Chloramphenicol

Chloramphenicol is a broad-spectrum antibiotic. It is active against a wide variety of organisms. It interferes with the production of proteins that the bacteria need to multiply. This inhibits the ability of the bacteria to grow and therefore stops the spread of the infection. Chloramphenicol is effective against a wide variety of microorganisms, but due to serious side-effects (e.g., damage to the bone marrow) in humans, it is usually reserved for the treatment of serious and life-threatening infections (e.g., typhoid fever). It is also used in eye drops or ointment to treat bacterial conjunctivitis.

Chloramphenicol’s antibiotic activity results from its interference with protein synthesis in invading microbes. However, it is a very toxic substance, its most serious and potentially lethal effect being depression of red blood cell production in bone marrow; cases of leukemia were also attributed to early use of chloramphenicol. Because of its toxicity, chloramphenicol is rarely prescribed for infections that can be treated by other antibiotics. It is used as an alternative therapy to treat typhoid fever, some forms of meningitis, and rickettsial infections such as Rocky Mountain spotted fever and typhus. Chloramphenicol is commonly used in biological research to study protein synthesis. Chloromycetin is a trade name for chloramphenicol.

Chloramphenicol is an antibiotic that was originally derived from the bacterium Streptomyces venezuelae and is now produced synthetically. Chloramphenicol is now synthesized chemically, but originally it was obtained from Streptomyces venezuelae. The structure is fairly simple and is formed by three parts; a benzene ring with a NO2 side chain, an acyl group and between these two groups the centre of the molecule, propanediol. The propanediol is responsible for the antimicrobial effect of Chloramphenicol. Substitution of the centre of the molecule results in the loss of antimicrobial activity, whereas changes in the benzene ring or acyl group do not. The bacteriostatic effect of Chloramphenicol is based on the inhibition of bacterial protein synthesis. The antibiotic binds reversibly to 70S ribosomes, but not to 80S ribosomes. Chloramphenicol also binds to mitochondrial ribosomes and is therefore toxic for eukaryotic cells. The binding site for Chloramphenicol, protein L16, lies on the 50S subunit of the ribosome. The binding of Chloramphenicol prevents the formation of a peptide bond between the amino acid end of acyl-tRNA and the already existing polypeptide chain. The Chloramphenicol binds close to the A-site, thus preventing peptidyltransferase from attaching to the amino acid end of the t-RNA. This prevents the formation of polypeptide chains longer than three amino acids.

Chloramphenicol binds reversibly with the large ribosomal subunit of bacteria and eukaryotes. The bacterial ribosome (and eukaryotic mitochondrial ribosome) is more sensitive, but protein synthesis is also decreased in patients receiving the drug as the concentration rises. It binds to the peptidyl transferase enzyme to inhibit transfer of the growing polypeptide to the next amino acid occupying the "Acceptor" site. The presence of chloramphenicol at this site may interfere with binding of lincosamides (e.g., clindamycin) and macrolides (e.g., erythromycin) which bind at or near the same site. This writer fails to see that this is an important problem because all are static in any case so overall inhibition of activity should not be a problem. However, it does point to the fact that one may be wasting drug and increasing the possibility of drug reactions by using a combination of these three drugs.

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