Tuberculosis is an ancient disease, having been around for over 30,000 years. In 2009 there were 9.4 million new cases of the disease and 1.7 million deaths globally due to this bacterial infection. This makes Mycobacterium tuberculosis the biggest world-wide killer among all bacteria, and worthy of in-depth of study.
Mycobacteria
Mycobacteria are single celled, rod shaped, Gram positive bacteria. They are aerobes, meaning that they require oxygen for respiration. They differ from other bacteria, as they have an unusually thick peptidoglycan cell wall which assists in their survival by preventing them from drying out.
There are many different strains of Mycobacteria, most of which are harmless soil bacteria, but the best known examples of this genus are the disease causing species, M. tuberculosis, the causative agent of tuberculosis (TB) and M. leprae, the causative agent of leprosy. Another notable species is M. bovis, which causes TB in cattle (and rarely in humans). The M. bovis form of TB can be acquired in humans through the consumption of non-pasteurised milk products.
Mycobacterium tuberculosis – A Human Pathogen
M. tuberculosis is an obligate aerobe, meaning that it cannot survive without a supply of oxygen. This requirement for a well aerated, oxygen rich growth environment explains why the bacteria are found in the upper lobes of the lungs in TB infected patients.
M. tuberculosis is capable of avoiding the human immune response by invading host cells, becoming an intracellular parasite. If an M. tuberculosis cell is engulfed by a macrophage (a type of white blood cell), it is capable of preventing the macrophage from fusing with a lysosome (a cellular organelle containing enzymes which break down foreign bodies and invading pathogens). It does this by producing a protein which is secreted by the macrophage, and alters the membrane of the lysosome, preventing binding, and therefore preventing the lysolytic enzymes from reaching their target. This means that M. tuberculosis can remain inside the macrophage unharmed, and hidden from the host’s immune system.
The thick cell wall of M. tuberculosis also contributes to its success as a pathogen. As well as being unusually thick, the cell wall contains high concentrations of hydrophobic (water hating) lipids, which helps to protect the bacterium not only from dessication (drying out) but also from acidic or alkaline compounds and antimicrobial drugs.
Studying M. tuberculosis
Mycobacteria are very slow growing, which means that they are difficult to culture in the laboratory. M. tuberculosis has a replication time of 15 to 20 hours, and it can take 4 to 6 weeks for visible colonies of the bacterium to grow. Colonies of the bacteria are small, and have an irregular outline due to the bacteria forming chains or serpentine cords.
Despite the practical difficulties associated with working with these bacteria, the genomes of several species of Mycobacteria, including M. tuberculosis, have been sequenced by scientists at the Sanger Institute, Cambridge, UK.
This research was funded by the Wellcome Trust and the UK Department for Food and Rural Affairs (DEFRA), indicating the importance which is placed upon the study of this disease causing organism.
Dr Haileyesus Getahun, of the World Health Organisation’s Stop TB Department was awarded the 2011 Union Scientific Prize for his work on TB and HIV associated TB. The World Health Organisation has pledged to halve TB deaths and prevalence by 2015, meaning that research into M. tuberculosis, its transmission, virulence and treatment, will remain of interest and importance to the scientific community and health professionals for many years to come.
Sources
101 Illnesses you don’t want to get, by Michael Powell with Dr Oliver Fischer (published 2005)
NHS Choices: Tuberculosis. Accessed November 2011.
John Innes Centre, Norwich, UK Microbiology Department: Accessed November 2011.
The Sanger Institute: http://www.sanger.ac.uk/
Textbook of Bacteriology.net: Tuberculosis. Accessed November 2011.
World Health Organisation: WHO Coordinator honoured for his outstanding work on TB/HIV.
Accessed November 2011.
Vicki Dennison - Vicki has a Ph.D, is a qualified Science teacher, and has a passion for inspiring young people.
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