Ch 13 - The Staphylococci
A 54-year-old woman develops a right shoulder abscess with a strain of S. 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 S. aureus cell
(C) Action of β-lactamase - correct answer D
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 - correct answer E
A 36-year-old male patient has an abscess with a strain of S. 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 - correct answer A
, 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 also has had headache, muscle aches, and abdominal cramps with diarrhea. Her
blood pressure is 70/40 mm Hg. Pelvic examination shows she is having her menstrual period with a
tampon in place; otherwise, the pelvic examination is normal. Her kidney function test (serum urea
nitrogen and creatinine) results are abnormal, indicating mild renal failure. A blood smear for malaria is
negative. Her illness is likely to be caused by which of the following?
(A) A toxin that results in greatly increased levels of intracellular cyclic adenosine monophosphate
(cAMP)
(B) A toxin that degrades sphingomyelin
(C) A toxin that binds to the class II major histocompatibility complex (MHC) of an antigen-presenting
cell - correct answer C
Over a period of 3 weeks, a total of five newborns in the hospital nursery developed S. aureus infections
with S. aureus bacteremia. The isolates all had the same colony morphology and hemolytic properties
and identical antimicrobial susceptibility patterns, suggesting that they were the same. (Later molecular
methods showed the isolates were identical.) Which of the following should be done now?
(A) Prophylactic treatment of all newborns with intravenous vancomycin
(B) Protective isolation of all newborns
(C) Closing the nursery and referring pregnant women to another hospital
(D) Hiring all new staff for the hospital nursery
(E) Culture using mannitol salt agar of the anterior nares of the physicians, nurses, and others who cared
for the infected babies - correct answer E
The exfoliative toxins, TSST-1, and the enterotoxins are all superantigens. The genes for these toxins are
(A) Present in all strains of S. aureus
(B) Widely distributed on the staphylococcal chromosome
(C) On both the staphylococcal chromosome (TSST-1 and exfoliative toxins) and on plasmids
(enterotoxins)
(D) On the staphylococcal chromosome in a pathogenicity island
(E) On plasmids - correct answer D
A 54-year-old woman develops a right shoulder abscess with a strain of S. 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 S. aureus cell
(C) Action of β-lactamase - correct answer D
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 - correct answer E
A 36-year-old male patient has an abscess with a strain of S. 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 - correct answer A
, 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 also has had headache, muscle aches, and abdominal cramps with diarrhea. Her
blood pressure is 70/40 mm Hg. Pelvic examination shows she is having her menstrual period with a
tampon in place; otherwise, the pelvic examination is normal. Her kidney function test (serum urea
nitrogen and creatinine) results are abnormal, indicating mild renal failure. A blood smear for malaria is
negative. Her illness is likely to be caused by which of the following?
(A) A toxin that results in greatly increased levels of intracellular cyclic adenosine monophosphate
(cAMP)
(B) A toxin that degrades sphingomyelin
(C) A toxin that binds to the class II major histocompatibility complex (MHC) of an antigen-presenting
cell - correct answer C
Over a period of 3 weeks, a total of five newborns in the hospital nursery developed S. aureus infections
with S. aureus bacteremia. The isolates all had the same colony morphology and hemolytic properties
and identical antimicrobial susceptibility patterns, suggesting that they were the same. (Later molecular
methods showed the isolates were identical.) Which of the following should be done now?
(A) Prophylactic treatment of all newborns with intravenous vancomycin
(B) Protective isolation of all newborns
(C) Closing the nursery and referring pregnant women to another hospital
(D) Hiring all new staff for the hospital nursery
(E) Culture using mannitol salt agar of the anterior nares of the physicians, nurses, and others who cared
for the infected babies - correct answer E
The exfoliative toxins, TSST-1, and the enterotoxins are all superantigens. The genes for these toxins are
(A) Present in all strains of S. aureus
(B) Widely distributed on the staphylococcal chromosome
(C) On both the staphylococcal chromosome (TSST-1 and exfoliative toxins) and on plasmids
(enterotoxins)
(D) On the staphylococcal chromosome in a pathogenicity island
(E) On plasmids - correct answer D
