Polymyxin B is an antibiotic primarily used for resistant gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

 infections. It is derived from the bacterium Bacillus polymyxa. Polymyxin B is a mixture of two closely related compounds, polymyxin B1 and polymyxin B2. It has a bactericidal action against almost all gram-negative bacilli except the Proteus

Proteus

In Greek mythology, Proteus is an early sea-god, one of several deities whom Homer calls the "Old Man of the Sea", whose name suggests the "first" , as protogonos is the "primordial" or the "firstborn". He became the son of Poseidon in the Olympian theogony In Greek mythology, Proteus (Πρωτεύς)...

 group. Polymyxins bind to the cell membrane and alter its structure, making it more permeable. The resulting water uptake leads to cell death. They are cationic, basic proteins that act like detergents (surfactant

Surfactant

Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...

s). Side-effects include neurotoxicity and acute renal tubular necrosis. It is commonly used in the topical first-aid preparation Neosporin

Neosporin

Neosporin is the product branding & formulas now owned by Johnson & Johnson of an antibiotic over-the-counter topical created under Warner-Lambert Consumer Healthcare, now a part of Pfizer...

.

1. Family of polypeptides with attached fatty acid; cationic detergent
   Detergent
   A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
   
    at physiological pH, both hydrophilic and hydrophobic properties
2. Bactericidal for gram-negative
   Gram-negative
   Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
   
   ; little to no effect on gram-positive
   Gram-positive
   Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
   
   , since cell wall is too thick to permit access to membrane

Mechanism of action

1. Alters bacterial outer membrane permeability by binding to a negatively charged site in the lipopolysaccharide layer, which has an electrostatic attraction for the positively charged amino groups in the cyclic peptide portion (this site normally is a binding site for calcium and magnesium counter ions); the result is a destabilized outer membrane
2. Fatty acid portion dissolves in hydrophobic region of cytoplasmic membrane and disrupts membrane integrity
3. Leakage of cellular molecules, inhibition of cellular respiration
4. Binds and inactivates endotoxin
5. Relative absence of selective toxicity: nonspecific for cell membranes of any type, highly toxic

Research application

In addition to its antibiotic function, polymyxin B has been used to clear endotoxin contamination in reagents.