The hive mind bacteria is a type of microscopic life form dwelling on an uncharted planet.
A natural telepath, the microscopic life form had all the classic traits of individual neurons and had colonized deep in the planet’s upper crust, evolving with the other native life forms. Over the course of millions of years the planet-wide colony infected the other native forms, creating a symbiotic life cycle and taking root in their own neural relays until it eventually reached a kind of critical mass until all the life forms were controlled and answerable to the greater biomass. While on their own, no individual creature was sentient, the combined thoughts of every plant, insect and animal linked in a "telesymbiosis" with the bacteria brought about a hive consciousness. The whole planet effectively became a unified organism; flowers telling insects when to pollinate them, wounded animals alerting carrion eaters so the meat is not wasted.
Eventually however, alien races arrived on the planet and, not suspecting what they were walking into, began colonizing the planet, cutting down forests, killing animals for food and otherwise disrupting the order that had existed for millennia. The bacteria responded the way any large organism responds to a foreign infection, by becoming stronger and imposing its telesymbiosis on the aliens, making them lost in telepathic illusion and eventually starving to death.
This cycle kept up until 2262 when Lyta Alexander and G'Kar arrived on the world, tracking the course of an abandoned Psi Corps mothership. Eventually seeing through the illusion, Alexander, with her enhanced abilities, was able to overload the bacteria's neural links, effectively crippling the "hive mind."[1]