Activity of antimicrobial drugs evaluated by agar dilution and radiometric methods against strains of Nocardia asteroides isolated in Italy from immunocompromised patients.

Benzylpenicillin, amoxicillin, amoxicillin plus clavulanic acid, cephalothin, cephaloridine, cefotaxime, imipenem, erythromycin, clarithromycin, azithromycin, amikacin, ciprofloxacin and trimethoprim-sulfamethoxazole were tested in vitro by the agar dilution method against eleven strains of Nocardia asteroides isolated both from AIDS and other immunocompromised patients. Imipenem, amikacin and trimethoprim-sulfamethoxazole were shown to be the most active drugs with minimum inhibitory concentrations (MIC) values nearly always lower than concentrations achievable in blood. Ciprofloxacin, cephaloridine and cefotaxime were moderately active, while the remaining drugs were totally ineffective. When susceptibility was assessed by the radiometric method the MIC90 values were uniformly lower than those in the agar method, possibly due to lower inactivation of drugs during incubation. The two methods showed a good correlation only for imipenem, amikacin and ciprofloxacin. The results obtained by the radiometric method seem to indicate that, as for mycobacteria, this method may also give a more accurate evaluation of the antimicrobial susceptibility of Nocardiae.