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

Antibacterial drugs are called bacteristatic when they merely inhibit growth, and bactericidal when they 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. An essential quality of an antibiotic should be the ability to destroy the parasitic micro-organism without injury  to host cells.

The antibiotic penicillin acts 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 in- effective. 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 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 more effective against extracellular bacteria than intracellular bacteria. Tetracycline inhibits protein synthesis by interfering with binding of tRNA with ribosomes. Chloramphenicol inhibits protein 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 protein synthesis by acting as an analogue to the adenosine terminal of phenylalanyl tRNA and thus displacing aminoacyl tRNA.

Actionomycin D acts against bacteria at the level of nucleic 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 to basic proteins in eukaryotes. In very high concentrations actionmycin D blocks replication of DNA.