ARTICLE Antimicrobial susceptibility of rapidly growing mycobacteria using the rapid colorimetric method I. B. Ramis 1,2 & M. Cnockaert 2 & A. von Groll 3 & C. L. Nogueira 3 & S. C. Leão 3 & E. Andre 4 & A. Simon 4 & J. C. Palomino 2 & P. E. A. da Silva 1,5 & P. Vandamme 2 & A. Martin 2 Received: 28 January 2015 /Accepted: 12 March 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract Drug susceptibility testing (DST) of rapidly grow- ing mycobacteria (RGM) are recommended for guiding the antimicrobial therapy. We have evaluated the use of resazurin in Mueller–Hinton medium (MHR) for MIC determination of RGM and compared the results with those obtained with the reference standard broth microdilution in Mueller–Hinton (MH) and with the resazurin microtiter assay (REMA) in 7H9 broth. The MIC of eight drugs: amikacin (AMI), cefoxitin (FOX), ciprofloxacin (CIP), clarithromycin (CLA), doxycycline (DOX), linezolid (LZD), moxifloxacin (MXF) and trimethoprim-sulfamethoxazole (TMP-SMX) were eval- uated against 76 RGM (18 species) using three methods (MH, MHR, and REMA) in a 96-well plate format incubated at 37 °C over 3–5 days. Results obtained in the MH plates were interpreted by the appearance of turbidity at the bottom of the well before adding the resazurin. MHR and 7H9-REMA plates were read by visual observation for a change in color from blue to pink. The majority of results were obtained at day 5 for MH and 1 day after for MHR and 7H9-REMA. However, the preliminary experiment on time to positivity results using the reference strain showed that the resazurin can be added to the MH at day 2 to produce the results at day 3, but future studies with large sets of strains are required to confirm this suggestion. A high level of agreement (kappa 1.000–0.884) was obtained between the MH and the MHR. Comparison of results obtained with 7H9-REMA, on the other hand, revealed several discrepancies and a lower level of agreement (kappa 1.000–0.111). The majority of the strains were resistant to DOX and TMP-SMX, and the most active antimicro- bials for RGM were AMI and FOX. In the present study, MHR represented an excellent alternative for MIC determination of RGM. The results could be read reliably, more easily, and more quickly than with the classical MH method. Introduction Rapidly growing mycobacteria (RGM) are nontuberculous mycobacteria (NTM) that are widely distributed in the envi- ronment and isolated most frequently from soil and water [1]. In the last few years, reports of human infections caused by RGM have increased globally, especially in immunocompro- mised individuals, although RGM can also affect persons with a competent immune system [2, 3]. They can cause a wide variety of disseminated or localized infections, particularly pulmonary, skin, and soft tissue infections [4–6]. The treat- ment of choice for NTM infections is antibiotic chemotherapy, which requires multiple antimicrobial agents administered during a long period of time; consequently, the treatment is costly and often associated with drug-related toxicities [7, 8]. RGM are intrinsically resistant to several antibiotics and * I. B. Ramis ivynha_@hotmail.com 1 Centro de Desenvolvimento Tecnológico (CDTec), Universidade Federal de Pelotas, Pelotas, Brazil 2 Laboratory of Microbiology, Department of Biochemistry and Microbiology, Ghent University, Ghent, Belgium 3 Departamento de Microbiologia, Imunologia e Parasitologia, Escola Paulista de Medicina, Universidade Federal de São Paulo, São Paulo, SP, Brazil 4 Pôle de Microbiologie, Institut de Recherche Expérimentale et Clinique, UC Louvain, Brussels, Belgium 5 Laboratório de Micobacteriologia, Faculdade de Medicina, Universidade Federal do Rio Grande, Rio Grande, Brazil Eur J Clin Microbiol Infect Dis DOI 10.1007/s10096-015-2365-2