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Survival of the fattest: TB accumulates fat to survive - and spread

01.04.2008
Medical scientists from the University of Leicester, together with colleagues from St Georges, University of London, funded principally by the Medical Research Council (MRC) and The Wellcome Trust, have published details of a new breakthrough discovery on TB.

They have identified for the first time that the TB bug lays down body fat that may help it survive passing from one person to another and, in the process, the bacteria increase their resistance to the action of anti-TB drugs.

This finding challenges the established view that the TB bacteria coughed up in sputum by infected individuals are rapidly multiplying.

Lead investigator Professor Mike Barer, Professor of Clinical Microbiology in the Department of Infection, Immunity and Inflammation at the University of Leicester said: “Strenuous efforts are being made to reduce the global burden of tuberculosis, a disease which kills four people every minute. Our success so far has been limited for many reasons; one of these is our failure to control the spread of TB from one person to another. Very little is known about this vital part of the bacterium’s life cycle.

“If scientists could understand more about the transmission of TB between people, they might identify new therapeutic and preventative targets.”

The Leicester team discovered that, unlike TB bacteria growing in test tubes, many of the bugs in sputum are loaded with fat droplets. They went on to show with their colleagues in London that these ‘fat bacilli’ were in an inert non-growing state, a condition in which they are more likely to survive the process of passing from one person to another.

The findings are published today (April 1st) in the freely available Journal, Public Library of Science Medicine (www.plosmedicine.org). Additional funding for the study was provided by the British Lung Foundation and the Henry Smith Charity.

The discovery sheds light on the story of “persister bacteria” in TB - a mysterious population believed by many to be the reason why TB patients have to be treated for at least six months.

Professor Barer said: “These surprising findings have opened the door for us to develop new ways to stop TB from spreading and to treat it more effectively. We hope that our new ability to monitor these sleepy and resistant bacteria in sputum will enable us to treat the disease more quickly.

“This work has taken more than ten years to come to fruition and has taken dedicated work from the teams in Leicester and London. I am particularly delighted for my team in Leicester who fought long and hard to bring this story together.”

Professor Philip Butcher and his team at St George’s, University of London have exploited the genome sequence information of the TB bacteria generated by the Pathogen Sequencing Unit at the Wellcome Trust Sanger Institute. They studied all the genes that are expressed by the bacteria in sputum having being coughed up from the lungs of TB patients using microarrays or gene-chips, made available through the Wellcome Trust funded Bacterial Microarray Group at St George’s (B?G@S; http://bugs.sgul.ac.uk). Importantly the St George’s team have developed a novel way to study the small numbers of bacteria present in sputum and this discovery will open the way to investigate why bacteria in TB lungs are so hard to kill with antibiotics. “This work forms the foundation to develop a new drug that works effectively against these fat and lazy bacteria” said Professor Butcher.

Professor Barer added: “In the University of Leicester study we examined TB in sputum samples from infected patients to get a snapshot of the disease at the point of its transmission to a new person and ask how the characteristics of these bacilli compare with those of TB growing in the laboratory.”

The researchers found the presence of a fat deposits and related gene expression patterns which may help the TB bacterium to survive during transmission and establish a new infection.

Please find below an Editors’ Summary taken from the Journal Public Library of Science Medicine

Background.
Every year, nearly nine million people develop tuberculosis—a contagious infection usually of the lungs—and about two million people die from the disease. Tuberculosis is caused by Mycobacterium tuberculosis, bacteria that are spread in airborne droplets when people with the disease cough or sneeze. The symptoms of tuberculosis include a persistent cough, weight loss, and night sweats.

Diagnostic tests include chest X-rays, the tuberculin skin test, and sputum analysis. For the last of these tests, a sample of sputum (mucus and other matter brought up from the lungs by coughing) is collected and then taken to a laboratory where bacteriologists look for M. tuberculosis using special stains—tuberculosis-positive sputum contains ‘‘acid-fast bacilli’’—and also try to grow bacteria from the sample.

Tuberculosis can be cured by taking several powerful antibiotics for several months. It is very important that this treatment is completed to ensure that all the M. tuberculosis bacteria in the body are killed and to prevent the emergence of drug-resistant bacteria.

Why Was This Study Done?
Strenuous efforts are being made to reduce the global burden of tuberculosis but with limited success so far for many reasons. One barrier to success is the efficiency with which M. tuberculosis spreads from one person to another. Very little is known about this part of the bacteria’s life cycle. If scientists could understand more about the transmission of M. tuberculosis between people, they might identify new therapeutic and preventative targets. In the study, therefore, the researchers examines the acid-fast bacilli in tuberculosis positive sputum samples to get a snapshot of M. tuberculosis at the point of its transmission to a new person and ask how the characteristics of these bacilli compare with those of M. tuberculosis growing in the laboratory.
What Did the Researchers Do and Find?
The researchers collected sputum samples from patients with tuberculosis in the UK and The Gambia before they received any treatment, and looked for the presence of acid-fast bacilli containing ‘‘lipid bodies.’’ These small structures contain a fat called triacylglycerol. M. tuberculosis accumulates triacylglycerol when it is exposed to several stresses present during infection (for example, reduced oxygen or hypoxia) and the researchers suggest that the presence of this fat may help the bacteria survive during transmission and establish a new infection. They found that all the samples contained some lipid body–positive acid-fast bacilli.

Next, the researchers showed that M. tuberculosis grown in the laboratory under hypoxic conditions, which induce the bacteria to enter an antibiotic tolerant condition called a ‘‘nonreplicating persistent’’ (NRP) state, also accumulated lipid bodies. This result suggests that the lipid body– positive acid-fast bacilli in sputum might be in an NRP state. To test this idea, the researchers compared the pattern of mRNAs (the templates from which proteins are produced; the pattern of mRNAs is called the transcriptome and gives an idea of which proteins a cell is making under given conditions) made by growing cultures of M. tuberculosis, by M. tuberculosis maintained in the NRP state, and by the acid-fast bacilli in several sputum samples. The transcriptome of the sputum sample revealed production of many proteins made in the NRP state.

Finally, the researchers showed that the time needed to grow M. tuberculosis from sputum samples increased as the proportion of lipid body–positive acidfast bacilli in the sputum increased, just as one would suspect if the presence lipid bodies signifies nongrowing cells.

What Do These Findings Mean?
It has been generally assumed that the acid-fast bacilli in sputum collected from patients with tuberculosis are rapidly replicating M. tuberculosis released from infected areas of the lungs. By identifying a population of bacteria that contain lipid bodies and that are in an NRP-like state in all the samples of sputum examined from two geographical sites, this study strongly challenges this assumption. The characteristics of this population of bacteria, the researchers suggest, might help them survive the adverse conditions that M. tuberculosis encounters during transmission between people and might partly explain why complete clearance of M. tuberculosis requires extended treatment with antibiotics. To establish the clinical significance of these findings, future studies will need to examine whether antibiotic treatment affects the frequency of lipid body–positive M. tuberculosis bacteria in patients’ sputum and whether there is any relationship between this measurement and infectiousness, or clinical response to treatment.

Ather Mirza | alfa
Further information:
http://www.le.ac.uk
http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0050075

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