Cockroaches join the fight against infection: an improved insect host system for Francisella tularensis

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.