This proposed research aims to investigate the emergence and transmission of multidrug resistant tuberculosis (MDR-TB) in the Republic of Moldova, which has one of the most severe MDR-TB epidemics in the world. The study will focus on the transmission of two specific lineages of MDR-TB, the Beijing lineage and the Ural lineage, which we have identified as major drivers of the MDR-TB epidemic in Moldova. Our recent genomic epidemiology research in Moldova has revealed that several large transmission clusters of Beijing-lineage and one large cluster of Ural-lineage MDR Mycobacterium tuberculosis (Mtb) are currently spreading in the country. These lineages have different geographic distributions, with the transmission of multiple Beijing clades focused predominantly within the eastern region of Transnistria, while the Ural clade is more diffusely spread throughout the rest of the country. At this time, we have neither a sufficiently precise estimate of the speed by which each of these clades has grown and spread nor a clear understanding of the distinct genetic mechanisms favoring emergence and spread of these two MDR lineages. The first aim of this proposal is to compare the evolution and expansion of Beijing and Ural Mtb lineages in Moldova since 2010, using isolates and data systematically collected from all culture-positive patients prior to TB treatment. This will allow us to estimate the speed at which these two lineages have become prevalent in the population and to investigate how specific mutations accumulated within these lineages may contribute to their apparent evolutionary success. The second aim is to quantify and compare the transmission of these lineages using data collected during investigations of known contacts of TB patients in Moldova. This will allow us to estimate the relative reproductive potential of these lineages. The final aim is to develop transmission dynamic models to estimate the health impact and costs associated with interventions to interrupt MDR-TB transmission. These models will help to inform public health policy and guide resource allocation for MDR-TB control efforts in Moldova. This research addresses a significant public health crisis of transmitted MDR-TB using innovative phylogenomic, phylogeographic, regression and transmission modeling approaches. Our long-term partnership with investigators in Moldova ensures access to a unique longitudinal collection of Mtb isolates and associated clinical, laboratory and epidemiological data. The ~2000 Mtb whole genome sequences, extensions to analytical tools for inferring transmission and estimating the relationship between bacterial and host factors and transmission, and transmission dynamic models developed during this project will be made freely available to other researchers to ensure maximum use of the data and methods generated during this project.