Pharmacology · patient case question
All of the following antibiotics are bactericidal EXCEPT:
Patient case: Choosing and stopping drugs
demo: Series of prescribing decisions
chief: Several patients seen in one session raise questions about antibiotic choice, analgesia and anticoagulation.
history: One patient has a spreading odontogenic infection unresponsive to five days of empirical antibiotics. Another takes rivaroxaban for atrial fibrillation. A third developed a widespread blistering rash and mucosal ulceration a week after starting a new medication.
meds: Varies by patient.
findings: Not applicable.
Show answer and explanation
Correct answer: D. Doxycycline
DOXYCYCLINE is bacteriostatic — inhibits protein synthesis via 30S ribosomal subunit binding. Cephalexin, amoxicillin, and penicillin are all beta-lactams (cell wall synthesis inhibitors) — BACTERICIDAL.
- A. Penicillin V is a beta-lactam and bactericidal. The useful generalisation is that agents acting on the cell wall or on DNA kill, while most that act on the ribosome only inhibit.
- B. Correct, and this is the exception. Tetracyclines including doxycycline are bacteriostatic. They bind the 30S ribosomal subunit and block aminoacyl-tRNA attachment, halting protein synthesis and arresting growth without killing the organism, so host immunity must clear the infection.
- C. Cephalexin is a first-generation cephalosporin and a beta-lactam, so it is bactericidal through inhibition of cell wall cross-linking.
- D. Amoxicillin is a beta-lactam and bactericidal.