Polymyxin B is a “last-line” antibiotic primarily used to treat hospital-acquired gram-negative infections that are resistant to essentially all other conventional antibiotics. As an intensivist, I’ve had to use polymyxin B for two very resistant nosocomial bugs: Acinetobacter and Stenotrophomonas. It can also be used for resistant Enterobacteriaceae, Pseudomonas, Salmonella, Shigella, etc.
Polymyxin B competes with divalent cations like magnesium and calcium for phosphate groups within the lipopolysaccharides (LPS) of the bacterial outer cell membrane. This disrupts the membrane allowing intracellular components to leak out, leading to bacterial death. By this mechanism, gram-positive bacteria which lack an LPS outer membrane are inherently resistant to the effects of polymyxin B in addition to Proteus spp, Providencia spp, S. marcescens, M. catarrhalis, M. morganii, and Burkholderia cepacia.
Compared to polymyxin E (colistin), which is administered as a prodrug and metabolized to an active agent, polymyxin B is active from the get-go and not affected by renal function due to extrarenal clearance. On the other hand, colistin is preferred for urinary tract infections as the drug has higher urinary concentrations. Polymyxin routes also include ophthalmic, topical, inhaled (typically colistin), and intrathecal. In any case, monitor for hypersensitivity, neurotoxicity, and nephrotoxicity!
Unfortunately, even last resort agents like polymyxin are plagued with increasing antibiotic resistance. We need to foster better antibiotic stewardship by using the narrowest spectrum agent for the shortest duration whenever possible!
Drop me a comment with your experience using polymyxin B!