Antibiotics for human use have been developed from many types of natural organic compounds that have shown antibacterial activity. They include aminoglycosides, cephalosporins, glycopeptides, macrolides, nitrofurans, oxazolidinone, penicillins, polypeptides, quinolines, sulfonamides and tetracyclines. Stop me if I’m boring you.
No antibiotic has yet been developed from a protein, but the possibility has arisen with the recent detection of a novel antibacterial in a fungus that is widely studied in laboratories.
Coprinopsis cinerea, known in English as grey shag, is a small ink cap mushroom found across much of the world. It is edible, but may be seen as unappetising because it is most often found growing on heaps of dung or rotten straw or vegetable refuse. It must be used promptly because soon after it has been collected the caps start to deliquesce into black ink.
Because C cinerea is easy to cultivate in the laboratory, it has emerged as the model organism for scientific studies of large multicellular fungi, and particularly for genome sequencing studies. Its genome sequence was published in 2010. Unlike the vast majority of mushroom species, C cinerea will thrive on a simple laboratory medium. Research is also aided by the fact that in controlled laboratory conditions it can complete its life cycle in as little as two weeks.
Researchers in Switzerland have recently found that C cinerea produces a highly stable protein that has a broad spectrum of antibacterial activity. They have shown that it works by binding to a lipid that is an essential building block for bacterial cell walls. The bacteria die because they cannot build new cell walls.
The protein, which has been given the name copsin, belongs to a class of small proteins known as defensins. These are produced by many organisms — including humans — to combat micro-organisms that cause disease.
Copsin is of interest as a potential antibiotic for two main reasons. The first reason is that bacteria do not seem to be able to develop resistance to defensins. The second reason is that copsin is exceptionally stable. Unlike most proteins, it does not degrade when heated to 100C for several hours or when subjected to protein-degrading enzymes. The researchers believe that these properties are due to an extremely compact three-dimensional structure.
Whether defensins such as copsin will eventually find a clinical role as antibiotics remains a matter for conjecture, but, with the ever-increasing problem of bacterial resistance to antibiotics, no doubt research into antibacterial proteins will be accelerated.
Copsin has also generated interest because of a possible use in the food industry. Since it has been shown to kill many pathogens that can cause severe food poisoning, it could perhaps be used to protect non-heat treated foodstuffs, such as raw milk cheeses and dried meats.