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Fifty-two clinical isolates of Nocardia asteroides were tested by agar dilution for their susceptibility to 25 antimicrobial agents. In general, susceptibility could not be predicted based on the antibiotic class tested. However, the beta-lactams, including third-generation cephalosporins, were generally ineffective (MIC for 90% of the organisms [MIC90], between 64 and >256 ,ug/ml), whereas minocycline and doxycycline were generally effective (MIC90, 4 and 8 ,ug/ml, respectively). Cycloserine was not effective below 60 pug/ml. The MIC50 and MIC90 of sulfamethoxazole was 16 and 32 jxg/ml, respectively, and that of trimethoprim varied widely (16 and >256 ,ug/ml, respectively). Based on MIC90 data, only doxycycline, minocycline, sulfamethoxazole, and imipenem could be applied empirically.Nocardia infections are often treated with sulfonamides or trimethoprim-sulfamethoxazole (14,15 Susceptibility testing. Isolates to be tested were stored in glycerol at -20°C or in sterile milk at -70°C and then passed onto Saboraud dextrose agar before testing. The inoculum was prepared by inoculating brain heart infusion (BHI) broth from the Saboraud medium and incubating it at 35°C for 4 to 5 days. To increase the homogeneity of the organism suspensions, sterile glass beads (1.0 mm in diameter) were added to the culture tubes and cultures were mixed with a vortex mixer twice daily during incubation.Agar dilution susceptibility was performed with twofold dilutions of the antibiotics according to the recommendations of the International Collaborative Study on susceptibility testing (9). The turbidity of the inoculum was adjusted to equal a 0.5 McFarland standard, and the media were inoculated with a Steers replicator. This procedure produced an average inoculum size of 3 x 104 CFU. Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 were also included as controls. The inoculated plates were incubated at 35°C for 48 h and then examined for growth to determine the MIC. The MIC was defined as the lowest concentration of antimicrobial agent resulting in complete inhibition (sulfonamides, 80% inhibition) of visible growth. RESULTSA comparison of strain growth in different broth media was made. Although all the isolates grew in BHI and tryptic soy broths, growth was sustained longer and more nearly homogeneous suspensions were obtained in BHI broth. Two of the isolates did not grow in MH broth, and among those that did grow, cellular aggregation was marked.For N. asteroides, the range of MICs, the MIC for 50% of

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A total of 2,720 urine specimens from three laboratories were evaluated by Autobac (Pfizer Diagnostics) and were compared with simultaneous colony counts for evidence of bacteriuria. Of 599 specimens with a colony count of greater than or equal to 10(5) colony-forming units per ml, 93.8% were detected within 6 h. This detection rate increased to 97% of 447 positive urine specimens when only specimens from patients not on antimicrobials were evaluated. The majority (77.9%) of positive specimens were detected as early as 3 h. Those specimens with greater than or equal to 10(5) colony-forming units per ml, which were negative by Autobac at 6 h, included organisms which are frequently considered to be contaminants (diphtheroids, lactobacilli, alpha and gamma streptococci, yeasts, and Staphylococcus epidermidis), or were from patients who were being treated with antimicrobial agents. Of 2,121 urine specimens with colony counts of less than 10(5), 98.1% were correctly determined to be negative by Autobac at 3 h. This percentage decreased to 86.0 at 6 h. The majority of these false-positive specimens were those with colony counts of 10(4) to 10(5) colony-forming units per ml. There appeared to be no major difference in results from the three laboratories or among the four lots of broth used in this study.