Differentiation between the genera Mycobacterium, Rhodococcus and Nocardia by susceptibility to 5-fluorouracil

A test for susceptibility to 5-fluorouracil was useful for differentiating between species of rapidly growing mycobacteria, and for differentiating the genus Rhodococcus from the genus Nocardia. The majority of rhodococci tested were susceptible to 5-fluorouracil (20 micrograms ml-1), whereas the majority of nocardiae tested were resistant to it. Strains of Nocardia asteroides sensu stricto could be divided into two subgroups by their reaction to 5-fluorouracil.