Pathogenic Nocardia, Rhodococcus, and related organisms are highly susceptible to imidazole antifungals

Rhodococcus equi and species of Nocardia and Gordonia may be human opportunistic pathogens. We find that these, as well as several isolates from closely related genera, are highly susceptible to the imidazoles bifonazole, clotrimazole, econazole, and miconazole, whose MICs are less than or equal to1 mug/ml. In liquid cultures 1 mug of the drug/ml was bacteriostatic and 10 mug/ml was bactericidal. On solid media at 10 mug of azole/ml no resistant mutants could be isolated. An MIC of 1 to 15 mug/ml was observed with ketoconazole, whereas none of these organisms was inhibited by the triazoles fluconazole and voriconazole (100 mug/ml). Imidazoles may offer the prospect of treatment of nocardioform mycetomas and may provide the basis for the development of additional antimicrobial agents to combat these pathogens.