Background Sodalis glossinidius, a vertically transmitted microbial symbiont of the tsetse fly, is currently considered as a potential delivery system for anti-trypanosomal components that reduce or eliminate the capability of the tsetse fly host to transmit parasitic trypanosomes, an approach also known as paratransgenesis. An essential step in developing paratransgenic tsetse is the stable colonization of adult flies and their progeny with recombinant Sodalis bacteria, expressing trypanocidal effector molecules in tissues where the parasite resides.ResultsIn this study, Sodalis was tested for its ability to deliver functional anti-trypanosome nanobodies (Nbs) in Glossina morsitans morsitans. We characterized the in vitro and in vivo stability of recombinant Sodalis (recSodalis) expressing a potent trypanolytic nanobody, i.e. Nb_An46. We show that recSodalis is competitive with WT Sodalis in in vivo conditions and that tsetse flies transiently cleared of their endogenous WT Sodalis population can be successfully repopulated with recSodalis at high densities. In addition, vertical transmission to the offspring was observed. Finally, we demonstrated that recSodalis expressed significant levels (ng range) of functional Nb_An46 in different tsetse fly tissues, including the midgut where an important developmental stage of the trypanosome parasite occurs.ConclusionsWe demonstrated the proof-of-concept that the Sodalis symbiont can be genetically engineered to express and release significant amounts of functional anti-trypanosome Nbs in different tissues of the tsetse fly. The application of this innovative concept of using pathogen-targeting nanobodies delivered by insect symbiotic bacteria could be extended to other vector-pathogen systems.