Bacterium are creatures in Parasite Eve. An amoeba like creature that is all about long-ranged attacks. They attack by firing a red ball at Aya from afar which will poison her if they come into contact, but they can be easily sidestepped. If you encounter the Bacterium with other NMCs, it is best to target the other NMCs first. This is so that Aya can heal and wait for the PE gauge to fill.