Slowing the decay to extinction: Francisella tularensis persistence in aquatic environments is prolonged by cold temperatures and reduced microbial complexity
Francisella tularensis is a zoonotic pathogen where all human cases are independently acquired from environmental sources. As a facultative intracellular pathogen, F. tularensis relies on eukaryotic cells for replication. However, between these disease events it must persist in the environment. The specific mechanisms of this persistence are likely different between F. tularensis subspecies. To better understand persistence outside of mammalian hosts associated to aquatic habitats, we studied planktonic virulent strains of F. tularensis subspecies tularensis (Type A) and subspecies holarctica (Type B) for one year under various axenic aquatic conditions. In cold, nutrient-poor treatments, the Type B strain (FSC200) could cause disease in mice and remained culturable on laboratory media, whereas the Type A strain (Schu S4) caused disease although with delayed onset and became non-responsive to laboratory growth conditions. The experiment was repeated and extended to one and a half year; Type B perpetuate disease to mice but with delayed onset while Type A no longer perpetuated disease. The rate of bacterial decline was strongly dependent on subspecies, temperature and the number of bacteria in the seeded population. We further explored the persistence of Type A and Type B populations in pond water that corroborate the findings in axenic conditions and show that both strains perpetuate disease from a metabolically inactive persistent state. This study provides time frames for the persistence of planktonic F. tularensis subspecies tularensis and holarctica populations in aquatic conditions, contributing to our understanding of the host-independent survival as a component of the perpetuation of tularemia.