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Another common contaminate from improper wort souring is brewer's yeast (''S. cerevisiae''). Brewer's yeast is not greatly inhibited by ''Lactobacillus''. Conversely, ''Lactobacillus'' is greatly inhibited by the presence of active ''S. cerevisiae'' <ref name="Hubbe">[https://www.facebook.com/groups/MilkTheFunk/permalink/1407620509266159/ Effect of mixed cultures on microbiological development in Berliner Weisse (master thesis). Thomas Hübbe. 2016.]</ref>. The warmer temperatures encourage ''S. cerevisiae'' to ferment the wort quickly. This often results in a beer that isn't sour because the ''Lactobacillus'' are out-competed by the yeast. Signs that yeast has contaminated the wort include the typical signs of yeast fermentation: the presence of a krausen, a gravity shift of more than 1.005 gravity points (or 0.5-1.0° Plato), and looking at a sample of the wort under a microscope. Sources of yeast contamination can come from poor sanitation, but they can also come from the yeast manufacturer themselves. See see [[100%25_Lactobacillus_Fermentation|100% ''Lactobacillus'' fermentation]] for more information.
Another source of contamination, which is arguably desired, happens when using a "wild" source of ''Lactobacillus''. For example, when culturing ''Lactobacillus'' using non-plating techniques from grains, fruit, or from some other fermented food such as kefir, sauerkraut, etc., yeast and other microbes can carry over from the culturing process (see [[Alternative Bacteria Sources]]). The only way to guarantee that only ''Lactobacillus'' from a "wild" source such as these this ferment the wort, is to isolate the microbe using plating techniques (see [[Wild Yeast Isolation]]). Otherwise, there is a chance that wild yeast will also survive the culturing process. See also: * [https://www.facebook.com/groups/MilkTheFunk/permalink/2502846349743564/?comment_id=2502876486407217&reply_comment_id=2506010409427158&comment_tracking=%7B%22tn%22%3A%22R2%22%7D Dr. Bryan Heit's explanation for 'krausen' caused by oxidative yeast and why it does not correspond with a drop in gravity.]
===Preventing Contamination===