Classification of Antibiotics

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Chapter: Medicinal Chemistry : Antibiotics

Antibiotics are classified on the basis of their mechanism of action and by its chemical nature.


Antibiotics are classified on the basis of their mechanism of action and by its chemical nature.


Classification Based on Mechanism of Action

1.           Agents that inhibit the synthesis of bacterial cell wall: These include the penicillins and cephalosporins that are structurally similar and dissimilar agents, such as cycloserine, vancomycin, bacitracin and the imidazole antifungal agents.

2.           Agents that act directly on the cell membrane of the microorganisms, affecting permeability, and leading to leakage of intracellular compounds: These include polymyxin, polyene antifungal agents, nystatin, and amphotericin B that bind to cell wall sterols.

3.           Agents that affect the function of 30s and 50s ribosomal subunits to cause reversible inhibition of protein synthesis: These include tetracyclines, erythromycins, chloramphenicol, and clindamycin.

4.           Agents that bind to the 30s ribosomal subunit and alter protein synthesis: These include aminoglycosides that leads to cell deaths eventually.

5.           Agents that affect nucleic acid metabolism: Such as rifamycins, which inhibit DNA dependent RNA polymerase.


Classification Based on Chemical Structure

1.           β-lactam antibiotics

2.           Aminoglycoside antibiotics

3.           Tetracycline antibiotics

4.           Polypeptide antibiotics

5.           Macrolide antibiotics

6.           Lincomycins

7.           Other antibiotics

1. β-lactam antibiotics

These consists of two major class of agents, that is penicillins and cephalosporins.

a. Penicillins

Penicillin, the most important antibiotic, was first extracted from the mould Penicillium notatum. Subsequently, a mutant of a related mould, P. chrysogenum, was found to give the highest yield of penicillin and is employed for the commercial production of this antibiotic. Penicillin belongs to a group of antibiotics called β-lactam antibiotics . The basic structure of the penicillins consists of a thiazolidine ring fused with a β-lactam ring, which is essential for antibacterial activity. These two rings constitute the fundamental nucleus of all the penicillins, namely, 6-amino penicillanic-acid (6-APA) A variety of semisynthetic penicillins are produced by altering the composition of the side chain attached to 6-APA nucleus. Both the 6-APA nucleus and side chain are essential for the antibacterial activity.


Penicillins are named in the following ways:

a. Chemical abstract

1.           The penicillins are described as 4-thia-1-azabicyclo (3.2.0) heptanes.

2.           Benzylpenicillin is 6-(2-phenylacetamido)-3, 3-dimethyl-7-oxo-4-thia-1-azabiclo(3.2.0)heptane2-carboxylic acid.

b. Penam

In order to simplify the unsubstituted bicylic ring system of penicillin, it is given the name penam. Accordingly, the penicillins are 6-acylamino-2, 2-dimethyl penam-3-carboxylates.

c. Pencillanic acid derivatives

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