Many bacteria cause disease by invading cells and creating a
safe niche in which to replicate. Cells respond to the infection
by activating the immune system, and a chief challenge for
bacteria is to avoid immune detection. Prior research had shown
that bacteria inside the cytosol (the cell's expansive gel-like
compartment) could be detected, but how bacteria within the
cytosol are recognized has not been clear. In the new work, Dr.
John Brumell and colleagues at The Hospital for Sick Children
studied the fate of Salmonella bacteria within the cytosol.
These bacteria normally occupy vacuoles in host cells, but under
some conditions they leave the vacuole and enter the cytosol. In
this foreign environment, the bacteria were recognized by the
ubiquitin system, a protein machine that applies molecular tags
to cellular proteins to target them for destruction by the
proteasome, essentially a molecular shredding device inside
mammalian cells. These findings suggest that bacterial proteins
are being destroyed by the