Towards symbiont-based visceral leishmaniosis control: Delftia tsuruhatensis TC1 shows anti-Leishmania infantum effect in Phlebotomus perniciosus

Introduction Visceral leishmaniosis caused by Leishmania infantum is a grave disease solely transmitted by vector sand flies with a high risk of mortality when left untreated. The introduction of symbiont bacteria with anti-parasitic properties within vector species represents a promising biological control strategy for vector-borne diseases. In this context, the symbiont bacterium Delftia tsuruhatensis strain TC1 (TC1) has previously demonstrated an in vivo antiparasitic effect in the sand fly Phlebotomus duboscqi against Leishmania major , the causative agent of cutaneous leishmaniosis. The current research aims to evaluate the anti- L. infantum effect of TC1 within Phlebotomus perniciosus , a major vector of visceral leishmaniosis in Southern Europe. Methods Newly emerged Ph. perniciosus females were fed on a sugar meal spiked with TC1. Bacterial midgut colonization was evaluated through plating at different time points after bacteria ingestion in selective media and molecularly verified. TC1-fed sand flies were offered an L. infantum infectious blood meal 24 h after spiked-sugar was removed. The anti- L. infantum effect of TC1 was evaluated by quantifying the number of parasites per midgut and evaluating the infection rate of the stomodeal valve in comparison with control groups. Additionally, the anti- L. infantum effect of TC1 cell-free supernatant and harmane, a β-carboline alkaloid secreted by TC1 and previously reported to display in vitro anti-leishmanial activity, were evaluated in vitro through dose-response curves assays and by quantification of promastigotes within the midgut of sand flies. Leishmania infantum viscerotropic strains were used during assays. Results TC1 was confirmed to colonize and persist within the midgut of female Ph. perniciosus up to at least 21 days after ingestion. TC1-fed sand flies showed a decrease in the number of promastigotes per midgut, as well as a reduction in the infection rate of the stomodeal valve. Both harmane and TC1-supernatant showed an anti- L. infantum effect under in vitro conditions, although no reduction in parasite counts was detected within sand flies. Discussion/conclusion TC1 reduces the proliferation and development of L. infantum within the vector Ph. perniciosus . Our results show the potential of TC1 as a promising tool for controlling L. infantum in visceral leishmaniosis endemic areas in the future.