Examination of in vitro epithelial cell lines as models for Francisella tularensis non-phagocytic infections.

Francisella tularensis (F. tularensis), the causative agent of tularemia, has long been known to invade and occupy non-phagocytic epithelial cells. Many epithelial cell infection models have been developed to study this process; however, due to the lack of consensus on infection methods and precise experimental procedures to evaluate invasion and replication, selection of appropriate models to use based on the literature is challenging. To evaluate in vitro non-phagocytic cell infection models, we chose 8 epithelial cultured cell lines from published models to infect with F. tularensis subspecies novicida (F. novicida) and compared the results to a recently developed model that used the mouse hepatocyte BNL CL.2 cell line. We utilized classical gentamicin-based invasion assays to determine total intracellular bacterial loads and employed microscopic examination with staining techniques that distinguished between intracellular and extracellular bacteria to provide an accurate assessment of the proportion of invaded host cells and the degree of bacterial replication. We found that COS-7 cells exhibited the greatest invasion rates; CMT-93 cells contained the largest intracellular bacterial loads; ad HEK-293s were capable of invasion and replication rates at high levels, but required shorter infection incubation times. Although COS-7, CMT-93 and HEK-293 cell lines may be suited to study certain aspects of invasion or replication, we found that BNL CL.2 cells appeared the most appropriate to study the overall pathogenesis of F. novicida when examined in toto.