Host-microbe interactions between the tsetse fly (Glossina morsitans) and its secondary symbiont Sodalis glossinidius and their impact on trypanosome parasites

Tsetse flies (Glossina spp.) are medically and agriculturally important vectors that transmit Trypanosoma spp. parasites responsible for human sleeping sickness and animal African trypanosomiasis. The tsetse fly is colonized by a low diversity microbiota with only two symbiotic bacterial species (family Enterobacteriaceae) present in the midgut and other tsetse tissues: an anciently associated mutualist Wigglesworthia glossinidia and a more recently established secondary symbiont Sodalis glossinidius – the latter which appears in many tsetse tissues in close proximity to the trypanosome parasite. So far, it has been experimentally challenging to study the specific role of these bacterial symbionts on the tsetse fly biology. In contrast to the obligate symbiont W. glossinidia, our current knowledge on how S. glossinidius affects tsetse at the molecular level is very limited. Here, important questions that remain to be answered are how the S. glossinidius population is controlled in the tsetse fly and whether S. glossinidius can affect the vector competence for the trypanosome parasite –which is still a highly controversial issue in the literature. In this PhD study, we successfully established for the first time a tsetse fly line that is specifically devoid of S. glossinidius (Chapter 3). This proved to be a useful tool in the investigation of how S. glossinidius influences the tsetse fly biology. Comparison of host biological parameters showed that S. glossinidius presence/absence did not affect fly survival but symbiont presence resulted in a decreased female fecundity. Through comparative transcriptomic analysis, we found that S. glossinidius presence/absence did not strongly affect the expression of tsetse innate immunity-related genes. Only when exposing S. glossinidius-free flies to at least 106 colony-forming units of S. glossinidius, a moderate immune activation could be observed. This was in sharp contrast when flies were exposed to pathogenic E. coli bacteria or to a closely related non-tsetse derived Sodalis strain - both leading to high immunity gene expression levels. Moreover, our results indicate the ability of S. glossinidius bacteria to have limited host immune detection as well as to resist host immune activity, two mechanisms likely to be involved in the maintenance of the tsetse-S. glossinidius association (Chapter 4). Using a comparative metabolomic analysis we identified increased levels of pterins and queuine related compounds, suggesting the symbiont might be providing these for the tsetse host, but also depletions of carbohydrates (e.g. sorbitol) and a purine precursor (i.e. inosine monophosphate; Chapter 5). Here, we also assessed the possible impact of S. glossinidius on the vector competence for African trypanosome parasites. We found that S. glossinidius does not affect the susceptibility of young flies to establish an infection with Trypanosoma brucei or T. congolense. In contrast, older S. glossinidius-harbouring flies showed an increased resistance against infection when flies were exposed to a nutritional stress. The trypanosome infection was increased in flies maintained on the above mentioned purine precursor, making us postulate that the S. glossinidius symbiont and the trypanosome parasite are potentially in a nutrient competition in the tsetse fly environment. In sum, this PhD contributes to the improvement of our knowledge on the interactions of the S. glossinidius symbiont with its tsetse fly host (including immunity and metabolism) and provides novel insights in how S. glossinidius might impact the tsetse fly vector competence for the African trypanosome parasite.