A fat droplet-laden tubercle bacillus in a patient's sputum
Many tuberculosis bacteria shield themselves in protective fat, which may keep them safe as they pass from person to person, research shows.
The UK finding may eventually lead to new treatments for the disease, which currently requires a six-month course of antibiotics.
It may also shed light on why efforts to combat TB, are increasingly being undermined by drug resistance.
The study appears in the journal Public Library of Science Medicine.
TB kills two million people around the world each year, and efforts to combat the disease have been stymied by the fact that scientists know very little about the life cycle of the bacterium which causes it.
The bacterium which causes the disease is also mutating to neutralise current drug therapies.
Researchers at the University of Leicester and St Georges, University of London, analysed all the genes active in TB samples taken from infected patients.
The researchers found that, unlike TB bacteria growing in test tubes, many of the bugs in sputum are loaded with fat droplets.
They appear to be an inert, non-growing state, which might help them better to survive the adverse conditions they are likely to encounter during person-to-person transmission.
Kills two million people globally each year
Caused by Mycobacterium tuberculosis, bacteria that are spread in airborne droplets when people with the disease cough or sneeze
Symptoms include a persistent cough, weight loss, and night sweats
Diagnostic tests include chest X-rays, the tuberculin skin test, and sputum analysis
Can be cured by taking several powerful antibiotics for several months
The discovery may explain why TB patients require a long course of treatment before they cease to pose an infection risk to others.
It had been thought that all bacteria contained in infected sputum was released from infected areas of the lungs, and while highly infectious, might also be vulnerable to attack by drugs.
Lead researcher Professor Barer, from Leicester, 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."
Professor Philip Butcher, from St George's, said: "This work forms the foundation to develop a new drug that works effectively against these fat and lazy bacteria."
Dr John Moore-Gillon, of the British Lung Foundation, which co-funded the study, said: "This research helps us understand just why the TB bug is an extremely tough cookie built to survive.
"It adds to our knowledge of why TB bacteria are so difficult to eradicate from the body and why drug resistant strains develop."
Professor Peter Ormerod, a respiratory expert at Blackburn Royal Infirmary, agreed the research raised the prospect of new drugs which could combat TB more quickly.
He said some TB bacteria were already known to be latent, but they tended to remain in the body's tissues, often within specialised immune system cells, rather than be coughed out in the sputum.