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 </=1 micro g/ml. In liquid cultures 1 micro g of the drug/ml was bacteriostatic and 10 micro g/ml was bactericidal. On solid media at 10 micro g of azole/ml no resistant mutants could be isolated. An MIC of 1 to 15 micro g/ml was observed with ketoconazole, whereas none of these organisms was inhibited by the triazoles fluconazole and voriconazole (100 micro g/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.