Polymyxins are antibiotics. Polymyxins B and E (also referred to as colistin) are utilized in the treatment of Gram-negative bacterial infections. They work mostly by ending the bacterial cell wall. They are a part of a broader class of molecules called nonribosomal peptides. They are produced in nature by Gram-positive bacteria like Paenibacillus polymyxa. Polymyxin antibiotics are relatively neurotoxic and nephrotoxic, in order that they are usually used only as a final resort if modern antibiotics are ineffective or are contraindicated. Typical uses are for infections caused by strains of multiple drug-resistant Pseudomonas aeruginosa or carbapenemase-producing Enterobacteriaceae. Polymyxins have less effect on Gram-positive organisms and are sometimes combined with other agents (as with trimethoprim/polymyxin) to broaden the effective spectrum. Polymyxins B is not absorbed from the gastrointestinal tract, so they are only administered orally if the goal is to disinfect the GI tract. Another route of administration is chosen for systemic treatment, e.g., parenteral (often intravenously) or by inhalation. They are also used externally as a cream or drops to treat otitis (swimmers ear), and as a component of triple antibiotic ointment to treat and stop skin infections.