Pharmacology Antimicrobial Therapy Principles Questions and Answers 1 — Questions and Answers
Question 1: A patient with infective endocarditis caused by Enterococcus faecalis is treated with a combination of ampicillin and gentamicin. What is the primary rationale for using this specific combination?
- To cover for potential anaerobic co-pathogens.
- To reduce the nephrotoxic effects of gentamicin.
- To achieve a synergistic bactericidal effect. (Correct answer)
- To prevent the emergence of resistance to ampicillin.
Correct answer: To achieve a synergistic bactericidal effect.
This is a classic example of antimicrobial synergy. Ampicillin, a beta-lactam, inhibits cell wall synthesis, which increases the permeability of the bacterial cell to the aminoglycoside, gentamicin. This enhanced uptake allows gentamicin to more effectively reach its intracellular target (the 30S ribosome), resulting in a bactericidal effect that is greater than the individual agents alone.
Question 2: A bacterial strain isolated from a wound culture is found to be resistant to ampicillin but susceptible to amoxicillin-clavulanate. What is the most likely mechanism of resistance to ampicillin in this strain?
- Alteration of the drug's target site (penicillin-binding proteins).
- Production of a beta-lactamase enzyme. (Correct answer)
- An active efflux pump that removes the antibiotic from the cell.
- Impaired drug entry due to a porin channel mutation.
Correct answer: Production of a beta-lactamase enzyme.
The addition of clavulanate, a beta-lactamase inhibitor, restores the activity of the beta-lactam antibiotic (amoxicillin). This indicates that the bacterium's resistance mechanism is the production of a beta-lactamase enzyme, which inactivates the antibiotic by hydrolyzing the beta-lactam ring.
Question 3: Which of the following best describes the primary pharmacodynamic property of beta-lactam antibiotics, such as piperacillin, which dictates their dosing strategy?
- Their efficacy is best correlated with the duration that the drug concentration remains above the minimum inhibitory concentration (T > MIC). (Correct answer)
- Their bactericidal activity is primarily dependent on achieving a high peak concentration relative to the MIC (Cmax/MIC).
- They exhibit a long and significant post-antibiotic effect (PAE) against most pathogens.
- They are most effective when dosed intermittently to minimize the risk of target site saturation.
Correct answer: Their efficacy is best correlated with the duration that the drug concentration remains above the minimum inhibitory concentration (T > MIC).
Beta-lactam antibiotics demonstrate time-dependent killing. Their effectiveness is maximized by keeping the drug concentration above the minimum inhibitory concentration (MIC) for as long as possible during the dosing interval (fT > MIC). This principle supports the use of frequent intermittent dosing or continuous infusions to optimize therapeutic outcomes.
Question 4: Sulfonamide antibiotics, such as sulfamethoxazole, exhibit selective toxicity against bacteria. Why are human cells typically unaffected by this mechanism?
- Human cells possess an active efflux pump that removes sulfonamides.
- Human cells have an 80S ribosome, while bacteria have a 70S ribosome.
- Human cells lack a peptidoglycan cell wall.
- Human cells utilize pre-formed folic acid from dietary sources. (Correct answer)
Correct answer: Human cells utilize pre-formed folic acid from dietary sources.
Sulfonamides competitively inhibit dihydropteroate synthase, an enzyme essential for the de novo synthesis of folic acid in bacteria. Human cells lack this pathway and instead acquire folic acid from the diet, making them insusceptible to the action of sulfonamides.
Question 5: A physician notes that an aminoglycoside has a significant post-antibiotic effect (PAE). Which of the following statements best describes the clinical implication of the PAE?
- The drug is likely to cause a disulfiram-like reaction with alcohol.
- Bacterial growth suppression continues for a period after the plasma concentration has fallen below the MIC. (Correct answer)
- The drug must be administered via continuous infusion to be effective.
- The drug is safe to use in patients with renal impairment without dose adjustment.
Correct answer: Bacterial growth suppression continues for a period after the plasma concentration has fallen below the MIC.
The post-antibiotic effect (PAE) is the phenomenon of persistent suppression of bacterial growth even after the antibiotic concentration has fallen below the minimum inhibitory concentration (MIC). This property, characteristic of drugs like aminoglycosides, allows for less frequent, high-dose administration schedules (once-daily dosing), which can maximize efficacy and minimize certain toxicities.
Question 6: A 68-year-old patient with a complicated urinary tract infection is being treated with ciprofloxacin, a fluoroquinolone. This class of antibiotics achieves its bactericidal effect by targeting which of the following?
- The 50S ribosomal subunit to inhibit protein translocation.
- Peptidoglycan cross-linking in the bacterial cell wall.
- DNA gyrase and topoisomerase IV. (Correct answer)
- Dihydropteroate synthase in the folic acid pathway.
Correct answer: DNA gyrase and topoisomerase IV.
Fluoroquinolones exert their bactericidal effect by inhibiting two key bacterial enzymes: DNA gyrase (topoisomerase II) and topoisomerase IV. Inhibition of these enzymes prevents the relaxation of supercoiled DNA and separation of daughter DNA strands, which are essential for DNA replication and cell division.
Pharmacology Antimicrobial Therapy Principles Questions and Answers 1
A patient with infective endocarditis caused by Enterococcus faecalis is treated with a combination of ampicillin and gentamicin.
What is the primary rationale for using this specific combination?