Therapy of Rhodococcus equi disseminated infections in nude mice

Rhodococcus equi is a facultative, intracellular, gram-positive coccobacillus increasingly reported as an opportunistic pathogen in human immunodeficiency virus-positive patients. However, the optimal drug regimen for treating R. equi pulmonary or systemic infections is not yet known. Therefore, a model of intravenously infected nude mice with disseminated infection was created to study the efficacy of antibiotics alone or in combination as determined by the reduction of bacterial CFU per gram in the lungs and spleen after 4 and 11 days of treatment. The studied antibiotics possessing low MICs against R. equi strains were amikacin, ciprofloxacin, erythromycin, imipenem, minocycline, rifampin, and vancomycin. Vancomycin, imipenem, and rifampin were the most effective agents in monotherapy. On the other hand, amikacin, ciprofloxacin, erythromycin, and minocycline alone were not active in this model. The most active drug combinations were those including vancomycin. No antibiotic-resistant mutants were selected in vivo with treatment involving any drugs used alone or in combination. Although the treatment recommended until now for R. equi infections is rifampin plus erythromycin, this study suggests that antibiotic combinations which include vancomycin may be the most effective in vivo.