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Unit 1

Clinical micro S. aureus

QuestionAnswer
What differential media is best for recovering Staphylococcus aureus and why? Mannitol Salt Agar (MSA) is the best media for recovering S. aureus because it is selective for Gram positive cocci and differentiates S. aureus because it ferments mannitol. This distinguishes it from CoNS, which does not ferment mannitol.
Staphylococcus aureus streaked on a Toluidine blue DNase plate will have what result? S. aureus colonies will have a pink color surrounding them on a Toluidine blue DNase plate (positive). This result indicates the production of active DNase exoenzymes by the species.
Which antibiotic is now used in the place of Oxacillin to test for Methicillin-resistance in Staph species? Cefoxitin. It has been shown that Cefoxitin is more sensitive than Oxacillin, and therefore provides a better indicator for detection of Methicillin-resistant strains.
What was the original drug of choice for treating Staphylococcus aureus infections? Penicillin was the original drug of choice for treating serious S. aureus infections. Because most strains are now resistant to Penicillin, Methicillin is now typically used.
What is the term for inflammation of the inner layer of the heart (often caused by Staphylococcus aureus)? Endocarditis. The endocardium, or the inner layer of the heart, can become inflamed by an S. aureus infection. The most common structures involved are the heart valves.
True or false: Staphylococcus aureus can break down fibrin clots? True. S. aureus is fibrinolytic, which means that it can destroy a fibrin clot and thereby allow infection to spread to the tissues.
What is MRSA and what is the drug of choice to treat it? MRSA stands for Methicillin-Resistant Staphylococcus Aureus. It is caused by strains of S. aureus that have become resistant to Methicillin. The drug of choice is Vancomycin.
What is the name of the "spreading factor" produced by Staphylococcus aureus that contributes to the spread of infection? Hyaluronidase. This enzyme is produced by S. aureus and helps to spread the infection.
Staph aureus strains that express the mecA gene are methicillin-resistant. However, the gene may only be expressed by some of the cells in a given population. What is this phenomenon called? Heteroresistance. Two subpopulations may exist within a culture, one that is susceptible to the antibiotic, and one that is resistant. Because the resistant population grows more slowly, it may be overlooked.
The condition caused by a Staphylococcus aureus infection in which an exfoliative toxin is produced and results in peeling skin and a "burn-like effect" is called what? Scalded Skin Syndrome. It usually occurs in neonates and is also called Ritter disease.
Created by: nkniep