They were first encountered in 2369 by the Federation science vessel USS Yosemite, sent to study the plasma streamer. The crew accidentally transported the microbes aboard along with a sample of plasma. When they subjected the plasma to a resonance frequency scan, the microbes responded by rupturing the container and a number of them infected the Yosemite crew. Lieutenant Joshua Kelly attempted to remove the microbes using the transporter's biofilter, but he was unsuccessful and the crew members became trapped in the pattern buffer.
Later, when the USS Enterprise-D arrived to investigate the fate of the Yosemite, Lieutenant Reginald Barclay became infected by the microbes while he was beaming from the Yosemite back to the Enterprise. Barclay also saw large versions of the microbes in the matter stream. Afterward, he began experiencing pain and blue flashes over his body, and examination revealed that he had somehow been exposed to the same high-energy plasma as Kelly.
Once the microbes were discovered, the Enterprise crew found a way to remove them by keeping Barclay suspended in mid-transport until his molecular structure began to degrade. At that point, the nucleonic particle emissions enabled the biofilter to isolate the microbes. During this process, Barclay recognized the microbes he saw in the matter stream as the missing Yosemite crew members, enabling their rescue. The microbes were subsequently returned to the streamer. (TNG: "Realm of Fear")