BPC stands for bacteria per click.
When the player taps on the main screen, he will produce bacteria in the amount that is at least equal to the current BPC. The game starts with the player having just 1 BPC and it is primarily increased by purchasing Bacteria guns from the bacteria colonies menu.
BPC sources add some number to the base bacteria production per tap.
They include Bacteria gun, which can be purchased from the bacteria colonies menu, and Bacteria cannon, which can be purchased for diamonds in the premium section.
BPC modifiers increase the current BPC by some percentage. They include:
Game mechanics that temporarily boost BPC: