A 68-year-old man presents to the emergency department with a 4-day history of cough productive of rust-colored sputum. He is experiencing dyspnea, fever with chills, and sharp left-sided chest pain during inspiration and with coughing. Past medical history is significant for hypertension, peripheral arterial disease, and gout. He smokes 10 cigarettes per day, and he does not drink alcohol. He works part-time as a music teacher and as a janitor, and he lives with his wife and two cats. Vital signs show tachycardia and tachypnea with a normal blood pressure. Physical examination reveals at the left lung base upon auscultation. Cardiac examination reveals tachycardia and normal heart sounds. Abdominal examination is normal. A chest x-ray is ordered. Gram stain of the sputum reveals a gram-positive, lancet-shaped diplococci.
The patient was started on an antibiotic that inhibits peptidoglycan transpeptidase. Which of the following is the most likely antibiotic?
A) Amoxicillin
B) Doxycycline
C) Levofloxacin
D) Linezolid
E) Vancomycin
A) Amoxicillin
This patient has a bacterial pneumonia most likely caused by Streptococcus pneumoniae. The key hints to this are the rust-colored sputum and the gram-positive, lancet-shaped diplococci seen on the Gram stain. Cephalosporins and penicillins inhibit transpeptidase that catalyzes the final step in cell wall biosynthesis, the cross-linking of peptidoglycan. Amoxicillin has this mechanism of action and is active against Streptococcus pneumoniae. Doxycycline binds to the 30S subunit of the bacterial ribosome and prevents attachment of the aminoacyl-tRNA. Levofloxacin is a fluoroquinolone that inhibits prokaryotic topoisomerase II (also known as DNA gyrase). Linezolid binds to the 50S subunit of the bacterial ribosome and prevents the initiation complex. Vancomycin inhibits the synthesis/polymerization of peptidoglycan for the bacterial cell wall.
Key Learning Point
Penicillins and cephalosporins work by inhibiting transpeptidase that catalyzes the final step in cell wall biosynthesis, the cross-linking of peptidoglycan.