Pseudomonas fluorescens is a common nonpathogenic Gram-negative bacteria found in poop, soil, and water that gets its name from the fact that it secretes a fluorescent siderophore (iron-binding compound) called pyoverdin (fluorescein). Cultures of it glow under black light, making it the raver's bacterium of choice. P. fluorescens also produces an epoxide antibiotic, originally named pseudomonic acid A but commonly called mupirocin (trade names Bactroban or Centany), that is good at inhibiting the growth of and outright killing (at higher concentrations) Gram-positive bacteria, particularly staphylococci and streptococci.

Mupirocin is typically formulated as a topical cream, ointment, or spray used to treat infections of the skin (e.g. boils, impetigo, the likely consequence of not bothering to clean a dirty scrape on your knee, etc.). It can only be used topically (i.e. applied to the surface of the body) because it is rapidly and extensively broken down once it gets into the blood and interstitial fluid. Notably, it is capable of killing methicillin-resistant Staphylococcus aureus (MRSA), and has been used to treat nasal carriers of this bacterium in addition to those infected with it. Although resistance to mupirocin is more or less inevitable, limiting its use could prolong its usefulness until resistant bacteria become widespread.

The ability of mupirocin to extirpate bacteria that are resistant to other antibiotics is due to its novel mechanism of action, which involves the inhibition of the enzyme isoleucyl-tRNA synthetase, which is responsible for attaching the amino acid isoleucine to its corresponding transfer RNA . Bacteria need this amino acid to make proteins. Most antibiotics mess with either the bacterial cell wall or the synthesis of bacterial DNA or proteins, so hitting the bacteria at the amino acid level is something different.

- Parenti MA, Hatfield SM, Leyden JJ. Mupirocin: a topical antibiotic with a unique structure and mechanism of action. Clin Pharm. 1987 Oct;6(10):761-70. Review.

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