Multidrug-resistant (MDR) tuberculosis (TB), defined as resistance to isoniazid (INH) and rifampin (RIF), is a severe problem for TB control. Recent reports document the global emergence of highly resistant Mycobacterium tuberculosis
strains, particularly in countries of Eastern Europe (7
). The emergence of MDR TB highlights the need for drug susceptibility testing (DST), patient management, and drug resistance surveillance. Early diagnosis is essential for starting an effective treatment regimen and reducing its transmission in the population.
In countries with low resources, DST involves conventional culture methods with Löwenstein-Jensen (L-J) and Middlebrook agars and requires 3 to 6 weeks to yield results (1
). Faster methods which use liquid media, such as the BACTEC radiometric method and the mycobacteria growth indicator tube method, require radioisotopes (27
), expensive equipment and media, or commercial products not always available in most developing countries (34
). Recently, molecular methods for rapid detection of drug resistance have appeared (6
). However, their costs and the requirement for equipment and skilled personnel have precluded their routine implementation in countries with low resources.
Colorimetric assays employing oxidation-reduction indicators for DST have been previously used with mycobacteria (11
). A simple method employing Alamar blue for DST of M. tuberculosis
was recently described (23
). Resazurin, an oxidation-reduction indicator, has been used to assess viability and bacterial contamination and to test for antimicrobial activity (3
). Since Alamar blue has been recently identified as resazurin in cell cytotoxicity studies (22
), we have standardized and evaluated a microplate method which uses the reduction of resazurin for DST to INH and RIF in clinical isolates of M. tuberculosis
from low-income countries. The results were compared to those of the proportion method (PM) on L-J medium.
Eighty clinical isolates were studied: 65 from TB patients from a region of Peru with a high prevalence of MDR TB and 15 from the Tuberculosis Reference Laboratory at the Instituto Nacional de Laboratorios de Salud, La Paz, Bolivia. American Type Culture Collection (Manassas, Va.) reference strains were used as controls. INH and RIF (Sigma-Aldrich NV/SA, Bornem, Belgium) solutions were prepared at concentrations of 1 mg/ml in distilled water and 10 mg/ml in methanol, respectively, filter sterilized, and frozen until used. Resazurin sodium salt powder (Acros Organic N.V., Geel, Belgium) was prepared at 0.01% (wt/vol) in distilled water and filter sterilized; it can be stored at 4°C for 1 week. The resazurin microtiter assay (REMA) plate method was performed in 7H9-S medium containing Middlebrook broth, 0.1% Casitone, and 0.5% glycerol and supplemented with oleic acid, albumin, dextrose, and catalase (Becton-Dickinson). INH and RIF solutions were thawed and diluted in 7H9-S medium. Serial twofold dilutions of each drug in 100 μl of 7H9-S medium were prepared directly in 96-well plates at concentrations of 1.0 to 0.03 μg/ml for INH and 2.0 to 0.06 μg/ml for RIF. Growth controls containing no antibiotic and sterility controls without inoculation were also included. The inoculum was prepared from fresh L-J medium in 7H9-S medium adjusted to a McFarland tube no. 1 and then diluted 1:20, and 100 μl was used as an inoculum. The plates were covered, sealed in plastic bags, and incubated at 37°C in the normal atmosphere. After 7 days of incubation, 30 μl of resazurin solution was added to each well, incubated overnight at 37°C, and assessed for color development. A change from blue to pink indicates reduction of resazurin and therefore bacterial growth. The MIC was defined as the lowest drug concentration that prevented this color change. Breakpoint drug concentrations defining drug resistance were determined for each drug. The PM was performed on L-J medium according to standard procedures (1
The INH MICs for all 54 M. tuberculosis isolates determined to be resistant to INH by PM were at least 0.25 μg/ml with the REMA plate method, and the MICs were 1.0 μg/ml or higher for 45 (83%) of the isolates. MICs of 0.06 μg/ml or lower were found by using the REMA plate method for 25 out of 26 isolates determined to be susceptible to INH by PM (Table ). The only discordant isolate was determined to be susceptible by PM, but the MIC for it was higher than 1 μg/ml with the REMA plate method. On repeated testing, it was found to be susceptible by both methods. It was also susceptible by the mycobacteria growth indicator tube method. Based on these results, the tentative breakpoint concentration of INH was defined as 0.25 μg/ml.
MICs of INH and RIF for 80 M. tuberculosis isolates determined by using the REMA plate method
RIF MICs for all 49 isolates shown to be resistant to RIF by PM were at least 0.5 μg/ml with the REMA plate method, and the MICs for all but one (48 out of 49) were higher than 2 μg/ml. MICs for all 31 isolates shown to be susceptible by PM were 0.25 μg/ml or lower when tested by the REMA plate method (Table ). A tentative breakpoint concentration of RIF was defined as 0.5 μg/ml. For INH, 79 out of 80 results were concordant, yielding specificity and sensitivity of 96.2 and 100%, respectively, with predictive values for susceptibility and resistance of 100 and 98.2%. As all 80 results were concordant for RIF, the specificity, sensitivity, and predictive values were all 100%.
Timely detection of initial drug resistance is very important for the outcome of TB treatment (4
), and both the Centers for Disease Control and Prevention and the British Thoracic Society recommend that DST be performed on isolates from all new patients (15
Although DOTS (directly observed treatment short course) can reduce the emergence of drug resistance (37
), it has been found that patients with initial resistance to INH and RIF treated with standard DOTS developed resistance to other drugs (9
). DST on initial isolates from new patients is also an essential component of the DOTS-plus strategy (8
). In low-income countries DST methods use solid media, such as L-J and Middlebrook agars, on which the proportion of resistant bacteria can be quantitatively determined (13
), taking 3 to 6 weeks to obtain results (16
). Alternative methods employing liquid media have been described previously (27
). Here we describe the initial results with a rapid method that uses the reduction of resazurin as an indicator of mycobacterial growth. This REMA plate method is simple to perform and inexpensive, giving results after 1 week of incubation that are comparable to those of other DST methods which use liquid media (27
). MICs were higher than 2 μg/ml for 98% of RIF-resistant isolates, allowing easy discrimination of RIF resistance. All of these isolates were also resistant to INH, so testing for RIF resistance alone with the REMA plate method would have identified all of the MDR-TB isolates (19
). Receiver operating characteristic curve analysis (12
) gave areas under the curve of 0.973 for INH (95% confidence interval, 0.909 to 0.996) and 1.00 for RIF (95% confidence interval, 1.000), demonstrating an excellent capacity to differentiate resistant and susceptible isolates.
Colorimetric methods have been used for DST in yeast (25
) and bacteria (40
), including M. tuberculosis
). This REMA plate method is very similar to the Alamar blue assay, interpretation of results is very easy, and correlation with the PM was excellent. Since it is also tested in liquid medium, it has not been implemented as a direct DST method due to the contamination problems that may arise. Being a nonproprietary product and cheaper than Alamar blue, it could be easily implemented in low-resource settings; it has the added advantage that it doesn't require uptake by the bacterial cell (17
). One main concern with this type of test is biosafety; it has been shown, however, that the test can be easily adapted to a closed-tube format, thereby avoiding this problem (38
). In summary, we have found the REMA plate method to be a rapid, simple, and inexpensive technique for the detection of MDR TB isolates that could be adapted for DST with other anti-TB drugs, particularly in low-income countries.