Modification and elimination of the bacterial flora in cultures of Entamoeba invadens Rodhain, 1934.

1. Cysts, formed in a strain of E. invadens growing with a mixed bacterial flora, were sterilized of all but one species of bacterium (organism x ), by treatment with chemicals. Strains of E. invadens derived from these cysts were prepared, growing with organism x and either Bact. coli or B. megatherium . 2. A monobacterial strain of E. invadens , growing with Bact. coli alone, was obtained by maintaining a dibacterial strain, growing with organism x and Bact. coli , for 72 days in medium containing gentian violet. 3. A monobacterial strain of E. invadens , growing with Staph. albus , was prepared from cysts, produced in the monobacterial strain of E. invadens + Bact. coli , and sterilized by treatment with chemicals. 4. An axenic strain of E. invadens was prepared by maintaining the monobacterial strain of E. invadens + Staph. albus , in saline-liver medium containing antibiotics. This axenic strain has now been maintained for over 14 months in saline containing untreated, sterile liver, but no antibiotics.