Abstract
The yeast Wickerhamomyces anomalus has been investigated for several years for its wide biotechnological potential, especially for applications in the food industry. Specifically, the antimicrobial activity of this yeast, associated with the production of Killer Toxins (KTs), has attracted a great deal of attention. The strains of W. anomalus able to produce KTs, called “killer” yeasts, have been shown to be highly competitive in the environment. Different W. anomalus strains have been isolated from diverse habitats and recently even from insects. In the malaria mosquito vector Anopheles stephensi these yeasts have been detected in the midgut and gonads. Here we show that the strain of W. anomalus isolated from An. stephensi, namely WaF17.12, is a killer yeast able to produce a KT in a cell-free medium (in vitro) as well as in the mosquito body (in vivo). We showed a constant production of WaF17.12-KT over time, after stimulation of toxin secretion in yeast cultures and reintroduction of the activated cells into the mosquito through the diet. Furthermore, the antimicrobial activity of WaF17.12-KT has been demonstrated in vitro against sensitive microbes, showing that strain WaF17.12 releases a functional toxin. The mosquito-associated yeast WaF17.12 thus possesses an antimicrobial activity, which makes this yeast worthy of further investigations, in view of its potential as an agent for the symbiotic control of malaria.
Highlights
Wickerhamomyces anomalus is a Saccharomycetes yeast with wide biotechnological potential [1], traditionally used in the agro-food sector, as a biopreservation agent, suitable to improve feed and food safety
It has been shown to be associated with the malaria vector Anopheles stephensi, where the yeast is present in the midgut as well as in the gonads [25].The association between Anopheles mosquitoes and W. anomalus deserves to be further investigated, in order to explore the possibility that this yeast could be developed as an agent for the symbiotic control (SC) of malaria [26], in addition to other candidates for SC, such as the bacteria Asaia spp. and Wolbachia, and the fungus Metarhizium robertsii [27,28,29].The objective of this work is to investigate whether the W. anomalus strain isolated from An. stephensi displays killer activity against model microorganisms, and whether it produces a Killer Toxins (KTs) antimicrobial molecule within the mosquito body
The monoclonal antibody used for labelling was directed against a specific W. anomalus KTs (WaKTs), produced by the reference strain WaATCC 96603 and well characterised for its wide antimicrobial activity; Monoclonal Antibody KT4 (mAbKT4) has already been shown to cross-react with the toxin produced by other strains of killer yeasts [14,30]
Summary
Wickerhamomyces anomalus (formerly Hansenula anomala and Pichia anomala) is a Saccharomycetes yeast with wide biotechnological potential [1], traditionally used in the agro-food sector, as a biopreservation agent, suitable to improve feed and food safety. W. anomalus is highly tolerant to environmental stresses, and is adapted to a wide range of growth conditions, in terms of temperature (3–37uC), pH value (2–12) and osmolarity [3]. This robustness makes this yeast highly competitive in many different habitats. It has been isolated from very diverse sources, including flowering plants, fruit skins, dairy and baked food products, contaminated oil, salted foods, wastewater, marine environments, human tissues and even the gut of insects (flies, beetles and mosquitoes) [1]
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