Methods for detecting and monitoring Salmonella infection and chronic carriage in living mice using bioluminescent in vivo imaging

Salmonella enterica serovar Typhi primarily persists in chronic carriers by forming biofilms on gallstones in the gallbladder. We have developed a gallstone mouse model to study chronic carriage. To better understand the infection timeline and differentiate between mice that have maintained long-term gallbladder carriage from those that have cleared infection, we utilized bioluminescent S. Typhimurium and in vivo imaging to detect and track the organ-specific presence of bacteria in living mice. The mice infected with our bioluminescent S. Typhimurium showed luminescence in the abdomen as early as three days by comparison to the mice infected with non-luminescent wild type S. Typhimurium. With our methods, we achieve image resolution such that we can confidently identify the presence of S. Typhimurium in the gallbladder at >60 days post-infection. Using these methods we have determined that the minimum number of bacteria necessary to detect luminescence in the mice is 103 colony forming units (CFUs), and that 1 out of 6 initially infected mice will remain persistently infected for greater than 60 days, with gallbladder bacterial loads reaching upwards of 103 per mg of tissue. Given that our limit of detection of luminescence is 103 CFUs, our sensitivity is robust enough to identify the bacterial loads present in the average chronically infected mouse. Quantification of individual organs’ bacterial CFUs and comparison of luminescence between WT and luminescent S. Typhimurium validates that our technique is specific and sensitive enough to detect organ specific infection in our model of typhoidal chronic carriage.