Penicillin was discovered by chance, in 1928, after Alexander Fleming accidentally left a dish of staphylococcus bacteria uncovered for a few days. He returned to find the dish dotted with bacterial growth, apart from one area where a patch of mold (Penicillin notatum) was growing. The mold produced a substance, named penicillin by Fleming, which inhibited bacterial growth and was later found to be effective against a wide range of harmful bacteria. Penicillin was finally isolated by Howard Florey and Ernst Chain. Fleming, Florey and Chain received a Nobel prize in 1945, for their discovery which revolutionised medicine and led to the development of lifesaving antibiotics.
Variants of Penicillin used nowadays:

1> Benzylpenicillin, commonly known as penicillin G, is the gold standard penicillin. Penicillin G is typically given by a parenteral route of administration (not orally) because it is unstable in the hydrochloric acid of the stomach. Because the drug is given parenterally, higher tissue concentrations of penicillin G can be achieved than is possible with phenoxymethylpenicillin. These higher concentrations translate to increased antibacterial activity.
Specific indications for benzylpenicillin include:
* Cellulitis
* Bacterial endocarditis
* Gonorrhea
* Meningitis
* aspiration pneumonia, lung abscess
* Community-acquired pneumonia
* Syphilis
* Septicemia in children

2> Phenoxymethylpenicillin, commonly known as penicillin V, is the orally-active form of penicillin. It is less active than benzylpenicillin, however, and is appropriate only in conditions where high tissue concentrations are not required.
Specific indications for phenoxymethylpenicillin include:
* Infections caused by Streptococcus pyogenes
o Tonsillitis
o Pharyngitis
o Skin infections
* Prophylaxis of rheumatic fever
* Moderate-to-severe gingivitis (with metronidazole)
* Penicillin V is the first choice in the treatment of odontogenic infections.

3> Procaine benzylpenicillin (rINN), also known as procaine penicillin, is a combination of benzylpenicillin with the local anaesthetic agent procaine. Following deep intramuscular injection, it is slowly absorbed into the circulation and hydrolysed to benzylpenicillin — thus it is used where prolonged low concentrations of benzylpenicillin are required. This combination is aimed at reducing the pain and discomfort associated with a large intramuscular injection of penicillin. It is widely used in veterinary settings.
Specific indications for procaine penicillin include:
* Syphilis
It should be noted that in the United States, Bicillin C-R (a injectable suspension which 1.2 million units of benzathine penicillin & 1.2 million units of procaine penicillin per 4 mL) is not recommended for treating syphilis, since it contains only half the recommended dose of benzathine penicillin. Medication errors have been made due to the confusion between Bicillin L-A & Bicillin C-R. As a result, changes in product packaging have been made; specifically, the statement "Not for the Treatment of Syphilis" has been added in red text to both the Bicillin CR and Billin CR 900/300 syringe labels.
* Respiratory tract infections where compliance with oral treatment is unlikely
* Cellulitis, erysipelas
* Procaine penicillin is also used as an adjunct in the treatment of anthrax.

4> Benzathine benzylpenicillin (rINN), also known as benzathine penicillin, is slowly absorbed into the circulation, after intramuscular injection, and hydrolysed to benzylpenicillin in vivo. It is the drug-of-choice when prolonged low concentrations of benzylpenicillin are required and appropriate, allowing prolonged antibiotic action over 2–4 weeks after a single IM dose. It is marketed by Wyeth under the trade name Bicillin L-A. Specific indications for benzathine penicillin include
* Prophylaxis of rheumatic fever
* Early or latent syphilis