The Bacterium is a common type of enemy found only in the bonus Worm level.
They move in a predictable pattern as they patrol the surfaces of the terrain, rolling along contiguous walls. They do not detach from the walls except when the block they are clinging to is destroyed, for example by a bomb.
There are many narrow tunnels and passages in the Worm that can make these enemies very tricky to avoid.
Although bacteria are immune to damage from the shotgun and the whip, they will still bleed when hit. This is one of the methods to exploit the Kapala (besides a Mummy's vomit and corpses in Spikes), but it is much slower.
A Freeze Ray will still freeze them, allowing you to destroy them easily if you can manage to bring one into the Worm.
If they touch the Spelunker, they will self-destruct after dealing damage.