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The genus Mycobacterium includes more than 190 species, and many cause severe diseases such as tuberculosis and leprosy. According to the "World Health Organization", in year 2019 alone, 10 million people developed TB, and 1.4 million died. TB had been in decline in developed countries, but made its reappearance as an opportunistic pathogen targeting immuno-compromised AIDS victims. Also, non-tuberculosis mycobacteria (NTM) infections have emerged as a major infectious agent in recent times. NTM occupy diverse ecological niches and can be isolated from soil, tap water, and groundwater. This thesis has investigated the Mycobacterium species from a genomic perspective, focusing on the biology of virulence factors, mobile genetic elements, tRNAs, and non-coding RNAs and their evolutionary distribution and possible relationship with phenotypic diversity. 

As part of this study, we have sequenced 153 mycobacterial genomes, including type strains, environmental samples, isolates from hospital patients, infected fish, and outbreak samples in an animal facility at Uppsala University. We have provided a phylogenetic tree based on 387 (and 56) core genes covering most species (244 genomes) constituting the Mycobacterium genus. The core gene phylogeny resulted in 33 clades. Subsequently, we have covered different clade groups, such as, M. marinum, M. mucogenicum, M. chelonae and M. chlorophenolicum and investigated the NTM clade-specific genome diversity and evolution. 

Our examination of non-coding genes showed that the total number of tRNA genes per species varies between 42 and 90. Among the species with more than 50 tRNAs, additional tRNA genes are likely acquired through horizontal gene transfer (HGT), as supported by the presence of closely linked HNH endonuclease gene and GOLLD RNA. We have explored the presence of selenocysteine utility and the gene for selenoprotein "formate dehydrogenase" among 244 mycobacterial genomes. 

For the M. chlorophenolicum clade, we have explored genes with a role in the bioremediation process. Comparative genomics of M. marinum and M. chelonae clade groups suggest new clusters or subspecies. Mutational hotspots are relatively higher in M. marinum compared to that in M. tuberculosis and M. salmoniphilum. Relatively higher number of hotspots in M. marinum is likely related to its ability to occupy different ecological niches. Finally, the thesis uncovered IS elements, phage sequences, plasmids, tRNA, and ncRNA contributing to mycobacterial evolution.