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Colistin is the most prominent member of the polymyxin family of antibiotics that have been used since about 1950 to treat pneumonia, ear infections and other bacterial invasions. Officially named nebulized polymyxin E, the drug lost popularity in a few decades, in favor of other antibacterial agents like aminoglycosides that are less toxic to the kidneys and nervous system. Around the turn of the 21st century, drug-resistant strains of gram-negative bacteria led to a resurgence in the use of colistin, which the modern bacteria has been hard-pressed to conquer.
Originally isolated in Japan, polymyxin drugs are derived from the bacteria Bacillus polymyxa. They are considered gram-positive organisms that, when confronted by gram-negative bacterias like the cocci and bacilli families, latch onto the phospholipid shells of those cells and commence to destroy them one-by-one, inside and out. Though polymyxin E is commonly included in follow-up antibiotic treatments, so too is its cousin, polymyxin B.
Available by injection, inhaler, and oral suspension or pill, colistin is often turned to when other antibiotics have been ineffective at battling an infection from bacteria like Haemophilus influenzae, Pseudomonas aeruginosa, E. coli, Acinetobacter, Salmonella and Shigella. Some strains of these bacterias have proven drug-resistant to the most common antibiotics in recent decades, but not colistin. Scientists also have verified the drug's effectiveness in treating pneumonia in patients with cystic fibrosis — a presumably genetic condition marked by copious mucus in the lungs and digestive tract.
Two delivery systems for colistin are typically available — colistimethate sodium and colistin sulfate. The latter compound is administered primarily in response to digestive disorders. Colistimethate sodium is the workhorse assigned to kill bacteria in those with cystic fibrosis and others with bacterial infections not responding to ordinary antibiotic cocktails.
Colistin is usually reserved as a last resort due to it being not only neurotoxic to the nervous system, but also nephrotoxic to the kidneys. These characteristics can lead to symptoms ranging from a loss of sensation, headache and wooziness to a rash, respiratory discomfort and vertigo — particularly if the patient is taking a high or prolonged dosage. Due to these potential difficulties, other antibiotics are more likely to be the first line of defense. Depending on the type and location of the infection, doctors may prescribe an aminoglycoside, penicillin, tetracycline, fluoroquinolone or macrolide antibiotic. Only after determining that another type of antibiotic is it likely that a doctor will recommend a polymyxin.