Characterization of Nocardia, Rhodococcus and Gordona species by in vitro susceptibility testing.

Representative strains of Gordona, Nocardia and Rhodococcus were tested against 26 antimicrobial agents using the disc diffusion method. A distinct susceptibility profile was noted for most species. Nocardia asteroides, N. brasiliensis, N. otitidiscaviarum, N. transvalensis and N. vaccinii were rarely susceptible to the antibiotics tested; N. brevicatena and N. farcinica varied in their susceptibility depending on the strain tested. Only the antibiotic amikacin was active against all nocardiae. In contrast, the Gordona and Rhodococcus strains showed considerable susceptibility, in particular to the beta-lactam antibiotics. Amoxicillin + clavulanic acid and gentamicin were active against all of the gordonae and rhodococci. The differences in susceptibility patterns may prove useful in characterizing the genera Gordona, Nocardia and Rhodococcus and in separating species within these taxa.