Opacification of middlebrook agar as an aid in distinguishing Nocardia farcinica within the Nocardia asteroides complex

Abstract

Among 58 aerobic actinomycetes isolated from different sources and geographical locations, none of 23 Nocardia asteroides isolates, all 18 N. farcinica isolates. 1 of 5 N. otitidiscaviarum isolates, and 1 of 4 Rhodococcus species isolates opacified Middlebrook 7H10 medium. Within the N. asteroides complex, this characteristic, together with growth at 45°C and resistance to each of erythromycin, cefotaxime, and tobramycin, provides a simple means of distinguishing N. farcinica from N. asteroides.