London, Feb 4 (IANS) Scientists now know the reason why tuberculosis resurfaces years or decades after initial therapy. The bugs can hide in the patients' bone marrow, safe from the action of drugs, says a new finding.

Stanford University researchers have discovered that the disease is capable of "infiltrating" cells in the bone marrow, where it finds "protection" from treatment.

The cells have qualities like a natural resistance to drugs, infrequent division and a privileged immune status which could allow them to survive various types of treatment, the scientists explained, the journal Science Translational Medicine reports.

The researchers reported that they had found active TB bacteria in the cells of human patients who had been treated for the disease, according to the Telegraph.

Possibly a host of other infectious diseases may use a similar "wolf-in-stem-cell-clothing" tactic to hide away from therapies, they suggested, although any new treatments will take many years to develop.

Traces of the bacteria were identified in mesenchymal stem cells, which can produce specialised cell types including bone, fat and cartilage. Although the stem cells are typically found in the bone marrow, they are also capable of moving to the lungs - the ideal environment for TB bugs.

After testing their theory on mice in a previous study, the researchers carried out a small clinical trial where bone marrow biopsies were taken from a group of patients who had been treated for TB and who had no traces of the bacteria in their respiratory tract.

In the bulk of the patients, bacterial DNA could still be found in bone marrow stem cells, and in two of these cases, living bacteria were found.

Bikul Das, who led the study, said: "Not only is this strong evidence that the tuberculosis can remain dormant in stem cells, it shows that the living bacteria could be recovered from these cells after a long period of time."

Dean Felsher, co-senior author, added: "Other infectious agents might use stem cells in a similar manner. We'd like to further characterise whether and how these stem cells provide a protective niche for other infectious agents."