A Bioluminescent Francisella tularensis SCHU S4 Strain Enables Noninvasive Tracking of Bacterial Dissemination and the Evaluation of Antibiotics in an Inhalational Mouse Model of Tularemia

Abstract

bBioluminescence imaging (BLI) enables real-time, noninvasive tracking of infection in vivo and longitudinal infection studies. In this study, a bioluminescent Francisella tularensis strain, SCHU S4-lux, was used to develop an inhalational infection model in BALB/c mice. Mice were infected intranasally, and the progression of infection was monitored in real time using BLI. A bioluminescent signal was detectable from 3 days postinfection (3 dpi), initially in the spleen and then in the liver and lymph nodes, before finally becoming systemic. The level of bioluminescent signal correlated with bacterial numbers in vivo, enabling noninvasive quantification of bacterial burdens in tissues. Treatment with levofloxacin (commencing at 4 dpi) significantly reduced the BLI signal. Furthermore, BLI was able to distinguish noninvasively between different levofloxacin treatment regimens and to identify sites of relapse following treatment cessation. These data demonstrate that BLI and SCHU S4-lux are suitable for the study of F. tularensis pathogenesis and the evaluation of therapeutics for tularemia. Francisella tularensis is a Gram-negative intracellular bacterium and is the etiological agent of the disease tularemia. Tularemia has a low infectious dose by the aerosol route (50% infectious dose [ID 50 ], Ͻ10 CFU), causing a disabling illness without rapid treatment, and is considered a potential biothreat agent. The preferred treatments for tularemia are limited to a few antibiotics, including fluoroquinolones (1, 2), tetracycline (3), streptomycin (4, 5), and gentamicin (6, 7). The lack of a licensed vaccine, reported treatment failures, even with preferred antibiotics (8), and a 2% mortality rate despite treatment (5) necessitate the development and evaluation of new therapies.Conventional methods of assessing bacterial growth and dissemination in vivo require the infection of large cohorts of animals and the culling of groups of animals at set time points throughout a study. This strategy not only uses large numbers of animals but is also labor-intensive and subject to tissue sampling bias. Noninvasive imaging techniques, such as bioluminescence imaging (BLI), can report spatial and temporal aspects of infectious diseases in vivo both longitudinally and in real time (9). BLI typically relies on the detection of light produced by luciferase-catalyzed oxidation reactions, which are encoded in the pathogen of interest, by use of a sensitive charge-coupled device (CCD) camera (10). Pivotal experiments by Contag et al. have demonstrated the usefulness of BLI in the noninvasive study of bacterial pathogenesis and the evaluation of antibiotic treatments for Salmonella enterica serovar Typhimurium (11). Subsequently, bioluminescent reporters have been introduced into other bacteria, including Mycobacterium tuberculosis (10, 12), Staphylococcus aureus (13, 14), Burkholderia mallei (15), and Yersinia pestis (16,17). Further, BLI has been used to evaluate vaccines (18), antibiotics (19-21), and supportive therapies (22) for a ran...

Stochastic dynamics of Francisella tularensis infection and replication

“…The mean fraction of bacteria in each organ was used to obtain the following weights: w MLN = 0.8, w liver = 0.11, w spleen = 0.05 and w kidney = 0.04. The larger weight assigned to the MLNsis reasonable, since bacteria may be drained rapidly through the lymphatic system to the MLN [63,64]. The aim of this section is to make use of the experimental data (see Fig 9) and the mathematical cohort model described in the previous section, to learn about the selected model parameters with an approximate Bayesian computation (ABC) rejection sampling algorithm [42].…”