polymyxin n : any of several toxic antibiotics obtained from a particular soil bacterium

English

Noun

1. Any of several toxic antibiotics, derived from the soil bacterium Bacillus polymyxa, used to treat infections by gram-negative bacteria

Polymyxins are antibiotics, with a general structure of a cyclic peptide with a long hydrophobic tail. They disrupt the structure of the bacterial cell membrane by interacting with its phospholipids. They are produced by the Gram-positive bacterium Bacillus polymyxa and are selectively toxic for Gram-negative bacteria due to their specificity for lipopolysaccharide in the outer membrane of the cell wall of these organisms.

Polymyxins B and E (also known as colistin) are used in the treatment of infections with gram-negative bacteria.

Mode of action

After binding to lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria, polymyxins disrupt both the outer and inner membrane. The hydrophobic tail is important in causing membrane damage, suggesting a detergent-like mode of action.

Removal of the hydrophobic tail of polymyxin B yields polymyxin nonapeptide, which still binds to LPS but does no longer kill the bacterial cell. However, it still detectably increases the permeability of the bacterial cell wall to other antibiotics, indicating that it still causes some degree of membrane disorganization.

Gram-negative bacteria can develop resistance to polymyxins through various modifications of the LPS structure that inhibit the binding of polymyxins to LPS.

Clinical use

Polymyxin antibiotics are considered relatively neurotoxic and nephrotoxic and, though effective, are therefore used only if less toxic antibiotics cannot. Typical use cases are infections with strains of Pseudomonas aeruginosa or Enterobacteriaceae species that are highly resistant to other types of antibiotics such as cephalosporins.

Polymyxins are not absorbed from the gastrointestinal tract and therefore have to be applied intravenously.

Use in biomedical research

Polymyxins are used to neutralize or absorb LPS contaminating samples that are intended for use in e.g. immunological experiments. Minimization of LPS contamination can be important because LPS can evoke strong reactions from immune cells and therefore distort experimental results.

See also

• Polysporin

References

External links