Rapid susceptibility testing for slowly growing nontuberculous mycobacteria using a colorimetric microbial viability assay based on the reduction of water-soluble tetrazolium WST-1

Rapid susceptibility testing for slowly growing nontuberculous mycobacteria (NTM) using a colorimetric microbial viability assay based on the reduction of the water-soluble tetrazolium salt {2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt (WST-1)} using 2,3,5,6-tetramethyl-1,4-benzoquinone as an electron mediator was developed. Using the Clinical and Laboratory Standards Institute (CLSI) method, a long-term incubation time (7–14 days) was required to determine the minimum inhibitory concentrations (MICs) of the slowly growing NTM. The MICs for a variety of different antibiotics against the slowly growing NTM were determined by the WST-1 colorimetric method and compared with those obtained using the broth microdilution methods approved by the CLSI. Good agreement was found between the MICs determined after 3–4 days using the WST-1 colorimetric method and those obtained after 10–14 days using the broth microdilution method. The results suggest that the WST-1 colorimetric assay is a useful method for the rapid determination of the MICs for the slowly growing NTM.