Antibiotics

Substances competing for the active site of an enzyme in a metabolic pathway are called antimetabolities. Antibiotics are antimetabolities isolated from living cells. Antibitotics have been defined as substances of microbial origin with antimicrobial activity in minute quantities. Thus an antibiotic is a substance produce by one living organism which is harmful to another. It should, be noted however, that some antibiotics, e.g., chloromphenical, are now produced commercially by chemical synthesis. Antibiotics against bacteria are produced by a number of microorganisms, including fungi and bacteria.

Antibacterial drugs are called bacteristatic when they merely inhibit growth, and bactericidal when they a have an irreversible lethal effect. An antibiotic commonly acts by (1) inhibiting cell wall synthesis, (2) inhibiting protein synthesis by altering the bacterial ribosome and (3) inhibiting mRNA synthesis on the DNA template. A essential quality of an antibiotic should be the ability to destroy the parasitic microorganism without injury to host cells. The action of certain commonly used antibiotics at cell level will now be taken up.

The antibiotic penicillin act by inhibiting cell wall synthesis in bacteria. The cell continues to grow without the protective cell wall, and finally undergoes osmotic lysis. Since mammalian cells do not have cell walls, penicillin does not harm the host. Certain bacteria undergo mutation and become resistant to penicillin. Some bacteria can synthesize the enzyme penicillinase which breaks down penicillin and makes it ineffective. Some bacteria even thrive on penicillin by utilizing it as a source of nutritional carbon.

Certain antibiotics act by inhibiting protein synthesis in bacteria, at the same time not affecting protein synthesis in the host. This is because of the differences between the 70S bacterial ribosome and the 80S eukaryote ribosome. Streptomycin acts by inhibiting protein synthesis by altering the structure of the 30S ribosomal subunit. This results in the mis-reading of the mRNA code. Streptomycin is rapidly bactericidal. Tetracycline inhibits protein synthesis by interfering with binding of tRNA the with ribosomes. Chloramphenicol (chloromycetin) inhinbits proteins synthesis in bacteria by interacting with the 50S ribosomal subunit. The tetracyclines and chloramphenicol are conventionally called broad-spectrum antibiotics because of their wide-ranging antimicrobial activity.

Puromycin blocks proteins synthesis by acting as an analogue to the adenosine terminal of phenylalanyl tRNA, and thus displacing aminoacyl tRNA.

Actinomycin D acts against bacteria at the level of nuceic acid synthesis. It binds directly to double – stranded DNA and inhibits synthesis of mRNA on the DNA template. It has no effect on host cells because DNA is bound the basic proteins in eukaryotes. In very high concentrations actionmycin D blocks replication of DNA.