Antimicrobial Susceptibility of Nocardia Species

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Abstract
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.