Mupirocin (pseudomonic acid) is a natural substance produced by Pseudomonas fluorescens. It is rapidly inactivated after absorption,and systemic levels are undetectable. It is available as an ointment for topical application.
Mupirocin is active against gram-positive cocci, including methicillin-susceptible and methicillin-resistant strains of Staphylococcus aureus. Mupirocin inhibits staphylococcal isoleucyltRNA synthetase. Low-level resistance, defined as a minimum inhibitory concentration (MIC) of up to 100 mcg/mL, is due to point mutation in the gene of the target enzyme. Low-level resis-tance has been observed after prolonged use. However, local con-centrations achieved with topical application are well above this MIC, and this level of resistance does not lead to clinical failure. High-level resistance, with MICs exceeding 1000 mcg/mL, is due to the presence of a second isoleucyl tRNA synthetase gene, which is plasmid-encoded. High-level resistance results in complete loss of activity. Strains with high-level resistance have caused hospital-associated (nosocomial) outbreaks of staphylococcal infection and colonization. Although higher rates of resistance are encountered with intensive use of mupirocin, more than 95% of staphylococcal isolates are still susceptible.
Mupirocin is indicated for topical treatment of minor skin infections, such as impetigo . Topical application over large infected areas, such as decubitus ulcers or open surgical wounds, is an important factor leading to emergence of mupiro-cin-resistant strains and is not recommended. Mupirocin effec-tively eliminates S aureus nasal carriage by patients or health care workers, but results are mixed with respect to its ability to prevent subsequent staphylococcal infection.