Penicillins are a group of beta lactum containing bacteriocidal antibiotics. Being the first among the antibiotics to be discovered, penicillins are historically important. The basic structure of all the penicillins consisits of a lactum ring and a thizolidine ring fused together to form six amino penicillinic acid. Some semi synthetic penicillins are ampicillin, floxacillin, azlocillin, coxacillin etc.

Natural penicillins are effective against several Gram positive bacteria. They inhibit the bacterial cell wall synthesis such as peptoglyclans and cause cell death. Several semi synthetic penicillins that are resistant to beta lactamase have been developed and successfully used against a large number of Gram negative bacteria.

In the early days Penicillium notatum was used for the large scale production of penicillins. Currently Penicillium chrysogenum and its improved mutant strains are used.


The pharmaceutical uses of penicillin are associated with allergic reaction in
some individuals. To overcome these allergic problems cephalosporins were developed.
They have improved stability against beta lactumases and are more active against Gram negative bacteria. Cephalosporins are broad spectrum antibiotics with low toxicity. Basically cephalosporins have a beta lactum ring with fused with a dehydrothiazine ring.

Cephalosporin C was first discovered in the cultures of fungi Cephalosporium acremonium.


Aminoglycosides are oligosaccharides or carbohydrate antibiotics. They contain an aminocyclo hexanol moiety which is bound to other amino sugars by glycosidic linkages. More than one hundred aminoglycosides are known that is streptomycin, neomycin, kanamycin, gentamycin, hygromycin and sisomycin.

Aminoglycosides are very potent antibiotics and act against Gram positive and Gram negative bacteria besides mycobacteria. At the molecular level aminoglycosides bind to 30S ribosomal subunit and block protein biosynthesis. It is used for treatment of severe and chronic infections. Streptomycin was the first aminoglycosides that was successfully used to treat tuberculosis. Aminoglycoside antibiotic are produced by Actinomyces sp.


Tetracycline are broad spectrum antibiotics with wide spread medical use. They are effective against Gram positive and Gram negative bacteria besides other organisms like mycoplasms, chlamydias, rickettsias. Tetracyclines are used to combat stomach ulcers against Helicobacter pylori. They are the most commonly used antibiotics, next to cephalosporins and penicillins. Tetracyclins inhibit protein biosynthesis by blocking the binding of aminoacyl tRNA to ribosomal A site.

The basic structure of tetracycline is composed of a napthacene ring a four ring structure. The first tetracycline antibiotic that was isolated was chlortetracycline from the cultures of Streptomyces aureofaciens in 1945.


Macrolides are a group of antibiotic with large lactone rings or macro cyclic lactone rings. They consist of 12, 14 or 16 membered lactone rings with 1 to 3 sugars linked by glycosidic bonds. The sugars may be 6-deoxyhexoses or amino sugars.

Erytromycin and oleandomycin are 14 membered lactone ring containing macrolides while tylosin and leucomycin are examples for 16 membered macrolides. Polyene macrolides is the term applied for very large ring macrolides that may contain lactone rings in the range of 26 to 28. Example of a polyene macrolides are nystatin, amphotericin.

They are effective against Gram-positive bacteria and are frequently used to kill penicillin resistant organisms. Clarithromycin is currently used to combat stomach ulcers caused by H.pylori. The macrolides inhibit the protein biosynthesis by binding to 50S ribosomal subunit. The polyene macrolides functions as antifungal agents. Macrolides are produced by actinomycetes.


Chloramphenicol is a broad spectrum antibiotic that can act against Gram-positive and Gram-negative bacteria, besides rickettsia, actinomycetes and chlamydias. Chloramphenicol binds to the 50S ribosomal subunit and blocks the peptidyl transferase reaction in protein synthesis. But due to severe side effects chloramphenicol is seldom used and is kept for reserve use.
Chloramphenicol can be produced by Streptococcus Venezuelae and Streptococcus omiyanesis.


Griseofulvin is an antibiotic that acts specifically on fungi with chitinous cell walls. It is used in the traeatment of various fungal skin infections. Further griseofulvin is alslo employed in the treatment of plant diseases caused by Biotrytis and Aternaria solani.
It affects chitin biosynthesis.

Nucleoside Antibiotics

There are several antibiotics more than 200 or so which have nucleoside like structures. Some of them are puromycin, blastimycin. Nucleoside antibiotics have diverse structures and biological acitivities.

Puromycin is used to understand the ribosomonal function in protein biosynthesis. Neplanosincauses antiviral activity. Blastisidin is a fungicide antibiotic used in plant pathology

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