A 45-year-old man comes to the physician due to pain, swelling, and erythema affecting his right leg. He says that he suffered a minor cut to his leg a few days ago while cleaning his garage. Physical examination shows an indurated region surrounding a minor laceration that is draining a purulent exudate. He is diagnosed with cellulitis and started on the appropriate treatment. Gram stain of the exudate shows gram-positive cocci in clusters. The organism most likely responsible for this patient's infection synthesizes a protein as part of its peptidoglycan cell wall that does which of the following?

Question

A 45-year-old man comes to the physician due to pain, swelling, and erythema affecting his right leg.  He says that he suffered a minor cut to his leg a few days ago while cleaning his garage.  Physical examination shows an indurated region surrounding a minor laceration that is draining a purulent exudate.  He is diagnosed with cellulitis and started on the appropriate treatment.  Gram stain of the exudate shows gram-positive cocci in clusters.  The organism most likely responsible for this patient's infection synthesizes a protein as part of its peptidoglycan cell wall that does which of the following?

Quizplus · Accepted Answer

11ec160b_3876_e570_9249_cf219d298529_MD0001_00 Clusters of gram-positive cocci on Gram stain is a characteristic finding of staphylococcal species such as Staphylococcus aureus. Protein A is a virulence factor that forms part of the outer peptidoglycan layer of S aureus. Protein A binds with the Fc portion of IgG antibodies at the complement-binding site, preventing complement activation (Choice A). This results in decreased production of C3b, leading to impaired opsonization and phagocytosis. (Choice C) Hemolysin is secreted by staphylococci and causes hemolysis as well as the destruction of neutrophils, macrophages, and platelets. It is a secreted factor that is not bound to the cell wall. (Choice D) Streptococcus pneumoniae and Neisseria gonorrhoeae produce IgA proteases that cause cleavage of IgA antibodies, preventing them from interfering with bacterial adhesion to mucous membranes. (Choice E) Major histocompatibility complex (MHC) class II (found on an antigen presenting cells) normally interacts with processed antigens, presenting them to T-lymphocytes to initiate an immune response. In contrast, superantigens (enterotoxins, toxic shock syndrome toxin) interact with MHC class II and the T-cell receptor outside of standard antigen binding sites to initiate widespread and nonspecific activation of T- lymphocytes. Although superantigens are synthesized by staphylococci, they are not bound to the peptidoglycan cell wall. Educational objective: Protein A is a virulence factor found in the peptidoglycan cell wall of Staphylococcus aureus that binds to the Fc portion of IgG, leading to impaired complement activation, opsonization, and phagocytosis.

A 45-Year-Old Man Comes to the Physician Due to Pain