Mupirocin

Mupirocin is a topical antibiotic whose active constituent is pseudomonic acid, a substance first isolated in 1971 from the bacterium Pseudomonas fluorescens. The commercial product is a mixture in which pseudomonic acid A makes up the large majority, alongside smaller amounts of related forms ^[1].

The drug entered clinical use in the mid-1980s and is now registered in a great many countries, sold under brand names such as Bactroban ^[2]. It is a prescription medicine, appears on the World Health Organization's Model List of Essential Medicines, and is available generically. Mupirocin is used only on the surface of the body, being supplied as a skin cream or ointment and as a preparation for application inside the nostrils; it is not given by mouth or injection because it is broken down and bound in the body too rapidly to work systemically ^[3].

Its main uses fall into two groups. As a treatment, it is applied to superficial bacterial skin infections such as impetigo, small boils and infected minor wounds, chiefly those caused by Staphylococcus aureus and Streptococcus pyogenes ^[3]. As a preventive measure, it is a standard means of clearing S. aureus, including methicillin-resistant strains (MRSA), from the nose, where the bacteria are commonly carried; nasal decolonization with mupirocin is used to reduce infections, for example before surgery ^[3].

Because repeated and widespread use encourages bacteria to become resistant, treatment courses are generally kept short. Two forms of resistance are recognized: a low-level resistance arising from a mutation in the bacterial target enzyme, and a high-level resistance carried on mobile genetic elements by the mupA gene, the latter being the more troublesome because it can defeat treatment; overuse, including unrestricted access and prolonged application to wounds, has been linked to rising resistance ^[2][3].

Mupirocin is prescription only and for topical use. It is well tolerated, with side effects mostly limited to the site of application, such as burning, stinging, itching or dryness; the nasal ointment can cause headache, a runny nose or a bad taste. It should not be used in or near the eyes. The ointment base contains polyethylene glycol, so it is not meant for large open wounds or burns in people with kidney problems, since the vehicle could be absorbed. Overusing it can drive mupirocin resistance in staph, so courses are kept short, usually 5 to 10 days.

There is very little to report here, and that is the honest answer rather than a gap in the record. Less than about one percent of a topical dose crosses intact skin, and what does get through is hydrolyzed quickly to monic acid, which has no antibacterial activity and is cleared by the kidney. No systemic drug interaction has been established, and none appears in its approved labeling.

The interactions that do exist are local. Mupirocin applied to the same area as another topical preparation can be diluted below its effective concentration, which matters because its activity is concentration dependent. Its ointment base is polyethylene glycol, which is absorbed through broken skin and can accumulate when large areas of damaged skin are treated in someone with impaired kidney function.

The practical constraint on mupirocin is resistance rather than interaction; prolonged or repeated courses select for mupirocin resistant Staphylococcus aureus, and that is what limits how often it can usefully be used.

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Mupirocin, whose active principle is pseudomonic acid, was discovered by scientists at Beecham (a forerunner of GlaxoSmithKline) who isolated it from cultures of the soil bacterium Pseudomonas fluorescens, in which it functions as a natural antibacterial agent. The compound was first described in the early 1970s, and after development as a topical therapeutic it was introduced to clinical use in the mid-1980s under the brand name Bactroban.

Its mechanism proved unusual and elegant; it inhibits bacterial isoleucyl-tRNA synthetase, a target not shared by any other class of clinically used antibiotic, which explains its lack of cross-resistance. Formulated as a cream, ointment and nasal preparation, it became a standard treatment for impetigo and other superficial skin infections and, importantly, for clearing Staphylococcus aureus, including MRSA, from the nostrils of carriers. Mupirocin now appears on the World Health Organization's list of essential medicines and remains a cornerstone of hospital decolonization protocols worldwide.

Mupirocin is widely respected as a highly effective, well-tolerated topical antibiotic that punches above its weight in infection control. Its standout role is nasal decolonization; applied inside the nose, often alongside chlorhexidine washes, it reliably reduces Staphylococcus aureus carriage and has been shown to lower rates of surgical-site and other staphylococcal infections, which is why it features prominently in perioperative and MRSA-prevention bundles.

Because its enzyme target is unique among antibiotics, it sidesteps cross-resistance with other drug classes, and its topical use keeps systemic exposure and side effects minimal. The honest caveat is that resistance can and does emerge, especially with heavy or prolonged use, so stewardship matters and it is best deployed for defined courses rather than indefinitely. Within those sensible limits, it remains a trusted, essential-medicine-grade tool against some of the most troublesome skin and nasal pathogens.