Restriction fragment length polymorphism (RFLP) analysis of 209Mycobacterium tuberculosis clinical isolates obtained from newly detected pulmonary tuberculosis patients (151 male and 58 female; mean age, 41 years) in Estonia during 1994 showed that 61 isolates (29%) belonged to a genetically closely related group of isolates, family A, with a predominant IS6110 banding pattern. These strains shared the majority of their IS6110 DNA-containing restriction fragments, representing a predominant banding pattern (similarity, >65%). This family A comprised 12 clusters of identical isolates, and the largest cluster comprised 10 strains. The majority (87.5%) of all multidrug-resistant (MDR) isolates, 67.2% of all isolates with any drug resistance, but only 12% of the fully susceptible isolates of M. tuberculosis belonged to family A. These strains were confirmed by spoligotyping as members of the Beijing genotype family. The spread of Beijing genotype MDR M. tuberculosis strains was also frequently seen in 1997 to 1999. The members of this homogenous group of drug-resistant M. tuberculosis strains have contributed substantially to the continual emergence of drugresistant tuberculosis all over Estonia.An increase in tuberculosis (TB) morbidity accompanied by an appearance of multidrug-resistant (MDR) TB has been documented in Estonia since the early 1990s. After a decline in incidence from 417 per 100,000 population in 1953 to 26 per 100,000 in 1992, the incidence showed a steady increase, which reached 52 per 100,000 in 1999 (26). This twofold increase in morbidity was accompanied by an increase in drug-resistant TB and particularly in MDR TB (i.e., TB that is resistant to at least isoniazid and rifampin) and has become a serious problem in Estonia. In 1994 and 1998, MDR TB comprised 10% (13) and 14% (25) of the new pulmonary cases detected. This places Estonia among the countries with the highest MDR TB rates in the world (24).To understand the epidemiology of tuberculosis, molecular methods such as restriction fragment length polymorphism (RFLP) analyses of the infecting organisms have proven to be very powerful tools (11,20,27). The RFLP method can detect genotypic variations between strains by using repetitive DNA sequences as probes in Southern hybridization analyses (12).In outbreak investigations (1, 14, 16), mutual association of IS6110 RFLP patterns and resistance profiles has often been described. However, in population-based studies, the molecular epidemiology and drug resistance pattern of the M. tuberculosis isolates (constituting a genetically closely related group of bacteria) have been studied only in very few geographical settings (23).In the epidemiology of drug-resistant disease, molecular epidemiological studies of resistant strains could help identify current and past failures in TB control and allow tracking of the transfer path of the resistant strains (6). In this study we explore the spread of M. tuberculosis strains in Estonia by molecular epidemiological methods and demonstrate that transmissio...

For the first-line antituberculosis drugs, there is a clear correlation between the drug susceptibility testing (DST) results assayed in vitro and the clinical usefulness of the drug (25). Moreover, for many years there has been a generally accepted consensus on how laboratory testing of the drug susceptibility of Mycobacterium tuberculosis should be performed (5,6,14,24,27). For most second-line drugs and new alternative compounds for treatment of drug-resistant tuberculosis (TB), there is a lack of accepted standard techniques for drug testing as well as full understanding of the clinical interpretation of test results.Much recent attention has focused on assessing the global burden of multidrug-resistant (MDR) TB and predicting the future threat of the pathogen. At present, MDR TB continues to be a serious problem, particularly in developing countries in Asia (11, 12), but also in the Baltic region (9, 22) and in other parts of the former Soviet Union (3,10,26,29,33).The increase in MDR TB rates has led to pressing demands for appropriate treatment with second-line antituberculosis drugs, and accurate and reliable drug susceptibility testing, not only for individual case management, but also for drug resistance surveillance (13). Consequently, laboratories are challenged to provide reliable (and ideally rapid) drug susceptibility testing for first-and second-line drugs to ensure effective treatment of tuberculosis worldwide.The reliability of the Becton Dickinson MGIT 960 system for rapid testing of Mycobacterium tuberculosis susceptibility to front-line drugs has been evaluated in several single-and multicenter studies (1,2,4,5,19,21,31,32). However, in most of these studies, only critical concentrations of drugs were tested (1,19,32) and/or limited numbers of drug-resistant and MDR M. tuberculosis strains were included in the analysis (2,4,18,19,21,32). Drug-resistant isolates are likely to exhibit physiological variations in parameters such as growth compared to drug-sensitive strains.In seven out of the eight studies described above, the MGIT 960 system was evaluated against the BACTEC 460TB system, confirming good reproducibility between liquid-medium-based systems made by the same company. There is no published analysis available for the MGIT 960 system for second-line DST including significant numbers of drug-resistant isolates.In light of this, the primary aim of our study was to test a large number of M. tuberculosis isolates (132 MDR, 40 drugresistant, and 28 drug-sensitive M. tuberculosis isolates) against the critical and high concentrations of first-line drugs using a solid-medium-based resistance ratio method (RRM) as a reference method for comparison with the MGIT 960 system.