Mycobacterium tuberculosis H37Ra

The highly attenuated H37Ra (and the already sequenced virulent H37Rv) strains of M. tuberculosis were derived by serial passaging of the H37 clinical isolate through laboratory media. The genetic basis for the avirulent phenotype of M. tuberculosis H37Ra in animal models is not known. Since the H37Ra strain does not revert to a virulent phenotype on passaging through animals, it is likely that the former has acquired multiple point mutations or small deletions/rearrangements in genes crucial for virulence. Some differences between the two strain have been noted. Although both H37Rv and H37Ra are able to infect macrophages equally, only H37Rv and not H37Ra was able to grow intracellularly as well as cause significant mitochondrial damage leading to necrosis. The aggregation of mycobacteria into structures known as "cords" has long been associated with virulence. Comparative gene expression analysis recently identified 22 genes that were consistently expressed at higher levels in H37Rv (a cording strain) than in H37Ra (a non-cording strain) under a variety of growth conditions. It is an acid-fast, obligate aerobic, non-motile, rod-shaped bacterium, this is the causative agent of tuberculosis. Tuberculosis is, to this day, according to the WHO, the leading killer of adults, with approximately 2 million deaths annually worldwide. It is estimated that 8 million people are infected each year. A large part of its success as a pathogen is due to its ability to persist in a dormant or latent form for years or even decades, with a concomitant absence of clinical symptoms.