We developed two colorimetric methods for the detection of vancomycin-and oxacillin-resistant Staphylococcus aureus in <6 h: (i) a nitrate reductase assay and (ii) a resazurin microplate method. MICs agreed with results obtained by CLSI methods for oxacillin. However, detection of vancomycin resistance required a larger inoculum. These methods may be recommended for the detection of vancomycin-and oxacillin-resistant S. aureus.
Multi-drug resistant (MDR) Mycobacterium tuberculosis is still a serious public health problem all over the world. MDR tuberculosis (MDR-TB) caused by these strains has emerged within the last decade and rapid detection is critical for the effective treatment of patients. Recently, a resazurin microtiter assay plate for detecting MDR strains was developed. In this study, it was adapted to screw-cap tubes and the activity of isoniazid (INH) and rifampin (RIF) to 50 M. tuberculosis clinical isolates was tested by this method for the first time. Results were compared with the radiometric reference method for the susceptibility testing of M. tuberculosis complex. The results of both methods were in 100% and 96% agreement for RIF and INH, respectively. Specificity, sensitivity, positive predictive value and negative predictive value were 91.7%, 100%, 92.8% and 100% for INH, respectively. All of these values were 100% for RIF. Susceptibility testing results were obtained on the 8th day of incubation for 42 isolates and on the 9th day for the other eight strains. Our results indicate that this method is suitable for the early determination of INH and RIF resistance in developing countries because it is inexpensive, rapid and easy to perform.
In this study, blood agar was used instead of 7H10 agar for the susceptibility testing of 34 clinical isolates of Mycobacterium tuberculosis to isoniazid (INH) and rifampin (RIF) in accordance with the NCCLS. The BACTEC 460 TB system (Becton Dickinson, Sparks, Md.) was used as a "gold standard." Results for both media were in agreement for RIF and INH at 100 and 94.1%, respectively. For INH, the specificity, sensitivity, positive predictive value, and negative predictive value were found to be 71.4, 100, 93.1, and 100%, respectively, while these values were 100% for RIF. In addition, the results of the susceptibility test performed with blood agar were obtained on day 14 of incubation. In conclusion, results were obtained much earlier with blood agar (2 weeks) than with 7H10 agar (3 weeks), and the results of this study suggest that blood agar may be used as an alternative medium for the susceptibility testing of M. tuberculosis to INH and RIF.The increasing incidence of multidrug-resistant tuberculosis (MDR-TB) produces serious problems in developed and especially in developing countries. Detecting tuberculosis and identifying MDR Mycobacterium tuberculosis strains by conventional methods is difficult because of the low growth rate of the causative agent. Therefore, rapid and efficient methods are needed for the control of this disease (3-6, 13).Several manufacturers have directed considerable effort toward the development of rapid and efficient systems for the growth, detection, and susceptibility testing of mycobacteria. Two of these systems, the BACTEC 460 TB (Becton Dickinson, Sparks, Md.) and the BACTEC MGIT 960 (Becton Dickinson), have become available for the susceptibility testing of M. tuberculosis and are also recommended by the NCCLS. Although the time to detect M. tuberculosis from clinical specimens can be shortened and the results for susceptibility testing can be obtained in 4 to 7 days by the use of these systems, they are labor-intensive and expensive and generate radioactive waste (2-4, 12). Therefore, several methods based on liquid media have been developed by different investigators for the susceptibility testing of M. tuberculosis (2, 4-6, 10, 11, 13, 14).Drancourt et al. (7) investigated the effectiveness of blood agar for primary isolation of M. tuberculosis. They reported that M. tuberculosis can easily grow on blood agar in 1 to 2 weeks and that this medium has been routinely used instead of egg-based medium in the inoculation of 10,000 samples in a year for the diagnosis of tuberculosis, with the same results being obtained.In this study, we evaluated the performance of blood agar for susceptibility testing of 34 M. tuberculosis clinical isolates to isoniazid (INH) and rifampin (RIF) by using the proportion method.Bacterial isolates. Thirty-four clinical isolates of M. tuberculosis were examined in this study, and H37Rv and H37Ra were also included as control strains. Drug susceptibility patterns of all isolates were previously detected by the BACTEC 460 TB system, and a standard prot...
Vancomycin-resistant enterococci (VRE) are a serious challenge for physicians because of the limited treatment options for infections caused by this organism. Prevention of VRE transmission in hospitals requires early detection of infected or colonized patients. Therefore rapid and correct detection of vancomycin resistance is essential. In this study, we use the resazurin microplate method (RMM), which is a modification of the NCCLS and BSAC broth microdilution methods to rapidly determine the susceptibilities of clinical enterococci isolates to vancomycin. The alteration in the RMM was relevant to the final bacterial count. In this method, inoculum that was 10-fold higher than standard methods was used. A total of 80 enterococci, including 11 VRE isolates and 6 vancomycin intermediate isolates, were screened with this modified colorimetric broth microdilution method. After 4 h of incubation 30 microl of 0.01% resazurin solution were added to each well and the plates were reincubated for color change for 5-10 min. The MICs were obtained at the 4th h. The results were in exact agreement with the NCCLS and the BSAC microdilution methods. Absolute and essential agreements were 100% and there were no minor, major or very major errors. In conclusion, this modified colorimetric broth microdilution method can be used as a reliable, easy, cheap and rapid method for early detection of VRE. Moreover, this method has the potential of being used to test the susceptibilities of different bacteria to other antibiotics.
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