Establishment of an unfed strain of the green paramecium, Paramecium bursaria, and analysis of symbiotic bacterial communities controlling proliferation. Bacteria

The ciliate Paramecium bursaria harbors several hundred symbiotic algae in its cell body and is widely used as an experimental model for studying symbiosis between eukaryotic cells. Currently, various types of bacteria and eukaryotic microorganisms are used as food for culturing P. bursaria, and thus the culture conditions are not unified among researchers. To solve this long-standing problem, we have established cloned, unfed strains that can be cultured using only sterile medium without exogenous feeding. The growth of these unfed strains is suppressed in the presence of antibiotics, suggesting that bacteria are required for the growth of the unfed strains. Indeed, several kinds of bacteria of the orders Rhizobiales, Sphingomonadales, Rhodospirillales or Burkholderiales, which are able to fix atmospheric nitrogen and/or to degrade chemical pollutants, are detected in the unfed strains.