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Certain yeast strains, known as killer strains (K), secrete protein toxins into their environment that are capable of killing other, sensitive strains (S). The killer strains are not sensitive to their own toxin but can be killed by a toxin that they do not produce. Additionally, neutral strains (N) do not produce a toxin but are resistant. The action of a killer strain on a sensitive strain is easy to demonstrate in the laboratory on an agar culture medium at pH 4.2–4.7 at 20°C. The sensitive strain is inoculated into the mass of agar before it solidifies; then the strain to be tested is inoculated in streaks on the solidified medium. If it is a killer strain, a clear zone in which the sensitive strain cannot grow encircles the inoculum streaks (Figure 1.17).

FIGURE 1.16 Genome renewal of a homozygote yeast strain for the HO gene of homothallism, having accumulated recessive mutations during vegetative reproduction (Mortimer et al., 1994). (a and b), mutation of certain genes.

This phenomenon, known as the killer factor, was discovered in S. cerevisiae, but killer strains also exist in other yeast genera such as Hansenula, Candida, Kloeckera, Hanseniaspora, Pichia, Torulopsis, Kluyveromyces, and Debaryomyces. Killer yeasts have been classified into 11 groups according to the sensitivity reaction between strains as well as the nature and properties of the toxins involved. The killer factor is a cell interaction model mediated by the excreted protein toxin. It has given rise to much fundamental research (Tipper and Bostian, 1984; Young, 1987). Barre (1984, 1992), Radler (1988), and Van Vuuren and Jacobs (1992) have described the technological implications of this phenomenon for wine yeasts and the fermentation process.