Abstract
Francisella tularensis, a highly virulent bacteria that causes the zoonotic disease tularemia, is considered a potential agent of biological warfare and bioterrorism. Although the host range for several species within the Francisella is known, little is known about the natural reservoirs of various Francisella species. The lack of knowledge regarding the environmental fates of these pathogens greatly reduces the possibilities for microbial risk assessments. The greater wax moth (Galleria mellonella) is an insect of the order Lepidoptera that has been used as an alternative model to study microbial infection during recent years. The aim of this study was to evaluate G. mellonella as a model system for studies of human pathogenic and closely related opportunistic and non-pathogenic strains within the Francisella genus. The employed G. mellonella larvae model demonstrated differences in lethality between human pathogenic and human non-pathogenic or opportunistic Francisella species. The F. novicida, F. hispaniensis and F. philomiragia strains were significantly more virulent in the G. mellonella model than the strains of human pathogens F. t. holarctica and F. t. tularensis. Our data show that G. mellonella is a possible in vivo model of insect immunity for studies of both opportunistic and virulent lineages of Francisella spp., that produces inverse results regarding lethality in G. mellonella and incapacitating disease in humans. The results provide insight into the potential host specificity of F. tularensis and closely related members of the same genus, thus increasing our present understanding of Francisella spp. ecology.
Highlights
Francisella tularensis is a highly virulent bacteria that causes the zoonotic disease tularemia
The bacterial load of F. philomiragia in hemolymph upon larval death was noticeably lower than what was observed for other Francisella species, not exceeding 108 colony forming units (CFU)/mL, which corresponds to 3.5 × 106 bacteria per larva at the time of death (Figure 3). These results suggest that the bacterial load increased by 104 over 2 days in samples infected with F. novicida and F. hispaniensis, by 104 over 5 days in samples infected with F. t. holarctica, and 102 over 2 days in samples infected with F. philomiragia
Our findings suggest that phylogenetically close to the human pathogenic strains, the F. novicida, F. hispaniensis, and F. philomiragia are more adapted for rapid growth in the insect host model
Summary
Francisella tularensis is a highly virulent bacteria that causes the zoonotic disease tularemia. This pathogen is considered a potential agent of biological warfare and bioterrorism. As such, it is classified as a Tier 1 select agent (Federal Register, 2012). Both F. tularensis subspecies holarctica and subspecies tularensis cause disease in humans (Penn, 2010). Mediasiatica, which is primarily found in Central Asia, in humans is limited (Timofeev et al, 2017). Besides F. tularensis, the genus Francisella includes opportunistic pathogens that only cause disease in immunocompromised humans
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