Mupirocin, sold under the brand name Bactroban among others, is a topical antibiotic useful against superficial skin infections such as impetigo or folliculitis. It may also be used to get rid of methicillin-resistant S. aureus (MRSA) when present in the nose without symptoms. Due to concerns of developing resistance, use for greater than ten days is not recommended. It is used as a cream or ointment applied to the skin. Common side effects include itchiness and rash at the site of application, headache, and nausea. Long term use may result in increased growth of fungi. Use during pregnancy and breastfeeding appears to be safe. Mupirocin is in the carboxylic acid class of medications. It works by blocking a bacteria's ability to make protein, which usually results in bacterial death. Mupirocin was initially isolated in 1971 from Pseudomonas fluorescens. It is on the World Health Organization's List of Essential Medicines. In 2020, it was the 203rd most commonly prescribed medication in the United States, with more than 2million prescriptions. Medical uses Mupirocin is used as a topical treatment for bacterial skin infections (for example, boils, impetigo, or open wounds), which are typically due to infection by Staphylococcus aureus or Streptococcus pyogenes. It is also useful in the treatment of superficial methicillin-resistant Staphylococcus aureus (MRSA) infections. Mupirocin is inactive for most anaerobic bacteria, mycobacteria, mycoplasma, chlamydia, yeast, and fungi. Intranasal mupirocin before surgery is effective for prevention of post-operative wound infection with Staphylcoccus aureus and preventative intranasal or catheter-site treatment is effective for reducing the risk of catheter site infection in persons treated with chronic peritoneal dialysis. Resistance Shortly after the clinical use of mupirocin began, strains of Staphylococcus aureus that were resistant to mupirocin emerged, with nares clearance rates of less than 30% success. Two distinct populations of mupirocin-resistant S. aureus were isolated. One strain possessed low-level resistance (MuL: MIC = 8–256 mg/L), and another possessed high-level resistance (MuH: MIC > 256 mg/L). Resistance in the MuL strains is probably due to mutations in the organism's wild-type isoleucyl-tRNA synthetase. In E. coli IleRS, a single amino acid mutation was shown to alter mupirocin resistance. MuH is linked to the acquisition of a separate Ile synthetase gene, MupA. Mupirocin is not a viable antibiotic against MuH strains. Other antibiotic agents, such as azelaic acid, nitrofurazone, silver sulfadiazine, and ramoplanin have been shown to be effective against MuH strains. Most strains of Cutibacterium