The orange spotted cockroach (Blaptica dubia, Serville 1839) is a permissive experimental host for Francisella tularensis

Abstract

Francisella tularensis is a zoonotic bacterial pathogen that causes severe disease in a wide range of host animals, including humans. Well-developed murine models of F. tularensis pathogenesis are available, but they do not meet the needs of all investigators. Instead, researchers are increasingly turning to insect host systems to: (1) allow high-throughput that is cost-prohibitive or ethically-questionable in mammals; (2) enable studies of host-pathogen interactions when mammalian facilities are unavailable; and (3) provide valuable information about environmental persistence and transmission. However, the utility of previously-described insect hosts is limited because of temperature restriction, short lifespans, and concerns about the immunological status of insects mass-produced for other purposes. Here, we present a novel host species, the orange spotted (OS) cockroach (Blaptica dubia), that overcomes these limitations and is readily infected by F. tularensis. Intrahemocoel inoculation was accomplished using standard laboratory equipment and lethality was directly proportional to the number of bacteria injected. Disease progression differed in insects housed at low and high temperatures, a pattern indicative of a switch between virulence and transmission phenotypes. As in mammalian hosts, F. tularensis mutants lacking key virulence components were attenuated in OS cockroaches. Finally, antibiotics were delivered to infected OS cockroaches by systemic injection and controlled feeding; in the latter case, protection correlated with oral bioavailability in mammals. Collectively, these results demonstrate that this new host system should facilitate discovery of factors that control F. tularensis virulence, immune evasion, and transmission while also providing a platform for early stage drug discovery and development.

The utilization of Blaptica dubia cockroaches as an in vivo model to test antibiotic efficacy

“…We have previously established that Francisella tularensis , Burkholderia pseudomallei , and Burkholderia mallei are lethal to tropical cockroaches at very low doses and that the tissue tropism and genetic requirements for virulence in this model are similar to that seen in mammalian models 8 , 15 . Based on these findings and the fact that other major multi-drug resistant (MDR) bacteria, mycobacteria, and fungi are pathogenic toward other insect species, we hypothesize that a broad range of clinically relevant human pathogens will also establish productive infections in OS cockroaches.…”

“…This suggests that OS cockroaches may be intrinsically resistant to particular bacterial infections. However, these cockroaches succumbed to infection from E. coli and K. pneumoniae [among others 8 ] and may be useful for modeling certain mammalian infections and for preliminary antibiotic characterization studies.…”

“…Insects are now well recognized as biologically relevant alternative hosts for dozens of mammalian pathogens (both bacteria and fungi) and they are routinely used in molecular pathogenesis studies 8 – 22 . Unfortunately, incorporation of these models into the drug development pipeline has not yet occurred in a meaningful way.…”

The Madagascar Hissing Cockroach as an Alternative Non-mammalian Animal Model to Investigate Virulence, Pathogenesis, and Drug Efficacy

“…. For example, the experimental window for a hissing cockroach ranges from months to years allowing flexibility to the researchers, whereas that for a wax worm larva is only five days 19,20 . For a wax worm larva, the five day period also coincides with cocoon encasement; removal of cocoons is a labor intensive process that may cause physical trauma to the larvae 20 .…”

Genetic Determinants of Antibiotic Resistance in Francisella

“…These in vivo models allow for the more complex conditions of a host to be incorporated into the assay. While ticks, biting flies, and mosquitoes are known insect vectors for this disease, recently published insect models for Francisella virulence include Drosophila melanogaster (Ahlund et al, 2010;Moule et al, 2010), the orange spotted cockroach (Eklund et al, 2017), and the wax worm larvae, Galleria mellonella (Aperis et al, 2007;Sprynski et al, 2014;Propst et al, 2016b;Thelaus et al, 2018;Djainal et al, 2020). The correlation between virulence factors in humans and virulence in the waxworm is not exact (Thelaus et al, 2018), thus this model needs to be used with care in interpretation of the results, especially with respect to host-directed virulence.…”