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In terms of physiological pH, polymyxines are basically cationic, superficially active compounds, and they exhibit their bactericidal effect by reacting with phospholipid components of the cytoplasmatic membrane of susceptible bacteria, thus destroying the osmotic integrity of their cellular membrane, facilitating the release of many components, including potassium, from the cytoplasm into the surrounding medium. The detailed biochemical mechanism of their action is not yet known. Practically all polymyxines are active exclusively against aerobic Gram-negative microorganisms. In particular, they are active against P. aeruginosa, E. coli, K. pneumoniae, Enterobacter, Salmonella, Shigella, Haemophilus, and others. They do not affect coccal aerobic (staphylo-, strepto-, pneumo-, gono-, and meningococci) and anaerobic microorganisms, as well as most strains of Proteus bacterias causing tuberculosis and diphtheria.

These drugs are used for gastrointestinal diseases (colitis, enterocolitis, severe and chronic dysentery, sepsis, meningitis, pneumonia, infections of the urinary tract, and others caused by P. aeruginosa), when other antibiotics are ineffective. They are effectively used in the form of ointments for treating a few forms of eczema, boils, hidradenitis, and other skin diseases.

Parenteral use of polymyxines is limited due to their neuro- and nephrotoxic effects. They are the drugs of choice for some specific infections. However, they are used for serious, life-threatening infections such as bacteremia, which are caused by some strains of P. aeruginosa, as was already mentioned.

Bacitracin: Bacitracins are polypeptide antibiotics that are isolated from a culture fluid of B. licheniformis. Ten individual bacitracins have been isolated: bacitracins A, Ap B, C, D, E, Fj, F2, F3, and G. However, the drug itself, named bacitracin, that is used in medicine is a mixture of polypeptide antibiotics. All bacitracins are polypeptides that contain a thia-zole ring. However, bacitracin A, N-[[2-(1-amino-2-methylbutyl)-4,5-dihydro-4-thiazolyl] carbonyl]bacitracin F (32.7.11), makes up the main part of the isolated fractions

-CO-Leu-Glu—Ileu—Lys-Orn—Ileu NH-CO \ HOOC-Asp-Asp-His—Phe

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