The Red Flour Beetle as a Model for Bacterial Oral Infections

Abstract

Experimental infection systems are important for studying antagonistic interactions and coevolution between hosts and their pathogens. The red flour beetle Tribolium castaneum and the spore-forming bacterial insect pathogen Bacillus thuringiensis (Bt) are widely used and tractable model organisms. However, they have not been employed yet as an efficient experimental system to study host-pathogen interactions. We used a high throughput oral infection protocol to infect T. castaneum insects with coleopteran specific B. thuringiensis bv. tenebrionis (Btt) bacteria. We found that larval mortality depends on the dietary spore concentration and on the duration of exposure to the spores. Furthermore, differential susceptibility of larvae from different T. castaneum populations indicates that the host genetic background influences infection success. The recovery of high numbers of infectious spores from the cadavers indicates successful replication of bacteria in the host and suggests that Btt could establish infectious cycles in T. castaneum in nature. We were able to transfer plasmids from Btt to a non-pathogenic but genetically well-characterised Bt strain, which was thereafter able to successfully infect T. castaneum, suggesting that factors residing on the plasmids are important for the virulence of Btt. The availability of a genetically accessible strain will provide an ideal model for more in-depth analyses of pathogenicity factors during oral infections. Combined with the availability of the full genome sequence of T. castaneum, this system will enable analyses of host responses during infection, as well as addressing basic questions concerning host-parasite coevolution.

Infection of Tribolium castaneum with Bacillus thuringiensis: Quantification of Bacterial Replication within Cadavers, Transmission via Cannibalism, and Inhibition of Spore Germination

“…Depending on the insect population and the applied dietary spore concentration, B. thuringiensis bv. tenebrionis was able to cause considerable mortality in T. castaneum, with up to 100% mortality induced in a laboratory strain, whereas wild-collected populations generally showed higher resistance (13). Furthermore, we previously found that large amounts of spores can be recovered from infected cadavers (13), and in the present study, we investigated the number of spores that needs to be ingested to cause mortality and quantified the reproduction success of B. thuringiensis bv.…”

“…tenebrionis. Previously, we showed that in this system, the infection process is fast and mortality is rapidly induced in susceptible larvae (usually within the first 24 h after exposure) (13). Depending on the insect population and the applied dietary spore concentration, B. thuringiensis bv.…”

“…However, this bacterium is equipped with a diverse arsenal of virulence factors and other adaptations specifically designed for overcoming host defenses and growth and reproduction in cadavers (10,11), indicating that Bt coevolved with its insect hosts. Once ingested by the host, spores germinate in response to nutrients (12,13) and vegetative cells proliferate, contributing to tissue damage and host death (14)(15)(16). When nutrients within the cadaver are exhausted, bacteria resporulate, thereby ensuring survival during unfavorable growth conditions and enabling further transmission upon ingestion by a new host.…”

Fungal disease dynamics in insect societies: Optimal killing rates and the ambivalent effect of high social interaction rates

“…Pathogen-induced mortality is typically a sigmoidal function that connects a baseline value for no pathogen exposure with doses inflicting maximal mortality, as determined in several laboratory-based studies (Hughes et al, 2004;Milutinović et al, 2013;Vestergaard et al, 1995). In principle, our model can capture any form of dose-dependent mortality by assigning a killing rate σ x to every exposure class x.…”

Sustainable farming of the mealworm Tenebrio molitor for the production of food and feed

“…In this context, the broad database available for other tenebrionid beetles is valuable because T. castaneum is considered as a model organism for the investigation of immune priming [134][135][136]. One advantage of T. castaneum is that oral priming is possible by adding pathogens or a corresponding supernatant directly to the diet [137]. The gut microbiota in T. castaneum plays an important role in immune priming, but a cold shock applied to the parents is required for transgenerational immune priming [138,139].…”