Testing of Mycobacterium tuberculosis susceptibility to ethambutol, isoniazid, rifampin, and streptomycin by using Etest

Abstract

Etest (AB BIODISK, Solna, Sweden) is a precise MIC method and the practical method of choice for the susceptibility testing of many fastidious organisms, including rapidly growing mycobacteria. Methods recommended by the National Committee for Clinical Laboratory Standards for the susceptibility testing of Mycobacterium tuberculosis include the Bactec (Becton Dickinson, Sparks, Md.) broth and agar proportion methods. A comparison of Etest with the Bactec broth method for testing the susceptibility of M. tuberculosis to four first-line antituberculous agents demonstrated equivalent interpretive results for 100% of the isolates tested. Agreements with agar proportion MICs, within ؎ 2 log 2 dilutions, were 90, 93, 100, and 94% for ethambutol, isoniazid, rifampin, and streptomycin, respectively. Etest MICs were easily read within 5 to 10 days of inoculation. Preparation of the inoculum with a turbidity equivalent to a McFarland 3.0 standard prepared from growth on an agar surface and with a broth with a Bactec growth index of >999 yielded equivalent results. Clinical isolates for which the MICs were reproducible were also identified as possible quality control strains. The Etest method appears to be an alternative method for testing the susceptibility of M. tuberculosis isolates to the four most commonly used therapeutic agents.

Nitrite impacts the survival of Mycobacterium tuberculosis in response to isoniazid and hydrogen peroxide

“…Although the assay is carried out under room air (21% oxygen), nitrate reductase activity is measured at the end of a prolonged incubation period, which may indicate that the medium in which Mtb is incubated becomes hypoxic. Alternatively, Published MICs are provided for reference (Wanger and Mills 1996). while aerobic cultures of Mtb do not express the nitrate transporter, narK2, nitrate may gradually diffuse across the plasma membrane and be reduced to nitrite.…”

Susceptibility testing of Mycobacterium tuberculosis: comparison of the BACTEC TB-460 method and flow cytometric assay with the proportion method

“…Several methods have been developed that decrease greatly the time required by the proportion method to obtain susceptibility test results [11][12][13][14][15][16][17][18][19][20]. These methods include a bioluminescence assay for detection of mycobacterial ATP [11], the Gen-Probe DNA hybridisation system [12], the luciferase reporter gene assay [14], high-performance liquid chromatography mycolic acid analysis [15], the Etest method [16] and a colorimetric method [17]. However, to date, only the BACTEC method has been recognised as a valid alternative [7,8].…”

“…In contrast, there was considerable disagreement between the proportion and BACTEC methods regarding susceptibility to isoniazid, with 17 (22%) discrepancies among the 78 tests. Nine of the discrepancies occurred with isoniazid 0.2 mg ⁄ L. Isolates 1,11,14,15,16,23 and 24 were resistant to isoniazid by the proportion method, but susceptible by the BACTEC method. The remaining isolates (5 and 12) were susceptible by the proportion method, but resistant by the BACTEC method.…”

“…Even the method used most frequently, BACTEC-460, requires incubation for 4-12 days [4][5][6][7][8]. Therefore, newer methods are being developed constantly to facilitate more rapid availability of susceptibility results [9][10][11][12][13][14][15][16][17][18].…”

Molecular Mechanisms of Intrinsic Streptomycin Resistance in Mycobacterium abscessus

“…In Mycobacterium fortuitum, another member of the RGM, an aminoglycoside, 3Љ-O-phosphotransferase [encoded by aph(3Љ)] was described as a streptomycin resistance determinant (25), while no streptomycinmodifying enzymes have been identified in other mycobacteria, like Mycobacterium tuberculosis or Mycobacterium smegmatis (26). These organisms are naturally susceptible to streptomycin (27)(28)(29). Streptomycin was the first drug used to treat tuberculosis (30).…”