Survival of Tetrahymena thermophila at Low Initial Cell Densities. Effects of Lipids and Long‐Chain Alcohols

. Cells of the ciliate Tetrahymena thermophila failed to establish cultures in lipid‐free standard synthetic nutrient medium if the initial population density was 250 cells per ml or less. These cells died within 10 h, but were saved and formed dense cultures if their medium was supplemented with 10 μg per ml of either certain phospholipids, 1,2‐di‐, 1‐monoglycerides, fatty acids, long‐chain alcohols, or sterols. Cell multiplication was followed in cultures in which the standard synthetic medium was supplemented with a selection of the compounds listed above. It was observed that the cells in the supplemented cultures in their exponential phases of growth had about the same average doubling times as control cells starting multiplication at 10‐fold higher initial cell densities in lipid‐free medium. These cells have been grown for decades in lipid‐free synthetic nutrient media at short (ca. two‐three h) doubling times. Therefore lipids have been considered nutritionally non‐essential for growth and multiplication of these cells. We propose that those compounds that rescue the cells at low cell densities act as “proliferation signals,” sensu lato . This effect of lipids and long‐chain alcohols has so far remained unnoticed.