Trash to one is treasure to another.

Macrophages, a kind of cell in the immune system, usually chew up foreign microorganisms. However, disease-causing mycobacteria--the microorganisms that cause tuberculosis and other illnesses--transform these would-be enemies into safe and comfortable homes for themselves.

To gain entry to a macrophage, mycobacteria salvage a protein that their mammalian hosts have discarded, report Eric J. Brown of Washington University School of Medicine in St. Louis and his colleagues in the Aug. 22 Science.

Harmful mycobacteria invade macrophages by seizing a protein that their mammalian hosts had used to target other bacteria for uptake and destruction by macrophages.

"If we could block this process, we could greatly reduce the efficiency with which mycobacteria get into macrophages--and prevent disease," says Brown.

The study adds to the many features that distinguish mycobacteria from each other and presumably contribute to their different abilities to cause disease. Unlike harmless bacteria, the disease-causing strains can counteract the hostile environment within macrophages.

"Age-old studies show that mycobacteria that don't cause disease can also get into macrophages--by a different mechanism--yet they don't survive," says Lalita Ramakrishnan of Stanford University School of Medicine. "Clearly, harmful mycobacteria have developed special ways to persist inside these cells. It makes a lot of sense that they've also devised a way to ensure entry into this safe environmental niche."