2020 Volume 30 Issue 1-2 Pages 38-46
In parasitic unicellular eukaryotes, the tolerance to the pH (1-1.5) of gastric acid is a crucial survival strategy so that they can proliferate in the intestinal tract. We found that the resting cysts of non-parasitic soil ciliate Colpoda cucullus Nag-1 showed a strong tolerance to both extremely low and high pH. The purpose of this study was to explore the tolerance mechanism to extreme pH. Most cysts were alive after exposure to 0.1 M HCl (pH 1) for 4h, or after exposure to 0.1 M NaOH (pH 13) for 3h. Such tolerance to extreme pH is acquired gradually over several days after encystment induction. The resting cysts were reversibly dehydrated by osmotic pressure when they were transferred from water to 0.1 M HCl or 0.1 M NaOH. This result suggests that H+/Cl- and Na+/OH- may diffuse through the cyst wall to reach the plasma membrane. Acid tolerance was reduced in the presence of protonophore (CCCP), suggesting that less permeability of the cyst plasma membrane to H+ may be responsible for acid tolerance.