Mice orally infected with a luminescent bacterial pathogen ( Listeria monocytogenes ). The mouse on the left was subsequently fed with a probiotic (Lb. salivarius UCC118), which inhibited the pathogen, thus quenching the light. The mouse on the right was instead given a placebo, which had no effect on infection or light production. The image was taken by Pat Casey using the IVIS Imaging System 100 Series from Xenogen.