Interesting item in SciDev.Net about using genetically modified E. coli to detect landmines.
The bacteria have the gene for an enzyme called luciferase (this is present in fireflies, for one), which naturally produces light in some bacteria and fireflies. When TNT is present, this gene is switched on, and the bacteria glow. This can happen within hours of the bacteria being introduced to the region under investigation. Plants can be modified in the same way, but take weeks or months to produce results.
The work is not yet commercially available - European Union regulations on genetically modified bacteria need to be observed.
There is a protein in the cell membrane that senses TNT, according to the item. The work was done at the University of Edinburgh, by Alistair Elfick at the Centre for Biomedical Engineering.
Luciferase has also been used in bioluminiscent imaging, to detect tumour cells, although I can't immediately find anything published about its application to landmines.