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Chapter 13: The Staphylococci

A 54-year-old woman develops a right shoulder abscess with a strain of Staphylococcus aureus that is resistant to nafcillin. She was treated with a 2-week course of intravenous vancomycin and improved. Three weeks later (week 5), the infection recurred, and she was given 2 more weeks of intravenous vancomycin and again improved. Four weeks later (week 11), the infection recurred and the patient was again started on intravenous vancomycin. The MICs for vancomycin for the S aureus isolates were as follows: initial isolate (day 1), 1 μg/mL; week 5, 2 μg/mL; and week 11, 8 μg/mL. The patient failed to improve with the third course of vancomycin, and alternative therapy was used. The mechanism that best explains the relative resistance of the patient’s strain of S aureus to vancomycin is

(A) Acquisition of the vanA gene from another microorganism

(B) Active transport of vancomycin out of the Staphylococcus aureus cell

(C) Action of β-lactamase

(D) Increased cell wall synthesis and alterations in the cell wall structure

(E) Phosphorylation and resultant inactivation of the vancomycin

An 11-year-old boy develops a mild fever and pain in his upper arm. A radiograph of his arm shows a lytic lesion (dissolution) in the upper part of the humerus with periosteal elevation over the lesion. The patient is taken to surgery, where the lesion is debrided (dead bone and pus removed). Culture from the lesion yields gram-positive cocci. A test shows that the organism is a Staphylococcus and not a Streptococcus. Based on this information, you know the organism is

(A) Susceptible to nafcillin

(B) β-Lactamase positive

(C) A producer of protein A

(D) Encapsulated

(E) Catalase positive

A 36-year-old male patient has an abscess with a strain of Staphylococcus aureus that is β-lactamase positive. This indicates that the organism is resistant to which of the following antibiotics?

(A) Penicillin G, ampicillin, and piperacillin

(B) Trimethoprim–sulfamethoxazole

(C) Erythromycin, clarithromycin, and azithromycin

(D) Vancomycin

(E) Cefazolin and ceftriaxone

Seven days ago, a 27-year-old medical student returned from Central America, where she had spent the summer working in a clinic for indigenous people. Four days ago, she developed an erythematous sunburn-like rash. She ...

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