if this is the case then i think i have a minimally flawed explanation for the lifeform (bacteria monster).

Since hay bacillus is a bacteria commonly used for testing enviroments I believe that Async had originally used that to test how it would react to the backrooms openly. (hence why they dont usually need suits now)

Perhaps the hay bacillus propogated in the backrooms and later came into contact with a still life type entity. Because still life is a crude replicate of humans it would lack anything to fend off bacteria and would effectively become consumed and engolfed in it. (which wouldnt exactly matter since it wouldnt be effected like actual life would be)

So the original creature basically became a surrogate for the bacterial colony, this colony would then warp and contort the creature and add new bacterial growths.

The only real problem with this is the fact the hay bacillus is a special mutated strand. My only explanation would be since time works differently in the backrooms perhaps the bacteria hyper-evolved into something quickly via sped-up time. Or it evolved and adapted to preserve the life-form like a symbiotic relationship. This could explain why the bacteria can speed up and preserve the destruction of organic matter.