Survey of Genomes - Mycobacterium tuberculosis

Consumption, phthisis (pronounced ti-a-sis), lung fever, and the white plague are just some of the names used over the centuries for a disease that still infects upwards of 1/3 of all humans on Earth - tuberculosis. Most people don’t yet know that they are infected and may never show active symptoms. Anna Pallante from the 2019 Hiram College Genetics course tells us what we can learn from the genome of its causative agent - Mycobacterium tuberculosis.

Welcome to Genomics Revolution. This is Anna Pallante and this episode will focus on the genome of Mycobacterium tuberculosis H37Rv. Mycobacterium tuberculosis is a pathogenic bacteria from the family Mycobacteriaceae that was discovered in 1882 by Robert Koch (1). Koch was able to isolate the bacteria from tissue samples from animals that were suffering from tuberculosis. However, he had to develop and use new staining and culturing techniques in order to do so. When the tissue samples were stained with methylene blue, M. Tuberculosis turned bright blue while the surrounding tissue and other bacteria were all turned brown (1). All infected samples showed the bright blue rod shaped bacteria that Koch originally named tubercle bacillus. In order to prove that this bacteria was truly responsible for tuberculosis, Koch grew the isolated bacteria on a solid medium and then inoculated guinea pigs. Soon after, the guinea pigs developed tuberculosis (1).

When Mycobacterium tuberculosis was first discovered, it was estimated that tuberculosis killed 1/7 of all humans (1). Much research was done to discover the cause of this disease as well as prevention and curative measures. While many antibiotics have been found to treat tuberculosis, it is still one of the top ten causes of death in the world, killing approximately 1.6 million people every year (2). Multi drug resistant tuberculosis has become a major health crisis and studies continue to be done in order to find genetic reasons for resistance and new medications that can fight resistant strains.

M. tuberculosis H37Rv was sequenced in 1998. The genome contains one circular chromosome with approximately 4.41 Mbp with a G + C content of 65.6%. The genome encodes about 3,924 proteins (3). 

Mycobacterium tuberculosis is incredibly adept at lipid metabolism and production. The genome encodes around 250 distinct enzymes that are involved in lipid metabolism (3). This is more than almost any other bacteria. For example, E.coli only has about 50 enzymes for lipid metabolism. This genome also encodes enzymes that allow M.tuberculosis to be proficient lipid producers. The genome encodes biosynthetic enzymes similar to other bacteria, plants, and animals which allows M. Tuberculosis to contain an example of almost every lipid biosynthetic system (3).

M. Tuberculosis is an incredibly successful pathogen, and its virulence comes mainly from the ESX-1 secretion system (4). This is a type VII secretion system which allows transport through the heavily lipidated cell wall through specialized membrane proteins (5). The ESX-1 system is comprised of a cluster of proteins that allows for transport and give the bacteria its virulence. While the system is not fully understood, study of the genome has revealed that the EspL protein is responsible for a great amount of ESX regulation (4). When the espL gene was inactivated through a mutation, M. Tuberculosis was no longer able to secrete ESX-1 substrates and the amounts of various Esp proteins involved in the system were altered (4).

Multi drug resistant and extensively drug resistant tuberculosis are a major health threat due to the increased mortality rates because the antibiotics used to treat them are ineffective. The genomes of antibiotic resistant strains of M. Tuberculosis are being studied and compared to the drug sensitive H37Rv strain in order to find the genetic reason for drug resistance. The pncA gene has been found to be a major factor in the resistance to pyrazinamide, which is a first line treatment of tuberculosis (6). A study revealed that 70% of multidrug resistant and 96% of extensively drug resistant M. Tuberculosis strains had mutations in the pncA gene (6). A total of 69 polymorphism in the pncA gene were identified in the study. Fifty two of these were single nucleotide polymorphisms, and 67 were nonsynonymous mutations (6). These findings and others provide the basis for determining the cause of drug resistance and offer a platform to build off of in the future. Thank you for listening.


References

(1)Koch, 1882. The Germ Theory of Disease 15: 109-115. The etiology of tuberculosis.

(2)World Health Organization 2018. Online: https://www.who.int/news-room/fact-sheets/detail/tuberculosis. Tuberculosis.

(3)Cole et al., 1998. Nature 393: 537-544. Deciphering the biology of Mycobacterium tuberculosis from the complete genome sequence.

(4)Sala et al., 2018. PloS Pathogens 14(12): e1007491. EspL is essential for virulence and stabilizes EspE, EspF, and EspH levels in Mycobacterium tuberculosis.

(5)Green and Mecsas, 2016. Microbiology Spectrum 4(1): 10.1128. Bacterial Secretion Systems – An overview.

(6)Allana et al., 2017. Emerging Infectious Diseases 23(3): 491-495. pncA Gene Mutations Associated with Pyrazinamide Resistance in Drug-Resistant Tuberculosis, South Africa and Georgia.