Questions: Antimicrobial Agents: Properties and Mechanisms of Action

Question 1

Some β-lactam-resistant bacteria (with altered penicillin-binding proteins) remain susceptible to vancomycin. Why?

• A. Vancomycin is a much larger molecule and cannot be excluded by the same efflux pumps that remove β-lactams
• B. Vancomycin binds the D-Ala-D-Ala substrate directly, bypassing the altered PBPs that β-lactams target
• C. Vancomycin targets the 30S ribosomal subunit, which is a completely different mechanism from cell wall synthesis
• D. Vancomycin is only used for gram-negative bacteria, which have different resistance mechanisms

Answer: (B)

β-Lactams and vancomycin both disrupt peptidoglycan cross-linking but at different molecular points. β-Lactams mimic the D-Ala-D-Ala substrate and covalently bind the transpeptidase (PBP). β-Lactam resistance often works by altering the PBP structure so the drug can't bind. Vancomycin, however, binds the D-Ala-D-Ala dipeptide itself — before PBPs even access it — physically blocking the substrate regardless of PBP structure. An altered PBP is irrelevant if vancomycin has already blocked its substrate. This is why the two drug classes have non-overlapping resistance mechanisms.

Question 2

Sulfonamides and trimethoprim are combined (as co-trimoxazole) because they produce synergistic bacterial killing. What is the mechanistic basis for this synergy?

• A. They have additive toxic effects on the bacterial membrane when used together
• B. Trimethoprim increases bacterial uptake of sulfonamides, improving intracellular concentration
• C. They sequentially block two steps in the same folate synthesis pathway, creating a double blockade that depletes folate more completely
• D. Each drug targets a different bacterial species, so the combination has a broader spectrum

Answer: (C)

Sulfonamides inhibit dihydropteroate synthase (blocking early folate synthesis) and trimethoprim inhibits dihydrofolate reductase (blocking the next step). When combined, they create a sequential double blockade of the same essential pathway. Even partial inhibition at each step multiplies to near-complete depletion of the pathway's output. This is mechanistic synergy — two drugs hitting the same pathway at sequential steps — rather than mere additive toxicity. Because humans obtain dietary folate and lack dihydropteroate synthase, both drugs are selective for bacteria.

Question 3

Aminoglycosides are bacteriostatic antibiotics — they inhibit protein synthesis and halt bacterial growth but do not kill the cells.

• T. True
• F. False

Answer: False

Aminoglycosides are bactericidal, not bacteriostatic. They bind the 30S ribosomal subunit and cause mRNA misreading, leading to incorporation of wrong amino acids. The resulting misfolded proteins are not merely non-functional — they are toxic. Misfolded proteins insert into the bacterial membrane and disrupt its integrity, accelerating drug uptake in a fatal positive feedback loop. The distinction matters clinically: bacteriostatic drugs (like tetracyclines and macrolides) require an intact immune system to clear infection; bactericidal drugs like aminoglycosides are preferred for immunocompromised patients or severe infections.

Question 4

Antifungal azoles are selectively toxic to fungi because mammalian cells rely on ergosterol rather than cholesterol as their primary membrane sterol.

• T. True
• F. False

Answer: False

This reverses the facts. Fungi use ergosterol; mammalian cells use cholesterol. Azoles target the ergosterol synthesis pathway (specifically lanosterol 14α-demethylase), which is present in fungi but not in mammals. The selective toxicity comes from the fact that humans do not synthesize ergosterol and the human version of the target enzyme has sufficiently different structure that azoles bind it with much lower affinity. So azoles are selective because fungi use ergosterol and humans use cholesterol — not the other way around.

Question 5

Explain why selectivity — rather than potency — is the central design criterion for antimicrobial agents, and give one example of how structural differences between pathogen and host are exploited.

Model answer: Potency alone is insufficient: a drug can be highly effective at killing bacteria but equally effective at killing the patient's own cells, making it useless therapeutically. The therapeutic window — the ratio of toxic dose to effective dose — depends entirely on selectivity. An antimicrobial must exploit a structural or biochemical difference between the pathogen and the host to cause selective damage. Example: β-lactam antibiotics target bacterial penicillin-binding proteins (transpeptidases that cross-link peptidoglycan), which are unique to bacteria — human cells have no cell wall and therefore no PBPs. This makes β-lactams highly selective with a wide therapeutic window, explaining their clinical dominance.

The same logic explains why broad-spectrum antifungals are harder to develop than antibacterials: fungi are eukaryotes, much more similar to human cells than bacteria are. The few exploitable differences (ergosterol vs. cholesterol, fungal cell wall β-glucan) are the basis for all current antifungal classes. Antivirals are even more challenging because viruses hijack host machinery — the fewer virus-specific enzymes, the narrower the target repertoire.