Anti-staphylococcal activity of indolmycin, a potential topical agent for control of staphylococcal infections

Objectives: We sought to investigate the anti-staphylococcal activity of indolmycin, with particular emphasis on comparing its activity with fusidic acid and mupirocin. Methods: Established procedures were used to examine the activity of indolmycin against a range of clinical isolates, including strains resistant to fusidic acid and mupirocin. Indolmycin-resistant mutants were recovered and characterized phenotypically and genotypically. Results: Indolmycin was bacteriostatic and demonstrated good activity against MSSA (methicillin-susceptible Staphylococcus aureus), MRSA (methicillin-resistant S. aureus) and VISA (vancomycin-intermediate S. aureus), including strains resistant to mupirocin or fusidic acid. Spontaneous indolmycin-resistant mutants occurred at a lower frequency than those selected by mupirocin or fusidic acid and exhibited no cross-resistance with the comparative drugs. High-level resistance (indolmycin MIC 128 mg/L) that was associated with an H43N mutation in tryptophanyl-tRNA synthetase (TrpS), the target enzyme of indolmycin, resulted in loss of bacterial fitness. However, the locus responsible for low-level indolmycin resistance (indolmycin MICs 8-32 mg/L) was not identified. Conclusions: Indolmycin is a potent anti-staphylococcal agent, which exhibits activity against mupirocin- and fusidic acid-resistant strains. Indolmycin might be a candidate for development as a topical agent in the treatment of staphylococcal infections and nasal carriage of MRSA.