Killer Strains are strains of Saccharomyces cerevisiae that contain a killer factor (sometimes also called a “zymocin”), rendering them resistant to infection by other yeasts. This was first discovered by Makower and Bevan in 1963. Killer strains release a glycoprotein-type toxin that is lethal to sensitive yeast strains. The killer factor has since been found in many other species of yeast as well. Of 964 strains of yeast from the National Collection of Yeast Cultures that were tested in 1975 by Philliskirk and Young, 59 strains were killers. Of those, only 3 were classified as ale strains, although more probably these strains were contaminants of otherwise normal brewing yeast. Yeast can be killer (immune), neutral, or sensitive. Some wine yeast strains are killer yeasts as well. To date, no killer factor has been detected in commercial brewers’ yeast strains. However, most brewery strains are sensitive to the killer glycoproteins, which can be carried by wild yeasts. Therefore, should a wild killer strain contaminate a fermentation, it can destroy the culture strain. The only reported case of such an occurrence in the brewing industry was in a continuous fermentation system, where the contaminant killed the brewing yeast culture and then dominated the fermenter. According to Priest and Campbell, if the killer is a Saccharomyces spp. strain, it is particularly difficult to detect the contamination early on. A brewer’s strain can be tested for sensitivity to killer strains in the laboratory by checking whether it can grow in the presence of the killer toxin. If it does, it is considered a neutral strain.