Raoultella planticola was a bacterium.
In 2017, Agrofuel Research Operations genetically modified an sample of the bacteria using the enzyme EC 31. It was meant to break down the root system of a plant. Due to a misplaced decimal point, the bacteria became world-ending by killing life forms completely and turning it into "gunk".
The bacteria produced ethanol, which filled the greenhouse, as well as the lab. When the lab's vent systems cycled, it would be released into the Earth's atmosphere and destroy the world within a year as predicted by the Monks. The Twelfth Doctor destroyed the bacteria and saved the Earth by blowing up half of the lab the bacteria was created in, the fire burning the bacteria away. (TV: The Pyramid at the End of the World)
While aiding the Doctor, his companion Nardole is contaminated by the bacteria. Due to his alien physiology, he survives but it leaves him too sick to do anything for six weeks. (TV: The Pyramid at the End of the World, The Lie of the Land)
Behind the scenes
Although the complete name of the bacteria is not given on-screen, the name "R. planticola" can be seen on the screen of Agrofuel's computer.