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Background  The peritrophic matrix (PM) is an acellular chitin-containing envelope which in most blood sucking insects encloses the ingested blood meal and protects the midgut epithelium. Type I PM present in sand flies and other blood sucking batch feeders is secreted around the meal by the entire midgut in response to feeding. Here we tested the hypothesis that in  Sergentomyia schwetzi  the PM creates a physical barrier that prevents escape of  Leishmania  parasites from the endoperitrophic space.    Methodology/Principal findings  Morphology and ultrastructure of the PM as well the production of endogenous chitinase in  S .  schwetzi  were compared with three sand fly species, which are natural vectors of  Leishmania . Long persistence of the PM in  S .  schwetzi  was not accompanied by different morphology or decreased production of chitinase. To confirm the role of the PM in refractoriness of  S .  schwetzi  to  Leishmania  parasites, culture supernatant from the fungus  Beauveria bassiana  containing chitinase was added to the infective bloodmeal to disintegrate the PM artificially. In females treated with  B. bassiana  culture supernatants the PM was weakened and permeable, lacking multilayered inner structure;  Leishmania  colonized the midgut and the stomodeal valve and produced metacyclic forms. In control females  Leishmania  infections were lost during defecation.    Conclusions/Significance  Persistence of the PM till defecation of the bloodmeal represents an important factor responsible for refractoriness of  S .  schwetzi  to  Leishmania  development.  Leishmania major  as well as  L .  donovani  promastigotes survived defecation and developed late-stage infections only in females with PM disintegrated artificially by  B .  bassiana  culture supernatants containing exogenous chitinase.

Refractoriness of Sergentomyia schwetzi to Leishmania spp. is mediated by the peritrophic matrix