In-vitro activity of mupirocin ('pseudomonic acid') against clinical isolates of Staphylococcus aureus

The in-vitro activity of mupirocin ('pseudomonic acid') was determined against clinical isolates of Staphylococcus aureus with various antibiotic resistance patterns including resistance to methicillin. All 750 isolates were inhibited by 2 mg/l of mupirocin and the MICs for 200 isolates tested on DST agar at pH 7.2 with an inoculum of 10(4) cfu ranged from 0.015 to 0.06 mg/l. Inoculum size, pH, test agar and the method of dissolving mupirocin all influenced the MICs obtained. The minimum concentrations of mupirocin to obtain a 99, and 99.9 and 99.99% kill after 24 h in Iso-Sensitest broth at pH 7.2 were 2-4, 16 and 32 mg/l, respectively. Sterile cultures were obtained when each of five strains were exposed to 1.0 mg/l or more of mupirocin for 120 h. Resistant variants with MICs of up to 4 mg/l were detected at a frequency of approximately 1 X 10(-9) and produced chrome-yellow colonies. Each of five strains could be trained to grow in the presence of 40 mg/l of mupirocin. We conclude that although apparently bacteriostatic, mupirocin has slow bactericidal activity against Staph. aureus and that 2% mupirocin may well be effective for topical treatment of skin infections caused by Staph. aureus and contribute to the control of multiply-resistant strains during hospital outbreaks.