A Wickerhamomyces anomalus Killer Strain in the Malaria Vector Anopheles stephensi

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.

Isolation of aWickerhamomyces anomalusyeast strain from the sandflyPhlebotomus perniciosus, displaying the killer phenotype

“…As previously reported for the mosquito An. stephensi (Cappelli et al , ), the present results show that W. anomalus from P. perniciosus inhibits the growth of selected fungi, and can thus be regarded as a killer yeast that is probably able to produce a killer toxin. Further studies should investigate whether killer toxin‐producing strains of W. anomalus are commonly found in wild populations of P. perniciosus and if they possess anti‐ Leishmania activity.…”

“…(Diptera: Drosophilidae) (Zacchi & Vaughan‐Martini, ), Doubledaya bucculenta beetles (Coleoptera: Erotylidae) (Toki et al , ) and from different mosquito species (Ricci et al , ). Interestingly, one of the strains isolated from the malaria mosquito vector Anopheles stephensi (Diptera: Culicidae) has been shown to produce a killer toxin active against other yeast species (Cappelli et al , ). This result may suggest a potential involvement of W. anomalus in protection against pathogens in mosquitoes, as previously reported for the crab Portunus trituberculatus (Decapoda: Portunidae) (Wang et al , )…”

“…Some W. anomalus strains have demonstrated antifungal activity and, more generally, have been shown to inhibit harmful microorganisms in a great variety of habitats (Polonelli, ; Cappelli et al , ). To establish whether the strain Wa _1F1 isolated from P. perniciosus possesses antifungal activity, susceptible yeast strains were submitted to specific killer phenotype assays.…”

Mosquitoes can harbour yeasts of clinical significance and contribute to their environmental dissemination

“…Interestingly, insects may represent reservoirs of yeasts that are somehow involved in the host's nourishment and/or defence. An example is the killer yeast Wickerhamomyces anomalus , isolated from beetles, sand‐flies and several mosquito species, where it exerts important functions for the development or the antimicrobial protection of the host (Ricci et al ., ; Toki et al ., ; Cappelli et al ., ; Martin et al ., ; Valzano et al ., ). Notably, our findings in laboratory reared colonies supported a previous study in wild Culex sp.…”

The mycobiota of the sand fly Phlebotomus perniciosus: Involvement of yeast symbionts in uric acid metabolism

“…or viruses). Indeed, as in the case of other yeast species (e.g., Wickerhamomyces anomalus , symbiont of arthropod vectors, including sand flies), M. guilliermondii likely compete for nutrients or could be a source of enzymes, or could protect the host insect against infectious agents (Wang et al ., ; Cappelli et al ., ). As already demonstrated for bacteria, yeasts can also benefit insects by promoting the development of other mutualistic and beneficial organisms in the community, while decreasing the presence of insect pathogens (Zindel et al ., ).…”