Gene expression and pathogenicity in mycobacteria

Tuberculosis (TB) is the leading cause of death worldwide due to an infectious disease, a situation exacerbated by its deadly synergy with AIDS. Annually approximately 2 million people die from TB. Even in the UK and other western countries there is a continuing significant increase in the disease in recent years. The situation is not helped by the limited effectiveness, at least in some part of the world, of the widely used vaccine BCG. Moreover there is a great threat from bacteria that are resistant to treatment with conventional anti-TB drugs, resulting in a disease that is essentially non-treatable unless new antibiotics are discovered. Thus strategies for developing new approaches to the treatment and prevention of TB are a high public health priority. Research at the National Institute for Medical Research is aimed at trying to gain a better understanding of the way in which the bacterium which causes TB is able to infect people and how that process might be interrupted. Using molecular genetics the researchers are identifying some of the key molecules produced by the bacterium and the infected cells, which enable the infection to either progress or be eliminated. These molecules are then being used to screen for novel anti-TB drugs in collaboration with MRC-Technology.

Mycobacteria are a group of bacteria that include some of the most important causes of human infectious disease. Different mycobacterial species are the causes of tuberculosis (TB), the leading cause of death worldwide due to an infectious disease, and leprosy, a disease that results in deformity and disability in millions of people. The objective of this work is to understand the mechanisms which underlie the pathogenicity of the causative agent of human TB, Mycobacterium tuberculosis and to search for new therapeutic agents against it. In this project we are attempting to identify key molecules of M. tuberculosis which enable it to survive and grow inside macrophages, the host cells which are invaded by the bacteria in these diseases. We can also monitor the ways in which the macrophages respond to this invasion by the bacteria. By understanding this interaction we hope to develop new approaches for treating and/or preventing these diseases. This work involves the use of genetic approaches to identify molecules which are important for regulating gene expression and for growth and survival of M. tuberculosis inside macrophages. By introducing mutations into bacteria we can ask whether such changes reduce the ability of the bacteria to cause disease; if so, then the genes which we have mutated can be characterised to give us a better understanding of their function and thus to provide important information about how the bacteria cause TB. We are also able to analyse changes in gene expression that occur within macrophages following infection in order to identify key molecules which are produced by the host and which may influence the outcome of the infection. The changes in gene expression, in both bacteria and host cells, are monitored using whole genome DNA microarrays, either a spotted microarray for M. tuberculosis, or an Affymetrix microarray for host (mouse or human) cells. Having identified candidate genes in M. tuberculosis, deletions of them are derived using homologous recombination techniques, and the effect of the deletion is investigated both by infecting macrophages in vitro and by infecting mice. Identification of the key molecules involved in determining the outcome of infection is a starting point for identifying potential new drug targets. We can produce recombinant protein molecules and develop assays as the basis for screens for new anti-TB drugs. These high throughput screens are performed in collaboration with MRC-Technology. Since the emergence of multiple drug-resistant strains of M. tuberculosis and limitations in the effectiveness of the existing vaccine BCG represent an enormous public health challenge, such approaches are extremely important for the future control of TB and related diseases.