This game is based on the rules of the cellular automaton ‘Life’ devised by John Horton Conway in 1970.
(You can read more about how it works in greater details here: http://en.wikipedia.org/wiki/Conway's_Game_of_Life )

In 'Bacteria' you have a square game field that is in some form of a stable state. You need to add in a certain number of cells so that, after a few steps, the field will be halted (in colored mode) or even cleared (in "hard" black & white mode). You could draw in single points, or use standard 'Life' patterns like "Gliders" (they move at an angle) and "Lightweight spaceships (LWSS)" (they move in a straight line).

Depending on the difficulty level, the player can use a smaller area to add new cells.