Properties of Staphylococcus aureus
- Ability to grow aerobically and anaerobically, over a wide range of temperatures, and in the presence of a high concentration of salt; the latter is important because these bacteria are a common cause of food poisoning.
- Polysaccharide capsule that protects the bacteria from phagocytosis.
- Cell surface proteins (protein A, clumping factor proteins) that mediate adherence of the bacteria to host tissues
- Catalase that protects staphylococci from peroxides produced by neutrophils and macrophages
- Coagulase converts fibrinogen to insoluble fibrin that forms clots and can protect S. aureus from phagocytosis
- Hydrolytic enzymes and cytotoxins:
- Lipases, nucleases, and hyaluronidase that causes tissue destruction
- Cytotoxins (alpha, beta, delta, gamma, leukocidin) that lyse erythrocytes, neutrophils, macrophages, and other host cells
- Enterotoxins (many antigenically distinct) are the heat-stable and acid-resistant toxins responsible for food poisoning
- Exfoliative toxins A and B cause the superficial layers of skin to peel off (scalded skin syndrome)
- Toxic shock syndrome toxin is a heat- and protease-resistant toxin that mediates multiorgan pathology.
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Epidemiology of Staphylococcus aureus
- The common cause of infections both in the community and in the hospital because the bacteria are easily spread person-to-person and through direct contact or exposure to contaminated bed linens, clothing, and other surfaces
- Antibiotic-resistant strains (e.g., MRSA) are widely distributed in both the hospital and community.
Clinical Disease of Staphylococcus aureus
S. aureus Pyogenic Diseases
- Impetigo: localized skin infection characterized by pus-filled vesicles on a reddened or erythematous
base; seen mostly in children on their face and limbs
- Folliculitis: impetigo involving hair follicles, such as the beard area
- Furuncles (boils) and carbuncles: large, pus-filled skin nodules; can progress to deeper layers of the skin and spread into the blood and other areas of the body
- Wound infections: characterized by erythema and pus at the site of trauma or surgery; more difficult to treat if a foreign body is present (e.g., splinter, surgical suture); the majority of infections both in the community and hospital are caused by MRSA; recurrent bouts of infections are common
- Pneumonia: abscess formation in the lungs; observed primarily in the very young and old and frequently following viral infections of the respiratory tract
- Endocarditis: infection of the endothelial lining of the heart; the disease can progress rapidly and is associated with a high mortality rate
- Osteomyelitis: the destruction of bones, particularly the highly vascularized areas of long bones in
- Septic arthritis—infection of joint spaces characterized by a swollen, reddened joint with the accumulation
of pus; the most common cause of septic arthritis in children
S. aureus Toxin–Mediated Diseases
- Food poisoning: after consumption of food contaminated with the heat-stable enterotoxin, the onset of severe vomiting, diarrhea, and stomach cramps is rapid (2 to 4 hours) but resolves within 24 hours. This is because the intoxication is caused by the preformed toxin present in the food rather than an infection where the bacteria would have to grow and produce toxin in the intestine
- Scalded skin syndrome: bacteria in a localized infection produce the toxin that spreads through the blood and causes the outermost layer of the skin to blister and peels off; almost exclusively seen in very young children
- Toxic shock syndrome: bacteria in a localized infection produce the toxin that affects multiple organs; characterized initially by fever, hypotension, and a diffuse, macular, erythematous rash. There is a very high mortality rate associated with this disease unless antibiotics are promptly administered and the local infection managed.
- Microscopy: useful for pyogenic infections but not for bacteremia (too few organisms present), food poisoning (intoxication), or scalded skin syndrome and toxic shock syndrome (toxin production at the localized site of infection and bacteria typically not in affected organ tissues)
- Culture: organisms recovered on most laboratory media
- Nucleic acid amplification tests: a sensitive method for rapid detection of MSSA and MRSA in clinical specimens
- Identification tests:
- Catalase: separates Staphylococcus (+) from Streptococcus and Enterococcus (−)
- Coagulase: separates S. aureus (+) from other species of Staphylococcus (−)
- Protein A: separates S. aureus (+) from other species of Staphylococcus (−)
Treatment, Control, and Prevention
- Localized infections managed by incision and drainage
- Antibiotic therapy indicated for systemic infections; empiric therapy should include antibiotics active against MRSA
- Oral therapy can include trimethoprim-sulfamethoxazole, clindamycin, or doxycycline
- Vancomycin is the drug of choice for intravenous therapy
- Treatment is symptomatic for patients with food poisoning although the source of infection should be identified so other individuals will not be exposed.
- Proper cleansing of wounds and use of disinfectant help prevent infections
- Thorough hand washing and covering exposed skin helps medical personnel prevent infection or spread to other patients
- No vaccine is currently available
Patrick R. Murray. 2018. Basic Medical Microbiology. 1st Edition. Elsevier. Philadelphia, PA. ISBN: 978-0-323-47676-8.