Subtle alterations in growth medium composition can dramatically alter the percentage of unsaturated fatty acids in the yeast Saccharomyces cerevisiae.

The essence of the scientific method is the production of reproducible results in repeated experiments. Cells of the yeast strain DBY747 normally contain 36% unsaturated fatty acids but suddenly, and initially inexplicably, lipid analysis revealed 72% unsaturated fatty acids in the same strain at the same growth temperature. A comparative lipid analysis of DBY747 grown in YEPD and in a number of different types and batches of Yeast Nitrogen Base media revealed two heretofore unreported phenomena. We provide mass spectroscopy and yeast bioassay evidence suggesting that the increase in lipid unsaturation can be attributed to the presence of the plasticizing agent dioctylphthalate in YNB and bactopeptone packaged in 'new' plastic containers first introduced by Difco some 3-4 years ago. We also demonstrate that L-methionine plays an important role in determining the percentage of unsaturated fatty acids in cells grown in laboratory-produced YNB. The results illustrate a novel aspect of methionine metabolism while at the same time highlighting the need for more stringent control to be exercised by the companies that formulate and package defined media. Copyright 2000 John Wiley & Sons, Ltd.