For ex vivo imaging, selected mice were euthanized at different times after infection for harvest of the following organs: liver, spleen, thymus, lung, kidney, stomach, small and large intestine, heart, ovary and uterus, testis, epididymis, seminal vesicles and preputial glands. The organs were placed in 6 (intestine) or 2 ml (all other organs) of PBS. Before imaging, 1 mL/ml h-coelenterazine 5 mM in ethanol and propylene glycol (1:1) was added [41] . Imaging was performed in a hermetically sealed chamber to avoid light. Images were acquired 10 min after the addition of hcoelenterazine. Integration period ranged from 0.5 to 60 s depending on the amounts of light emitted from various organs.