The USS Enterprise encountered the space amoeba in 2268 while en route to Starbase 6, after diverting to investigate the destruction of an inhabited planet in the Gamma 7A system and the all-Vulcan starship USS Intrepid.
The Enterprise crew discovered that the negative energy field was toxic to humanoid life. They also found that the organism would soon reproduce by cell division. In view of the danger posed by the organism and its multiplication across the galaxy, the Enterprise crew used an antimatter charge to destroy it. (TOS: "The Immunity Syndrome")
The space amoebe was created by Frank Van der veer of Van der Veer Photo Effects. It was made by placing a colored liquid (most likely mineral oil) of several layers between two closely spaced sheets of glass, and by moving the two glasses, making the liquid undulate a little - to look like the "creature" is alive. (Star Trek: The Original Series 365, p. 236)