Abstract
BackgroundMalaria control strategies are focusing on new approaches, such as the symbiotic control, which consists in the use of microbial symbionts to prevent parasite development in the mosquito gut and to block the transmission of the infection to humans. Several microbes, bacteria and fungi, have been proposed for malaria or other mosquito-borne diseases control strategies. Among these, the yeast Wickerhamomyces anomalus has been recently isolated from the gut of Anopheles mosquitoes, where it releases a natural antimicrobial toxin. Interestingly, many environmental strains of W. anomalus exert a wide anti-bacterial/fungal activity and some of these ‘killer’ yeasts are already used in industrial applications as food and feed bio-preservation agents. Since a few studies showed that W. anomalus killer strains have antimicrobial effects also against protozoan parasites, the possible anti-plasmodial activity of the yeast was investigated.MethodsA yeast killer toxin (KT), purified through combined chromatographic techniques from a W. anomalus strain isolated from the malaria vector Anopheles stephensi, was tested as an effector molecule to target the sporogonic stages of the rodent malaria parasite Plasmodium berghei, in vitro. Giemsa staining was used to detect morphological damages in zygotes/ookinetes after treatment with the KT. Furthermore, the possible mechanism of action of the KT was investigated pre-incubating the protein with castanospermine, an inhibitor of β-glucanase activity.ResultsA strong anti-plasmodial effect was observed when the P. berghei sporogonic stages were treated with KT, obtaining an inhibition percentage up to around 90 %. Microscopy analysis revealed several ookinete alterations at morphological and structural level, suggesting the direct implication of the KT-enzymatic activity. Moreover, evidences of the reduction of KT activity upon treatment with castanospermine propose a β-glucanase-mediated activity.ConclusionThe results showed the in vitro killing efficacy of a protein produced by a mosquito strain of W. anomalus against malaria parasites. Further studies are required to test the KT activity against the sporogonic stages in vivo, nevertheless this work opens new perspectives for the possible use of killer strains in innovative strategies to impede the development of the malaria parasite in mosquito vectors by the means of microbial symbionts.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-015-1059-7) contains supplementary material, which is available to authorized users.
Highlights
Malaria control strategies are focusing on new approaches, such as the symbiotic control, which consists in the use of microbial symbionts to prevent parasite development in the mosquito gut and to block the transmission of the infection to humans
Purification of killer toxin (KT) The KTs from W. anomalus strains were purified to test their ability to inhibit the development of P. berghei sporogonic stages
Three different strains of W. anomalus growing in conditions stimulating the production of toxin [14] were used: (1) the KT-producer WaF17.12 isolated from An. stephensi mosquitoes; (2) the KT-producer WaATCC 96603, and (3) the non KT-producer WaUM3
Summary
Malaria control strategies are focusing on new approaches, such as the symbiotic control, which consists in the use of microbial symbionts to prevent parasite development in the mosquito gut and to block the transmission of the infection to humans. Malaria is one of the most alarming infectious diseases threatening millions people, mostly in sub-Saharan regions [1, 2] It is caused by Plasmodium protozoan parasites and transmitted by Anopheles mosquitoes. Malaria control programmes involving chemical and pharmacological treatments are not always sustainable due to several factors, such as economic costs and logistic aspects Both vectors and parasites have enhanced resistance against many commonly used pesticides and medicines. Confirmed resistance of 125 mosquito species lead the Malaria Eradication Research Agenda to state that novel control strategies are urgently requested for malaria suppression [4] In this frame, a new tool called “Symbiotic Control” (SC) has been recently proposed. Several microbes have been proposed for malaria-SC, including the bacteria Asaia, Wolbachia, Pantoea agglomerans, Elizabethkingia meningoseptica and the fungi Metarhizium robertsii and Wickerhamomyces anomalus [7,8,9,10,11,12]
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