Comparative Evaluation of E-Test and Disk Diffusion Methods for Susceptibility Testing of Nocardia Species

Variations in antimicrobial susceptibility among different Nocardia species limit the options for therapy. It is very difficult to perform antimicrobial susceptibility testing of these bacteria due to their slow growth rate and problems in inoculum preparation. The aim of this study was to compare E-test and disk diffusion methods for the determination of antimicrobial susceptibilities of Nocardia isolates. Since E-test is considered as 90% consistent with the gold standard microdilution method recommended by Clinical and Laboratory Standards Institute (CLSI), it was chosen for comparison with disk diffusion and in order to determine the use of disk diffusion in routine practice. A total of 21 Nocardia strains isolated from clinical specimens (12 lung, 7 brain and 2 skin/soft tissue samples) were included in the study. Six of the isolates were identified as N.asteroides, six were N.farcinica, five were N.cyriacigeorgica and four were Nocardia spp. by conventional methods. Susceptibilities of strains to ampicillin, ampicillin-sulbactam, amoxicillin-clavulanic acid, ceftazidime, sefepime, imipenem, gentamicin, erythromycin, levofloxacin, moxifloxacin, trimethoprim-sulfamethoxazole, piperacillin-tazobactam, tigecycline, and linezolid were investigated by using E-test and/or disk diffusion methods. The results were interpreted according to the CLSI breakpoints for Staphylococcus spp. All of the strains were found to be resistant to ceftazidime, piperacillin-tazobactam and ampicillin, however susceptible to levofloxacin, moxifloxacin, trimethoprim-sulfamethoxazole tigecycline, and linezolid. The concordance between the methods in terms of susceptibility testing were 100% for ampicillin, ceftazidime, imipenem, gentamicin and linezolid; 85.7% for erythromycin, 76.2% for sefepime, 73.7% for moxifloxacin, 71.4% for piperacillin-tazobactam, 70% for ampicillin-sulbactam and 46.2% for amoxicillin-clavulanic acid. In conclusion, the therapy must be planned according to the results of antimicrobial susceptibility testing. Disk diffusion is not a reliable method due to the high rates of very major errors. E-test would be an alternative method being practical and easily evaluated, especially in routine laboratories in which the reference method could not be performed.