Yeasts Producing Killer Toxins: An Overview

The production of exotoxins with antimicrobial activity on susceptible microorganisms by yeasts is a relatively common phenomenon. Exotoxins (generally proteins or glycoproteins) that are able to kill susceptible cells belonging to the same or congeneric species have been defined as killer toxins. Since first discovered in Saccharomyces cerevisiae , killer strains have been isolated from several yeast genera, including Candida, Cryptococcus, Hanseniaspora , Kluyveromyces, Pichia , Torulopsis , Ustilago , Williopsis and Zygosaccharomyces. Many types of killer toxins have been reported and their genomes were mapped on double-stranded RNA (S. cerevisiae K1, K2, K28,Ustilage maydis and Hanseniaspora uvarum) , a linear double-stranded DNA plasmid (Kluyveromyces lactis, Pichia acaciae and Pichia inositovora) or carried on a chromosome (S. cerevisiae KHS, KHR and Williopsis mrakii). During the last two decades, secreted killer toxins and toxin-producing killer yeasts have found several applications. For instance in the food and fermentation industries,killer yeasts have been used to combat contaminating wild-type yeasts which can occur during the production of wine, beer and bread . Killer yeasts have also been used as bio-control agents in the preservation of foods , in the bio-typing of medically important pathogenic yeasts and yeast-like fungi , in the development of novel antimycotics for the treatment of human and plant fungal infections, and finally in the field of recombinant DNA technology.