Variation among Genome Sequences of H37Rv Strains of Mycobacterium tuberculosis from Multiple Laboratories

The publication of the complete genome sequence for Mycobacterium tuberculosis H37Rv in 1998 has had a great impact on the research community. Nonetheless, it is suspected that genetic differences have arisen in stocks of H37Rv that are maintained in different laboratories. In order to assess the consistency of the genome sequences among H37Rv strains in use and the extent to which they have diverged from the original strain sequenced, we carried out whole-genome sequencing on six strains of H37Rv from different laboratories. Polymorphisms at 73 sites were observed, which were shared among the lab strains, though 72 of these were also shared with H37Ra and are likely to be due to sequencing errors in the original H37Rv reference sequence. An updated H37Rv genome sequence should be valuable to the tuberculosis research community as well as the broader microbial research community. In addition, several polymorphisms unique to individual strains and several shared polymorphisms were identified and shown to be consistent with the known provenance of these strains. Aside from nucleotide substitutions and insertion/deletions, multiple IS6110 transposition events were observed, supporting the theory that they play a significant role in plasticity of the M. tuberculosis genome. This genome-wide catalog of genetic differences can help explain any phenotypic differences that might be found, including a frameshift mutation in the mycocerosic acid synthase gene which causes two of the strains to be deficient in biosynthesis of the surface glycolipid phthiocerol dimycocerosate (PDIM). The resequencing of these six lab strains represents a fortuitous "in vitro evolution" experiment that demonstrates how the M. tuberculosis genome continues to evolve even in a controlled environment.