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\section{Discussion}  \textit{F. tularensis} is a highly-virulent zoonotic pathogen that causes significant morbidity and mortality globally. To facilitate future advances in our understanding of this important bacterium, we sought to develop an improved insect host system that eliminates both undesirable  biological and logistical trade-offs that accompany other popular host species such as \textit{D. melanogaster} and \textit{G. mellonella}. While insects lack adaptive immune functions, their innate immune systems share similar regulation and effector mechanisms with mammalian innate immune systems \cite{25699030,24392358,23517918,23271509}. Because of this, insects can provide investigators with a host-pathogen interaction system capable of high-throughput that would be either financially or ethically unacceptable in mammals. Importantly, insects also provide investigators at institutions without easy access to mammalian housing facilities an alternative means by which to assess \textit{in vivo} host-pathogen interactions. Finally, insects and other arthropods can be important environmental reservoirs and vectors for numerous zoonotic pathogens, including \textit{F. tularensis}. Thus, insect host systems also aid in illuminating how these microorganisms evade arthropod immune systems during this part of the pathogenic cycle their lifecycle  without the necessity of rearing sanguinivorous insect species arthropods  in the lab. Here, we sought to identify an experimental host for \textit{F. tularensis} that is (1) commercially-available, (2) simple to rear in the laboratory, (3) tolerant of mammalian body temperatures, (4) long-lived with low background mortality, (5) large enough in size to allow consistent delivery of bacterial inoculations using standard needle-syringe combinations, and (6) vigorous enough to withstand multiple injections of bacteria and/or antibiotics. We found that the \textit{B. dubia} OS cockroach satisfied all of these requirements. Like wax moth larvae, OS cockroaches are commercially-available from a large number of suppliers that produce them for the pet industry (primarily used as food for captive reptiles). However, whereas wax moths are relatively difficult to rear in captivity, maintenance of an OS a  cockroach breeding colony in the laboratory is simple and straightforward \cite{7966174, 14272467}. Compared to other species, OS cockroaches are docile and easy to handle. They are relatively slow, remain immobile when placed on their back, do not climb vertical glass or plastic surfaces, and they do not fly. When properly maintained, their rearing containers do not produce an offensive odor.  Given adequate access to water, OS cockroaches can be maintained kept  at temperatures above 37°C (up to 40°C in our studies) indefinitely. OS cockroaches undergo incomplete metamorphosis, with each developmental stage (or instar) lasting between 20 and 45 days. In total, it takes approximately 6 months for OS cockroaches to reach adulthood. The juvenile cockroaches used in this study were infected during the 6th instar (next to last), leaving between 30 and 60 days of experimental observation before they would have molted into the adult stage. In contrast, we often observed that significant percentages (>25\%, considerable fractions (>25\% in some cases,  data not shown) of wax moth larvae pupated during a typical 7 day survival  experiment. Importantly, This is troublesome for host-pathogen interaction studies because wax worm immune responses are known to vary throughout the period leading up to pupation \cite{17198709}. Additionally,  deaths of OS cockroaches are extremely rare in our PBS-only or no-injection control groups (none occurred during the experiments presented here) and here), another characteristic of this model that sets it apart from  the wax worm model, which can also suffer from high levels of background mortality ***********. The  LD_{50} of a non-pathogenic bacterium, \textit{Escherichia coli} DH5$\alpha$, was determined to be nearly 10^{7} CFU (\textbf{Table 1}), indicating that OS cockroaches are capable of mounting protective immune responses to non-pathogenic microorganisms. However, intrahemoceol injection of \textit{F. tularensis} LVS resulted in dose-dependent lethality (\textbf{Figure 1}) with estimated LD_{50}s of two different laboratory stocks of 1.7 x 10^4 CFU and 3.5 x 10^4 CFU (with overlapping confidence intervals; \textbf{Table 1}). Thus \textit{F. tularensis} LVS is able to evade OS immunity and cause dose-dependent establish a lethal  infection in this experimental host. Temperature is known to regulate expression of \textit{F. tularensis} virulence factors \cite{18842136}. One of the advantages of insect models, in comparison with mammals, is the ability to experimentally manipulate the temperature at which host-pathogen interactions occur. When we varied the temperature at which infection took place, we observed that higher temperatures correlated with higher mortality (\textbf{Figure 2, Table 1}). Others have shown that temperature can effect insect immune pathways \cite{23834825, 26040308, 25657206, 24561359, 24002645}, and there may be some differences in the immune response of OS cockroaches infected at different temperatures. However, it is intriguing to consider that, since \textit{F. tularensis} can be spread by environmental arthropods \cite{24057273,21529386,20482589,18950590,22530023,21612530,20885922, 20810833, 21845949}, temperature may provide an important environmental cue that allows \textit{F. tularensis} to dampen virulence pathways that would otherwise kill these vectors before they have an opportunity to transmit the bacterium to another host mammal. Thus, the OS cockroach system is an attractive new platform with which to interrogate the important but understudied environmental stage of the \textit{F. tularensis} lifecycle and the switch between mammalian and arthropod hosts.