-lactams, macrolides, and aminoglycosides. Treatment may be complicated by antibiotic resistance.
S. aureus is identified in three percent of community-acquired bacterial pneumonias (6), and 18% of hospital-acquired pneumonias (7). S. aureus is a cause of secondary bacterial pneumonia associated with influenza, and influenza has been shown to increase the adherence of S. aureus to host cells (8). One study showed that 33% of children admitted to the PICU during the 2009 H1N1 pandemic had a secondary bacterial coinfection, with S. aureus being the most common pathogen (9). S. aureus is also frequently isolated from the respiratory tract of children with cystic fibrosis (10).
S.aureus can cause necrotizing pneumonia, characterized by necrosis, liquefaction, and cavitation of the lung parenchyma (11) – often accompanied by empyema and bronchopleural fistulae. Necrotizing pneumonia caused by community-acquired methicillin-resistant S. aureus (MRSA) strains which produce Panton valentine leukocidin (PVL) toxin has a mortality rate of 60% (12).
Treatment of pneumonia caused by S. aureus is based on testing for antibiotic susceptibility. Nafcillin, oxacillin, and cefazolin are often used to treat methicillin-sensitive S. aureus (MSSA), while vancomycin or linezolid is often used to treat MRSA (13).
S.aureus is one of the most common causes of food-borne disease worldwide (14). Illness is characterized by a short incubation period (2h-4h), nausea, vomiting, intestinal cramping, and profuse watery, non-bloody diarrhea (15). The condition is generally self-limited, and symptoms typically resolve within 12 to 24 hours.
Staphylococcal food poisoning, outbreak-related cases and rates in the United States, 1952 – 2010
S.aureus is the most common cause of toxic shock syndrome, a life-threatening syndrome resulting from staphylococcal toxin-1 (TSST-1). It is characterized by fever, hypotension, myalgia, macular erythema, desquamation (particularly of the palms and soles), and acute vomiting or diarrhea (16). Most cases are associated with the use of “super absorbent” tampons or staphylococcal wound infection. Case fatality rates of 5 to 10% are reported. The condition is generally treated with vancomycin in combination with clindamycin.
S.aureus is the leading cause of acute bacterial endocarditis. Of infections caused by S. aureus, endocarditis accounts for the highest mortality rates (17). Populations at high risk include IV drug users and patients with implanted medical devices such as prosthetic heart valves, grafts, pacemakers, and hemodialysis catheters (18). Treatment varies and depends on several factors, including antibiotic susceptibility, site of infection (left side versus right side), IV drug abuse status, and if a prosthetic valve is present (19).
Staphylococcus aureus can also cause mastitis, urinary tract infections, osteomyelitis, meningitis, septic arthritis, and many infections associated with medical devices and implants.
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