Polymyxin belongs to the group of polypeptide antibiotics and is produced by Bacillus polymyxa. Polymyxin B sulfate has a bactericidal effect against almost all gram-negative bacteria, except for the Proteus group. Polymyxins increase the permeability of the bacterial cell membrane. All gram-positive bacteria, fungi and gram-negative cocci, N. gonorrhoeae and N. meningitidis are resistant to polymyxin B.
Sensitivity test determination on the plate: If the Kirby-Bauer disk method is used, a disk with polymyxin B, 300 IU, should give a zone of more than 11 mm when determining a bacterial strain that is resistant to polymyxin B.