Nanovore

Nanovores were microscopic robots, apparently capable of "eating" absolutely anything.

Characteristics
Nanovores were microscopic robots, but were closer to colonial insects in habit. They lived in hives, spreading out and disassembling their surroundings to create more nanovores. They communicated by sounds and could be repelled by a sonic screwdriver, though they would adapt over time. (NSA: The Forever Trap)

History
A colony of nanovores were somehow caught on floor 139 of the Edifice. They devoured all the inhabitants and then spread to other floors. When the Doctor and Donna Noble came to floor 139, the nanovores went after them as well. The Doctor was able to repel them with his sonic screwdriver and was eventually able to escape in the elevator before they became immune. (NSA: The Forever Trap)