Antibacterial activity of mupirocin (pseudomonic acid), a new antibiotic for topical use

Mupirocin (pseudomonic acid A), an antibiotic produced by Pseudomonas fluorescens, showed a high level of activity against staphylococci and streptococci and against certain gram-negative bacteria, including Haemophilus influenzae and Neisseria gonorrhoeae, but was much less active against most gram-negative bacilli an anaerobes. Nearly all clinical isolates of Staphylococcus aureus and Staphylococcus epidermidis, including multiply resistant strains, were susceptible (mupirocin MIC, less than or equal to 0.5 microgram/ml). There was no cross-resistance between mupirocin and clinically available antibiotics, and the selection of resistant variants in vitro occurred at a low frequency. Mupirocin was highly bound (95% bound) to the protein of human serum, and activity was reduced 10- to 20-fold in the presence of human serum. The activity of mupirocin was not greatly influenced by inoculum size but was significantly enhanced in acid medium. In tests of bactericidal activity, MBCs were 8- to 32-fold higher than MICs and the antibiotic demonstrated a slow bactericidal action in time-kill tests, resulting in 90 to 99% killing after 24 h at 37 degrees C.