Recently Andrew rang in asking which animal eats bacteria.
Bacteria make up a major part of the world's biomass.
Bacteria are really tiny, and are very good at absorbing mineral nutrients from the environment, so they are at the bottom of just about every food chain on the planet. Then there's slightly bigger animals which also feed directly on bacteria. In the sea, filter-feeding animals like fan worms and shellfish can suck bacteria out of the water. There are even species of snail that float around in the water trailing big transparent nets of fine mucus, to trap bacteria and other organisms. On the land, insects like termites have tame bacteria living in their gut to digest wood, and slime moulds can engulf bacteria whole.