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Beer-spoiling yeasts and killer toxins

Some beer-spoiling yeast strains are capable of producing proteins (killer factors) that are toxic and are able to kill a wide range of other (sensitive) yeasts. Toxin-secreting strains are referred to as killer yeasts and their presence within the brewery can have devastating effects. Typically, a killer yeast will rapidly replace the production strain and become the dominant organism in the fermentation (Young, 1987). Although killer yeasts are not particularly frequent contaminants within the brewing industry, killer activity is surprisingly common across many yeast species. A survey of 148 species comprising 964 strains from the NCYC collection indicated that 59 produced killer factors, the majority of which were Saccharomyces strains (Philliskirk and Young, 1975). However, killer activity can also be observed in other species and genera, including Barnettozyma californica, Hanse- niaspora uvarum, Kluyveromyces lactis, Pichia spp., and Zygosaccharomyces bailii.

The killer phenotype is caused by several different mechanisms. In Saccharomyces strains and some other species including Z. bailii, it is believed to arise due to cytoplasmic infection by a doublestranded RNA (dsRNA) virus, while in other yeasts (including K. lactis) it is encoded for by a linear dsDNA plasmid, also located in the cytoplasm (Stark et al., 1990). A third variation is seen in yeast species including L. saturnus, P. kluyveri, and H. uvarum, whereby the toxin is encoded chromosomally within the nucleus (Kimura et al., 1993; Radler et al., 1985). The best categorized killer strains belong to the first type, the virally induced killer phenotypes. These are caused by an infection with dsRNA viruses belonging to the Totiviridae family, which are widely distributed amongst yeast and higher fungi. In the majority of instances, the virus is inherited cytoplasmically, spreading horizontally during sexual reproduction. In Saccharomyces strains there are at least four major types of killer virus known as ScV-M1, ScV-M2, ScV-M28, and ScV-Mlus. Each encodes a specific killer toxin referred to as K1, K2, K28 and Klus, respectively, as well as a self-protective immunity component (Schmitt and Breinig, 2006; Rodriguez-Cousino et al., 2011). The result of this is that members of each group are able to kill sensitive strains (as well as killer yeasts belonging to any of the other types), but are immune to toxins produced by strains of the same killer type. Other yeasts can give rise to different toxins, for example in Lindnera species four toxins have been observed known as HMK, K500, WmKT, and wicaltin (Theisen et al., 2000; Yamamoto et al., 1986), while Z. bailii killer yeast can produce an antifungal called zygocin (Radler et al., 1993). There is also considerable variation within species; Saccharomyces yeasts can also produce toxins designated KHR and KHS, which are encoded on the chromosomal DNA. Irrespective of their origin, viral toxins typically kill sensitive cells using one of several different modes of action, the most common of which appear to target either cell membrane function or DNA synthesis. This is certainly the case for K28, which causes inhibition of DNA synthesis, and K1, which causes disruption of the plasma membrane, resulting in the formation of ion channels and thus ion leakage. Other effects include cell cycle arrest in G1 as seen in response to zygocin and inhibition of ^-1-3-glucan synthesis, which is associated with the WmKT toxin.

 
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