Long-Term Survival of Virulent Tularemia Pathogens outside a Host in Conditions That Mimic Natural Aquatic Environments
Open Access
- 26 February 2021
- journal article
- research article
- Published by American Society for Microbiology in Applied and Environmental Microbiology
- Vol. 87 (6)
- https://doi.org/10.1128/aem.02713-20
Abstract
Francisella tularensis, the causative agent of the zoonotic disease tularemia, can cause seasonal outbreaks of acute febrile illness in humans with disease peaks in late summer to autumn. Interestingly, its mechanisms for environmental persistence between outbreaks are poorly understood. One hypothesis is that F. tularensis forms biofilms in aquatic environments. We utilized two fully virulent wild-type strains: FSC200 (Francisella tularensis subsp. holarctica) and Schu S4 (Francisella tularensis subsp. tularensis) and three control strains, the attenuated live vaccine strain (LVS; F. tularensis subsp. holarctica), a Schu S4 ΔwbtI mutant that is documented to form biofilms, and the low-virulence strain U112 of the closely related species Francisella novicida. Strains were incubated in saline solution (0.9% NaCl) microcosms for 24 weeks at both 4°C and 20°C, whereupon viability and biofilm formation were measured. These temperatures were selected to approximate winter and summer temperatures of fresh water in Scandinavia, respectively. U112 and Schu S4 ΔwbtI formed biofilms, but F. tularensis strains FSC200 and Schu S4 and the LVS did not. All strains exhibited prolonged viability at 4°C compared to 20°C. U112 and FSC200 displayed remarkable long-term persistence at 4°C, with only 1- and 2-fold log reductions, respectively, of viable cells after 24 weeks. Schu S4 exhibited lower survival, yielding no viable cells by week 20. At 24 weeks, cells from FSC200, but not from Schu S4, were still fully virulent in mice. Taken together, these results demonstrate biofilm-independent, long-term survival of pathogenic F. tularensis subsp. holarctica in conditions that mimic overwinter survival in aquatic environments. IMPORTANCE Tularemia, a disease caused by the environmental bacterium Francisella tularensis, is characterized by acute febrile illness. F. tularensis is highly infectious: as few as 10 organisms can cause human disease. Tularemia is not known to be spread from person to person. Rather, all human infections are independently acquired from the environment via the bite of blood-feeding arthropods, ingestion of infected food or water, or inhalation of aerosolized bacteria. Despite the environmental origins of human disease events, the ecological factors governing the long-term persistence of F. tularensis in nature between seasonal human outbreaks are poorly understood. The significance of our research is in identifying conditions that promote long-term survival of fully virulent F. tularensis outside a mammalian host or insect vector. These conditions are similar to those found in natural aquatic environments in winter and provide important new insights on how F. tularensis may persist long-term in the environment.Keywords
Funding Information
- The Swedish Ministry of Defence (A4042)
- DOD | Defense Threat Reduction Agency (HDTRA1-18-1-0037)
This publication has 76 references indexed in Scilit:
- Genome Sequence of Francisella tularensis subspecies holarctica Strain FSC200, Isolated from a Child with TularemiaJournal of Bacteriology, 2012
- Cyclic Di-GMP Stimulates Biofilm Formation and Inhibits Virulence of Francisella novicidaInfection and Immunity, 2012
- Roles for wbtC, wbtI, and kdtA Genes in Lipopolysaccharide Biosynthesis, Protein Glycosylation, Virulence, and Immunogenicity in Francisella tularensis Strain SCHU S4Pathogens, 2012
- Use of Acid Treatment and a Selective Medium To Enhance the Recovery of Francisella tularensis from WaterApplied and Environmental Microbiology, 2011
- Contributions ofFrancisella tularensissubsp.novicidaChitinases and Sec Secretion System to Biofilm Formation on ChitinApplied and Environmental Microbiology, 2010
- Francisella tularensisType A Strains Cause the Rapid Encystment ofAcanthamoeba castellaniiand Survive in Amoebal Cysts for Three Weeks PostinfectionApplied and Environmental Microbiology, 2009
- Diversity of Francisella Species in Environmental Samples from Martha’s Vineyard, MassachusettsMicrobial Ecology, 2009
- Francisella tularensis: an arthropod-borne pathogenVeterinary Research, 2008
- MglA and Igl Proteins Contribute to the Modulation ofFrancisella tularensisLive Vaccine Strain-Containing Phagosomes in Murine MacrophagesInfection and Immunity, 2008
- The complete genome sequence of Francisella tularensis, the causative agent of tularemiaNature Genetics, 2005