Penicillin
Penicillins (P, PCN or PEN) 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[2] and then purified.[3][4] A number of natural penicillins have been discovered, but only two purified compounds are in clinical use: penicillin G (intramuscular or intravenous use) and penicillin V (given by mouth). 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 different bacterial infections, though many types of bacteria have developed resistance following extensive use.
For other uses, see Penicillin (disambiguation).Clinical data
- In general: ℞ (Prescription only)
Liver
Between 0.5 and 56 hours
Kidneys
Ten percent 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 (another group of β-lactam) because the immunoglobulin E (IgE) cross-reactivity is only 3%.[5]
Penicillin was discovered in 1928 by Scottish scientist Alexander Fleming as a crude extract of P. rubens.[6] Fleming's student Cecil George Paine was the first to successfully use penicillin to treat eye infection (neonatal conjunctivitis) in 1930. The purified compound (penicillin F) 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.[7] 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.
Types
Natural penicillins
Penicillin G (benzylpenicillin) 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 (F, K, N, X, O, U1 or U6) are currently in clinical use.
Pharmacology
Entry into bacteria
Penicillin can easily enter bacterial cells in the case of Gram-positive species. This is because Gram-positive bacteria do not have an outer cell membrane and are simply enclosed in a thick cell wall.[45] Penicillin molecules are small enough to pass through the spaces of glycoproteins in the cell wall. For this reason Gram-positive bacteria are very susceptible to penicillin (as first evidenced by the discovery of penicillin in 1928[46]).[47]
Penicillin, or any other molecule, enters Gram-negative bacteria in a different manner. The bacteria have thinner cell walls but the external surface is coated with an additional cell membrane, called the outer membrane. The outer membrane is a lipid layer (lipopolysaccharide chain) that blocks passage of water-soluble (hydrophilic) molecules like penicillin. It thus acts as the first line of defence against any toxic substance, which is the reason for relative resistance to antibiotics compared to Gram-positive species[48] But penicillin can still enter Gram-negative species by diffusing through aqueous channels called porins (outer membrane proteins), which are dispersed among the fatty molecules and can transport nutrients and antibiotics into the bacteria.[49] Porins are large enough to allow diffusion of most penicillins, but the rate of diffusion through them is determined by the specific size of the drug molecules. For instance, penicillin G is large and enters through porins slowly; while smaller ampicillin and amoxicillin diffuse much faster.[50] In contrast, large vancomycin can not pass through porins and is thus ineffective for Gram-negative bacteria.[51] The size and number of porins are different in different bacteria. As a result of the two factors—size of penicillin and porin—Gram-negative bacteria can be unsusceptible or have varying degree of susceptibility to specific penicillin.[52]
Resistance
When Alexander Fleming discovered the crude penicillin in 1928, one important observation he made was that many bacteria were not affected by penicillin.[46] This phenomenon was realised by Ernst Chain and Edward Abraham while trying to identify the exact of penicillin. In 1940, they discovered that unsusceptible bacteria like Escherichia coli produced specific enzymes that can break down penicillin molecules, thus making them resistant to the antibiotic. They named the enzyme penicillinase.[63] Penicillinase is now classified as member of enzymes called β-lactamases. These β-lactamases are naturally present in many other bacteria, and many bacteria produce them upon constant exposure to antibiotics. In most bacteria, resistance can be through three different mechanisms – reduced permeability in bacteria, reduced binding affinity of the penicillin-binding proteins (PBPs) or destruction of the antibiotic through the expression of β-lactamase.[64] Using any of these, bacteria commonly develop resistance to different antibiotics, a phenomenon called multi-drug resistance.
The actual process of resistance mechanism can be very complex. In case of reduced permeability in bacteria, the mechanisms are different between Gram-positive and Gram-negative bacteria. In Gram-positive bacteria, blockage of penicillin is due to changes in the cell wall. For example, resistance to vancomycin in S. aureus is due to additional peptidoglycan synthesis that makes the cell wall much thicker preventing effective penicillin entry.[47] Resistance in Gram-negative bacteria is due to mutational variations in the structure and number of porins.[52] In bacteria like Pseudomonas aeruginosa, there is reduced number of porins; whereas in bacteria like Enterobacter species, Escherichia coli and Klebsiella pneumoniae, there are modified porins such as non-specific porins (such as OmpC and OmpF groups) that cannot transport penicillin.[65]
Resistance due to PBP alterations is highly varied. A common case is found in Streptococcus pneumoniae where there is mutation in the gene for PBP, and the mutant PBPs have decreased binding affinity for penicillins.[66] There are six mutant PBPs in S. pneumoniae, of which PBP1a, PBP2b, PBP2x and sometimes PBP2a are responsible for reduced binding affinity.[67] S. aureus can activate a hidden gene that produces a different PBP, PBD2, which has low binding affinity for penicillins.[68] There is a different strain of S. aureus named methicillin-resistant S. aureus (MRSA) which is resistant not only to penicillin and other β-lactams, but also to most antibiotics. The bacterial strain developed after introduction of methicillin in 1959.[44] In MRSA, mutations in the genes (mec system) for PBP produce a variant protein called PBP2a (also termed PBP2'),[69] while making four normal PBPs. PBP2a has poor binding affinity for penicillin and also lacks glycosyltransferase activity required for complete peptidoglycan synthesis (which is carried out by the four normal PBPs).[67] In Helicobacter cinaedi, there are multiple mutations in different genes that make PBP variants.[70]
Enzymatic destruction by β-lactamases is the most important mechanism of penicillin resistance,[71] and is described as "the greatest threat to the usage [of penicillins]".[72] It was the first discovered mechanism of penicillin resistance. During the experiments when purification and biological activity tests of penicillin were performed in 1940, it was found that E. coli was unsusceptible.[73] The reason was discovered as production of an enzyme penicillinase (hence, the first β-lactamase known) in E. coli that easily degraded penicillin.[63] There are over 2,000 types of β-lactamases each of which has unique amino acid sequence, and thus, enzymatic activity.[72] All of them are able to hydrolyse β-lactam rings but their exact target sites are different.[74] They are secreted on the bacterial surface in large quantities in Gram-positive bacteria but less so in Gram-negative species. Therefore, in a mixed bacterial infection, the Gram-positive bacteria can protect the otherwise penicillin-susceptible Gram-negative cells.[50]
There are unusual mechanisms in P. aeruginosa, in which there can be biofilm-mediated resistance and formation of multidrug-tolerant persister cells.[75]
A technician preparing penicillin in 1943
Penicillin was being mass-produced in 1944.
World War II poster extolling use of penicillin
