The emergence of drug resistant strains of Mycobacterium tuberculosis is of growing concern. Multi-drug resistant disease (MDR-TB), where the strain is resistant to both the major anti-tuberculosis drugs rifampicin and isoniazid, has been reported in all regions of the world. Incidences of MDR exceeding 10% of TB caseloads have been reported in parts of Central Asia, China, Eastern Europe, Russia and Africa [1]. The prognosis of patients with MDR-TB is poor and, unless alternative anti-tuberculosis drugs are administered, they are likely to remain infectious until death. Treatment of MDR-TB is both expensive and difficult to administer as it requires prolonged treatment of at least 18 months with 'second line' drugs that exhibit enhanced toxicity Early detection of MDR-TB is important not only for the patient, but also to limit transmission and the spread of drug resistant disease. Traditional phenotypic methods of detecting drug resistant disease are slow due to the protracted growth rate of M. tuberculosis, with results often taking weeks to obtain. Rapid molecular methodologies have been developed that detect mutations predictive of resistance to rifampicin. These tests examine the rpoB gene encoding the β-subunit of bacterial DNA dependent RNA polymerase and they have been demonstrated to have high accuracy for detecting resistant strains of M. tuberculosis [2]. In some settings resistance to rifampicin is highly predictive of MDR-TB [3] and these rapid tests may be used to investigate suspected MDR-TB cases or to monitor high risk patients such as those failing standard treatment regimens. However, the new molecular technologies have not been implemented in resource limited settings due to their high cost and the requirement for specialist skills and equipment. Most high prevalence countries continue to use slow culture-based methods to investigate suspected MDR-TB cases and new simple, rapid tests are needed that are affordable in these settings.
We have previously described a 'low-tech' rapid method for investigating the susceptibility of M. tuberculosis to rifampicin that uses mycobacteriophage D29 [4]. In this technology mycobacteriophages are allowed to infect the bacteria, successful replication and production of progeny phage being indicative of the presence of viable mycobacteria. Rifampicin disrupts phage replication by preventing synthesis of bacterial mRNA and when critical concentrations of this drug are present progeny phage will only be observed in those strains resistant to the drug [5]. A microwell plate version of this technology has been developed which allows high-throughput screening of M. tuberculosis isolates [6]. To assess the performance of this simple test in a low-income, high prevalence country, the method was transferred to a TB laboratory in Uganda. The study was part of a multidisciplinary project aimed at developing strategies for the management of MDR-TB in the Kampala region. A panel of stored strains was selected to undergo blinded testing by the phage assay. Results were compared to those obtained by BACTEC 460 system (Becton Dickinson, Sparks, Maryland, USA); a broth based phenotypic method routinely employed in this laboratory. Strains identified as resistant to rifampicin by either assay were investigated for mutations in an 81 bp segment of the rpoB gene by sequencing [7]. Strains with discordant susceptibility test results were investigated further by application of a second phenotypic test to assess minimal inhibitory concentrations of the drug. To appraise the usefulness of the phage assay in this setting we also assessed the rapidity of the test and cost of the reagents.
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