MCQ Microbiology Answer 40

The answer is: E

The antibiotics in these questions have significantly different modes of action. Recent evidence suggests that while penicillin inhibits the final cross-linking of the cell wall, it also binds to penicillin-binding proteins and inhibits certain key enzymes involved in cell-wall synthesis. The mechanism is complex. Amdinocillin, although classified as a penicillin, selectively binds to penicillin-binding protein-2 (PBP-2). Binding to PBP-2 results in aberrant cell-wall elongation and spherical forms, seen when E. coli, for example, is exposed to mecillinam.

Because amphotericin binds to sterols (such as cholesterol) in the cell membrane, its range of activity is predictable; that is, it is effective against microorganisms that contain sterol in the cell membrane (such as molds, yeasts, and certain amebae). These polyene antibiotics cause reorientation of sterols in the membrane, and membrane structure is altered to the extent that permeability is affected. If sterol synthesis is blocked in fungi, then amphotericin is not effective. This occurs when fungi are exposed to miconazole, another antifungal antibiotic.

Chloramphenicol is a bacteriostatic antibiotic. Its action does not kill the cell but only inhibits it. If chloramphenicol is removed from the culture, then protein synthesis is reinitiated. Bacterial ribosomes are spherical particles with a molecular weight of 3 3 106. Protein synthesis takes place on the ribosome by a complex process involving various ribosomal subunits, tRNA, and nRNA. Chloramphenicol, in contrast to the aminoglycosides and tetracycline, attaches to the 50S ribosome subunit. The enzyme peptidyl transferase, found in the 50S subunit, is inhibited. Removal of the inhibition—in this case, chloramphenicol—results in full activity of the enzyme.

Trimethoprim (TMP), a diaminopyrimidine, is a folic acid antagonist. Although TMP is commonly used in combination with sulfa drugs, its mode of action is distinct. TMP is structurally similar to the pteridine portion of dihydrofolate and prevents the conversion of folic acid to tetrahydrofolic acid by inhibition of dihydrofolate reductase. Fortunately, this enzyme in humans is relatively insensitive to TMP.

Category: Microbiology MCQs