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Protein A
83%
384/463
Staphylokinase
3%
12/463
Exfoliatin A
1%
5/463
Protein M
12%
54/463
Neurotoxin
0%
0/463
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The patient described above has a Staphylococcus aureus skin infection (impetigo) as evidenced by the description of the lesion and causative bacteria. Protein A is present on the surface of S. aureus and is able to bind to the Fc region of immunoglobulin molecules. This region of the immunoglobulin is normally bound by phagocytic cells in order to absorb opsonized bacteria; protein A disrupts this action and prevents phagocytosis. Impetigo is a skin infection characterized by honey-colored flaky lesions which is caused by Streptococcus pyogenes and Staphylococcus aureus. The latter is a gram-positive cocci that grows in clusters and is coagulase positive, catalase positive, and beta-hemolytic (unlike other Staphylococcus species). In contrast, S. pyogenes is also beta-hemolytic but catalase negative. Other diseases that result from S. aureus infection include gastroenteritis, toxic shock syndrome, scalded skin syndrome, osteomyelitis, and pneumonia. Cole et al. detail the diagnosis and treatment of impetigo. Although impetigo generally self-resolves within two weeks, treatment is used to improve pain, cosmetic apearance, and to prevent other sequelae (post-streptococcal glomerulonephritis). Topical antibiotics include mupirocin and fusidic acid and are superior to systemic antibiotics except in cases of extensive disease where amoxicillin/clavulanate, cephalosporins, and macrolides can be used. Kim et al. discuss the function of several S. aureus virulence factors and possible ways to counteract them. In discussing Staphylococcus protein A (SpA), the authors note that it is involved in reactions with several host proteins and functions to prevent opsonization. They investigate ways to counteract this virulence factor. Image A shows an example of an impetigo lesion. Image B shows the classic gold color when S. aureus is cultured on a blood agar plate. Incorrect Answers: Answer 2: Staphylokinase, like streptokinase, leads to the breakdown of fibrin clots. Answer 3: Exfoliatin A leads to the breakdown of desmosomes and can lead to scalded skin syndrome. Answer 4: Protein M is a surface protein on Streptococcus pyogenes that helps the bacterium avoid phagocytosis. Answer 5: Neurotoxin would be consistent with a Clostridium botulinum infection (botulinum toxin).
4.4
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