Characterization of bacteria expectorated during forced salivation of the Phlebotomus papatasi: A neglected component of sand fly infectious inoculums.

The infectious inoculum of a sand fly, apart from its metacyclic promastigotes, is composed of factors derived from both the parasite and the vector. Vector-derived factors, including salivary proteins and the gut microbiota, are essential for the establishment and enhancement of infection. However, the type and the number of bacteria egested during salivation is unclear. In the present study, sand flies of Phlebotomus papatasi were gathered from three locations in hyperendemic focus of zoonotic cutaneous leishmaniasis (ZCL) in Isfahan Province, Iran. By using the forced salivation assay and targeting the 16S rRNA barcode gene, egested bacteria were characterized in 99 (44%) out of 224 sand flies. Culture-dependent and culture-independent methods identified the members of Enterobacter cloacae and Spiroplasma species as dominant taxa, respectively. Ten top genera of Spiroplasma, Ralstonia, Acinetobacter, Reyranella, Undibacterium, Bryobacter, Corynebacterium, Cutibacterium, Psychrobacter, and Wolbachia constituted >80% of the saliva microbiome. Phylogenetic analysis displayed the presence of only one bacterial species for the Spiroplasma, Ralstonia, Reyranella, Bryobacter and Wolbachia, two distinct species for Cutibacterium, three for Undibacterium and Psychrobacter, 16 for Acinetobacter, and 27 for Corynebacterium, in the saliva. The abundance of microbes in P. papatasi saliva was determined by incorporating the data on the read counts and the copy number of 16S rRNA gene, about 9,000 bacterial cells, per sand fly. Both microbiological and metagenomic data indicate that bacteria are constant companions of Leishmania, from the intestine of the vector to the vertebrate host. This is the first forced salivation experiment in a sand fly, addressing key questions on infectious bite and competent vectors. Author summary: Female sand flies salivate during feeding on vertebrate blood and natural sugars. During salivation, they may release microorganisms associated with the salivary glands and digestive tract, i.e. viruses, bacteria, and other pathogens, into the feeding substrates. However, the type and the number of bacteria egested during salivation is unclear. Here, by sing the forced salivation experiment and targeting the 16S rRNA barcode gene, egested bacteria were characterized in sand flies. Culture-dependent and culture-independent methods identified the members of Enterobacter cloacae and Spiroplasma species as dominant taxa, respectively. The abundance of microbes in the saliva of each P. papatasi sand fly was determined to be around 9,000 bacterial cells. The lack of a vaccine and the failure of treatment in leishmaniasis provide many motivations to better understand the factors cause the inflammatory response. The findings of this study can improve our insight into measuring the effect of vector-derived bacteria on the improvement or deterioration of leishmaniasis. [ABSTRACT FROM AUTHOR]