For what it's worth, cracker64 has created Game of Life particles, which behave like living things according to John Conway's Game of Life. Not really what you're looking for, but related nonetheless.
pilojo Stop commenting on things you have no idea about. How exactly would this be a "hard" element to code? We're not creating a living thing here, you know.
1) Make something that moves like noble gas, but is green. 2) When it touches STKM, remove health. 3) Has a LIFE counter set for x frames. 4) When WATR, FIRE, ACID touches bacteria, bacteria dies. 5) If bacteria reaches 50 C, it dies. 6) Bacteria randomly spawns another bacteria.
Tell me which one of these bullets would take the "days of work" you always seem to think the simplest element would take. Try thinking before posting.
EDIT: Sorry for double-posting, completely forgot about edit. And I support bacteria.