W-L Differential (WLD) Medium (Criterion)
W-L Differential Medium is used for isolating bacteria encountered in brewing an industrial fermentation processes. WL Nutrient Medium can support the growth of bacteria, but unless the yeast cell titer is small the bacteria may not be detected. Consequently, WL Differential Medium was created to inhibit the growth of yeasts without inhibiting the growth of bacteria present in beers.
Principles WL Differential Media contain a variety of substances to promote the growth and selection of bacteria and yeast. Yeast extract is a source of trace elements, vitamins and amino acids. Pancreatic digest of casein provides nitrogen, amino acids, and carbon. Dextrose is the source of carbohydrate. Monopotassium phosphate buffers the media. Potassium chloride, calcium chloride, and ferric chloride are essential ions and help to maintain osmotic balance. Manganese sulfate and magnesium sulfate are sources of divalent cations. Bromocresol green is a pH indicator. Agar is the solidifying agent. Actidione (cycloheximide) is added as a selective agent in WL Differential Medium to inhibit the growth of yeasts and mold. Reliable counts for brewers' yeast are obtained with WL Nutrient Medium at pH 5.5. However, a pH adjustment to 6.5 is necessary for obtaining reliable counts of baker's and distiller's yeast.