Antimicrobial susceptibility of Nocardia species

Variations of inoculum, pH, and composition of the agar assay media each influenced the in vitro susceptibility of Nocardia organisms (30 strains of N. asteroides and 6 strains of N. brasiliensis) to one or more antimicrobial agents incorporated in solid agar and incubated at 37 C for 48 h. Among newer antimicrobial agents, doxycycline, minocycline, and 7-halogenated lincomycin analogues exhibited the most consistent in vitro inhibitory activity; among older agents, viomycin, erythromycin, and streptomycin exhibited more selective in vitro inhibitory activity, but against a significant percentage of strains tested. N. brasiliensis was conspicuously more susceptible to erythromycin and gentamicin and more resistant to viomycin and capreomycin than was N. asteroides. Cycloserine demonstrated no significant in vitro inhibitory activity when assayed in appropriately augmented alanine-free medium. The outstanding feature of this in vitro survey was the marked variation in susceptibility among individual strains of N. asteroides, which variation deserves more careful consideration in future clinical or experimental therapeutic trials.