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

Abstract

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 10⁴ 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 4mg/l were detected at a frequency of approximately 1 × l0⁻⁹ 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.