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.
When going into the valve room on the first floor of the Hospital a mini-boss version of the Bacterium is encountered. Unlike the regular Bacterium though it is twice the size and instead of long-ranged attacks it will use it's body to attack. When fighting the mini-boss Bacterium it will jump at Aya and try to crush her. There is no good way to dodge this attack but the best way to try and avoid it is to stay away from the corners of the room or to dodge to the side of the Bacterium when it is about to jump. Another thing to note is that this Bacterium is more resilient to cyanide, tranquilizer, and confusion but still shares the same weakness.