Colistin sulphate is an antibiotic from the polymyxin group, first discovered in the late 1940s. It is naturally produced by Bacillus polymyxa. Though initially used in the 1950s, its use declined due to side effect concerns. Its recent re-emergence highlights its significance in treating difficult bacterial infections.
How Colistin Works
Colistin targets the outer membrane of Gram-negative bacteria. Its positively charged molecules are attracted to the negatively charged lipopolysaccharide (LPS) components on the bacterial outer membrane. This interaction displaces calcium and magnesium ions that stabilize the membrane structure.
The disruption of the outer membrane increases its permeability, allowing colistin to interact with the underlying cytoplasmic membrane. This leads to a rapid leakage of intracellular contents, such as ions, ATP, and macromolecules, from the bacterial cell. Damage to both membranes ultimately results in bacterial cell death. This mechanism makes colistin effective against many bacteria resistant to other antibiotics.
When It Is Prescribed
Colistin is often prescribed as a “last resort” antibiotic for severe infections caused by multidrug-resistant (MDR) Gram-negative bacteria. These include pathogens like Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae, common in hospitals, causing life-threatening conditions such as pneumonia, bloodstream infections, and meningitis. This “last resort” status reflects the limited effective treatment options for these highly resistant strains.
Global antibiotic resistance has renewed focus on colistin, despite its known side effects. When standard antibiotics fail, colistin is one of the few remaining choices for challenging infections. Its use is carefully considered, often with infectious disease specialists, to ensure it is the most appropriate treatment based on infection severity and bacterial resistance.
Administering Colistin
Colistin is available in two primary forms: colistin sulphate and colistimethate sodium (CMS). Colistin sulphate is used for topical applications due to its direct activity, while CMS is preferred for intravenous (IV) administration. CMS is an inactive prodrug, converted in the body to active colistin, primarily via hydrolysis. This conversion allows for controlled release and reduces immediate toxicity compared to direct IV colistin sulphate.
Intravenous CMS is used for systemic infections. For lung infections, especially in cystic fibrosis patients with chronic Pseudomonas aeruginosa, colistin can be nebulized. Inhaled colistin delivers the antibiotic directly to the lungs, achieving high local concentrations while minimizing systemic exposure and side effects. Topical colistin sulphate treats localized infections like those affecting the skin, eyes, or ears, where direct application is effective without significant systemic absorption.
Understanding Potential Side Effects
Colistin treatment can lead to potential adverse effects, with nephrotoxicity (kidney damage) being the most common concern. This toxicity can manifest as decreased kidney function, often reversible upon drug discontinuation or dosage adjustment. Healthcare providers monitor kidney function through regular blood tests, like creatinine and blood urea nitrogen, to detect and manage damage early.
Neurotoxicity, affecting the nervous system, is another potential side effect, though less common than nephrotoxicity. Symptoms include dizziness, tingling around the mouth or extremities, blurred vision, and muscle weakness. These neurological effects are typically dose-dependent and often resolve after stopping the medication. Close observation by medical staff helps promptly identify and address emerging side effects, ensuring patient safety during colistin therapy.