Mechanism of action of chloramphenicol is -

Ans. is 'b' i.e., SOS ribosome inhibitor * The mechanism of action of antimicrobials can be divided into :1. Inhibition of cell wall synthesis# Drugs inhibiting cell wall synthesis are : b-lactam antibiotics (penicillin, cephalosporins, monobactoms, carbapenems), cyclosporine, bacitracin, vancomycin, fosfomycin.# All antibiotic inhibiting cell wall synthesis are bactericidal.2. Inhibition of cell membrane function# Drugs acting on cell membrane are polypeptides (polymyxin, colistin, tyrothricin), polyenes (amphotericin B, nystatin, hamycin), azoles (ketoconazole, fluconazole, itraconazole).# All antimicrobials that act on cell membrane are bactericidal.3. Inhibition of protein synthesis (translation)# Drugs acting by inhibiting protein synthesis are tetracyclines, chloramphenicol, aminoglycosides, erythromycin, clindamycin, linazolide.# All protein synthesis inhibitors are bacteriostatic except aminoglycosides and streptogramins which are bactericidal.# These drugs can be divided into :a) Based on the steps of translation (protein synthesis) on which drug act:i) Freeze initiation : Aminoglycosides.ii) Inhibit elongation : Tetracyclines, puromycin, chloramphenicol.iii) Inhibit translocation : Clindamycin, erythromycin.iv) Causing premature termination : Puromycin.b) Based on ribosome on which they act:i) 30 S ribosome: Tetracycline, streptomycin.ii) 50 S ribosome : Chloramphenicol, erythromycin, clindamycin, Linezolid, pleuromutilins (retapamulin).iii) Both 30S and 50S ribosomes : Aminoglycosides (except streptomycin).4. Interference with nucleic acid synthesis# These drugs may be :1. Drugs directly affecting nucleic acids (RNA or DNA)* DNA gyrase inhibitors - Quinolones (nalidixic acid and fluoroquinolones) and novobiocin.* Drugs destroying DNA - Metronidazole, nitrofurantoin.* Nucleotide/nucleoside analogues - get incorporated into DNA or RNA and lead to formation of faulty nucleic acid that may be unstable or nonfunctional. Drugs are - Acyclovir, Zidovudin, Idoxuridine.* DNA dependent RNA polymerase inhibitors - Rifampicin2. Drugs acting indrectly by affecting intermediary metabolism* They either inhibits folate synthase (sulfonamide, PAS, dapsone) or dihydrofolate reductase (Trimethoprim, pyrimethamine) so that tetrahydrofolate can not be synthesized, which is essential for the synthesis of DNA.