Certain yeasts can be tolerant or intolerant of various conditions or chemical compounds. These properties can be used to gain information about the possible identity of an unknown yeast – by attempting to plate it on a range of plates containing various additions, and observing which plates allow growth and which ones inhibit growth. Often the information gained from the differential plating can be combined with information on the yeast morphology gained through microscopy, to reach a tentative identification, or at least to eliminate some possibilities. While there are many possible types of media to be used in differential plating, in practice when looking at wine samples, using plates with and without the chemical cycloheximide is a simple way to gain useful information. The techniques used are exactly the same as those described in the section on viable plating, however the interpretation provides information on possible identification.
For example, Saccharomyces cerevisiae is the yeast species most commonly associated with alcoholic fermentation in wine. It is therefore the most common yeast isolated from wine samples. Saccharomyces cerevisiae is not tolerant of cycloheximide, so any yeast isolated from wine that is able to grow on a plate containing cycloheximide can be immediately classified as non-Saccharomyces, and may therefore be of concern to the winemaker.
On the other hand, if a wine sample is plated out on two types of media: one containing cycloheximide and one without cycloheximide, and yeast cells with spherical to ovoid morphology are found to grow only on the media without cycloheximide, it is very likely that the isolated yeast is Saccharomyces which may not be of such concern to the winemaker.
Differential plating can be particulary useful in attempting to identify the spoilage yeast Dekkera/Brettanomyces in wine samples. Dekkera/Brettanomyces is a yeast that is tolerant of cycloheximide, and that also has quite a distinctive elongated or variable morphology. This means that any elongated or variable morphology yeast isolated from wine and able to grow on a cycloheximide-containing media is quite likely to be Dekkera/Brettanomyces. Dekkera/Brettanomyces is capable of causing significant spoilage in wine, so the identification of a viable population of this yeast in a wine could trigger measures such as filtration or addition of extra SO2 to attempt to reduce the population and avoid spoilage. More information about Dekkera/Brettanomyces spoilage and control measures can be found in AWRI paper S 756, available here.