Tyrothricin is an

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

mixture which was isolated from ''

Bacillus brevis ''Brevibacillus brevis'' (formerly known as ''Bacillus brevis'') is a Gram-positive, aerobic, motile, spore-forming, rod-shaped bacterium commonly found in soil, air, water, and decaying matter. It is rarely associated with infectious diseases. ...

'' by Rene Dubos in the late 1930s. It was later shown by Dubos and Rollin Hotchkiss to be a mixture of two different antibiotics:

gramicidin Gramicidin, also called gramicidin D, is a mix of ionophoric antibiotics, gramicidin A, B and C, which make up about 80%, 5%, and 15% of the mix, respectively. Each has 2 isoforms, so the mix has 6 different types of gramicidin molecules. They c ...

and

tyrocidine Tyrocidine is a mixture of cyclic decapeptides produced by the bacteria ''Bacillus brevis'' found in soil. It can be composed of 4 different amino acid sequences, giving tyrocidine A–D (See figure 1). Tyrocidine is the major constituent of t ...

. Both gramicidin and tyrocidine are short

polypeptides Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A p ...

which disrupt the

cell membranes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

of some, primarily

Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bacte ...

, bacteria. Tyrothricin and its component antibiotics are too toxic to be taken internally but are sometimes used as topical antibiotics. Tyrothricin, and its component antibiotics, belongs to the pharmacologically related group of polypeptide antibiotic compounds including

colistin Colistin, also known as polymyxin E, is an antibiotic medication used as a last-resort treatment for multidrug-resistant Gram-negative infections including pneumonia. These may involve bacteria such as ''Pseudomonas aeruginosa'', '' Klebsiella ...

polymyxin B Polymyxin B, sold under the brand name Poly-Rx among others, is an antibiotic used to treat meningitis, pneumonia, sepsis, and urinary tract infections. While it is useful for many Gram negative infections, it is not useful for Gram positive infe ...

, and

bacitracin Bacitracin is a polypeptide antibiotic. It is a mixture of related cyclic peptides produced by ''Bacillus licheniformis'' bacteria, that was first isolated from the variety "Tracy I" ( ATCC 10716) in 1945. These peptides disrupt Gram-positive bac ...

. There is no cross-resistance to these three agents.

References

{{Antibiotics and chemotherapeutics for dermatological use Polypeptide antibiotics