Penicillin

Penicillins are a group of β-lactam antibiotics originally obtained from Penicillium moulds, principally P. chrysogenum and P. rubens. Most penicillins in clinical use are synthesised by P. chrysogenum using deep tank fermentation and then purified. A number of natural penicillins have been discovered, but only two purified compounds are in clinical use: penicillin G and penicillin V. Penicillins were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are still widely used today for various bacterial infections, though many types of bacteria have developed resistance following extensive use.
In the United States, 10% of the population claims penicillin allergies, but because the frequency of positive skin test results decreases by 10% with each year of avoidance, 90% of these patients can eventually tolerate penicillin. Additionally, those with penicillin allergies can usually tolerate cephalosporins because the immunoglobulin E cross-reactivity is only 3%.
Penicillin was discovered in 1928 by the Scottish physician Alexander Fleming as a crude extract of P. rubens. Fleming's student Cecil George Paine was the first to successfully use penicillin to treat eye infection in 1930. The purified compound was isolated in 1940 by a research team led by Howard Florey and Ernst Boris Chain at the University of Oxford. Fleming first used the purified penicillin to treat streptococcal meningitis in 1942. The 1945 Nobel Prize in Physiology or Medicine was shared by Chain, Fleming and Florey.
Several semisynthetic penicillins are effective against a broader spectrum of bacteria: these include the antistaphylococcal penicillins, aminopenicillins, and antipseudomonal penicillins.

Nomenclature

The term "penicillin" is defined as the natural product of Penicillium mould with antimicrobial activity. It was coined by Alexander Fleming on 7 March 1929 when he discovered the antibacterial property of Penicillium rubens. Fleming explained in his 1929 paper in the British Journal of Experimental Pathology that "to avoid the repetition of the rather cumbersome phrase 'Mould broth filtrate', the name 'penicillin' will be used." The name thus refers to the scientific name of the mould, as described by Fleming in his Nobel lecture in 1945:

I have been frequently asked why I invented the name "Penicillin". I simply followed perfectly orthodox lines and coined a word which explained that the substance penicillin was derived from a plant of the genus Penicillium just as many years ago the word "Digitalin" was invented for a substance derived from the plant Digitalis.

In modern usage penicillin is used more broadly to refer to any β-lactam antimicrobial that contains a thiazolidine ring fused to the β-lactam core and may or may not be a natural product. Like most natural products, penicillin is present in Penicillium moulds as a mixture of active constituents. The principal active components of Penicillium are listed in the following table:

Chemical name	UK nomenclature	US nomenclature	Potency relative to penicillin G
2-Pentenylpenicillin	Penicillin I	Penicillin F	70–82%
Benzylpenicillin	Penicillin II	Penicillin G	100%
p-Hydroxybenzylpenicillin	Penicillin III	Penicillin X	130–140%
n-Heptylpenicillin	Penicillin IV	Penicillin K	110–120%

Other minor active components of Penicillium include penicillin O, penicillin U1, and penicillin U6. Other named constituents of natural Penicillium, such as penicillin A, were subsequently found not to have antibiotic activity and are not chemically related to antibiotic penicillins.
The precise constitution of the penicillin extracted depends on the species of Penicillium mould used and on the nutrient media used to culture the mould. Fleming's original strain of Penicillium rubens produces principally penicillin F, named after Fleming. But penicillin F is unstable, difficult to isolate, and produced by the mould in small quantities.
The principal commercial strain of Penicillium chrysogenum produces penicillin G as the principal component when corn steep liquor is used as the culture medium. When phenoxyethanol or phenoxyacetic acid are added to the culture medium, the mould produces penicillin V as the main penicillin instead.
6-Aminopenicillanic acid is a compound derived from penicillin G. 6-APA contains the beta-lactam core of penicillin G, but with the side chains stripped off; 6-APA is a useful precursor for manufacturing other penicillins. There are many semi-synthetic penicillins derived from 6-APA and these are in three groups: antistaphylococcal penicillins, broad-spectrum penicillins, and antipseudomonal penicillins. The semi-synthetic penicillins are all referred to as penicillins because they are all derived ultimately from penicillin G.

Penicillin units

One unit of penicillin G sodium is defined as 0.600 micrograms. Therefore, 2 million units of penicillin G is 1.2 g.
One unit of penicillin V potassium is defined as 0.625 micrograms. Therefore 400,000 units of penicillin V is 250 mg.

The use of units to prescribe penicillin is largely obsolete outside of the US. Since the original penicillin was an ill-defined mixture of active compounds, the potency of penicillin varied from batch to batch. It was therefore impractical to prescribe 1 g of penicillin because the activity of 1 g of penicillin from one batch would be different from the activity from another batch. To address this problem, after manufacture, each batch of penicillin was standardised against a known unit of penicillin: each glass vial was then filled with the number of units required. In the 1940s, a vial of 5,000 Oxford units was standard, but the depending on the batch, could contain anything from 15 mg to 20 mg of penicillin. Later, a vial of 1,000,000 international units became standard, and this could contain 2.5 g to 3 g of natural penicillin. With the advent of pure penicillin G preparations, there is little reason to prescribe penicillin in units, although units are still used for benzathine benzylpenicillin in the United States.
The "unit" of penicillin has had three previous definitions, and each definition was chosen as being roughly equivalent to the previous one.

Oxford or Florey unit. This was originally defined as the minimum amount of penicillin dissolved in 50 ml of meat extract that would inhibit the growth of a standard strain of Staphylococcus aureus. The reference standard was a large batch of impure penicillin kept in Oxford. The assay was later modified by Florey's group to a more reproducible "cup assay": in this assay, a penicillin solution was defined to contain one unit/ml of penicillin when 339 microlitres of the solution placed in a "cup" on a plate of solid agar produced a 24 millimetre zone of inhibition of growth of Oxford Staphylococcus.
First International Standard. A single 8 gram batch of pure crystalline penicillin G sodium was stored at The National Institute for Medical Research in Mill Hill, London. One penicillin unit was defined at 0.6 micrograms of the International Standard. An impure "working standard" was also defined and was available in much larger quantities distributed around the world: one unit of the working standard was 2.7 micrograms. At the same time, the cup assay was refined, where instead of specifying a zone diameter of 24 mm, the zone size were instead plotted against a reference curve to provide a readout on potency.
Second International Standard. A single 30 gram batch of pure crystalline penicillin G sodium was obtained: this was also stored at Mill Hill. One penicillin unit was defined as 0.5988 micrograms of the Second International Standard.

There is an older unit for penicillin V that is not equivalent to the current penicillin V unit. The reason is that the US FDA incorrectly assumed that the potency of penicillin V is the same mole-for-mole as penicillin G. In fact, penicillin V is less potent than penicillin G, and the current penicillin V unit reflects that fact.

First international unit of penicillin V. One unit of penicillin V was defined as 0.590 micrograms of a reference standard held at Mill Hill in London. This unit is now obsolete.

A similar standard was also established for penicillin K.

Types

Penicillins consist of a distinct 4-membered beta-lactam ring, in addition to a thiazolide ring and an R side chain. The main distinguishing feature between variants within this family is the R substituent.
This side chain is connected to the 6-aminopenicillanic acid residue and results in variations in the antimicrobial spectrum, stability and susceptibility to beta-lactamases of each type.

Natural penicillins

Penicillin G was first produced from a penicillium fungus that occurs in nature. The strain of fungus used today for the manufacture of penicillin G was created by genetic engineering to improve the yield in the manufacturing process. None of the other natural penicillins are currently in clinical use.

Semi-synthetic penicillin

Penicillin V is produced by adding the precursor phenoxyacetic acid to the medium in which a genetically modified strain of the penicillium fungus is being cultured.

Antibiotics created from 6-APA

There are three major groups of other semi-synthetic antibiotics related to the penicillins. They are synthesised by adding various side-chains to the precursor 6-APA, which is isolated from penicillin G. These are the antistaphylococcal antibiotics, broad-spectrum antibiotics and antipseudomonal antibiotics.