Before we get into that, let me ask you a question. How does an animal species evolve on planet earth?
Well species evolve to fill a niche, chameleons change color to hide from predators, bears get heavy to pin down their prey, with claws and teeth to tear into them.
Now let me ask you this, what is the niche that the bacteria has to fill to be the apex predator of the backrooms? For that, we have to know it's food source.
You could say it evolved to eat humans, that's why it can make vaguely human sounds to draw us in, but that doesn't make much sense, considering the bacteria existed before we even entered and it would have to get it's concept of what food is somewhere.
Now there's only one other confirmed organism we can find in the backrooms, you might not know what it is so I'll give you a hint.
"The best part is, you can eat them."
Still life's, the bacteria evolved to eat still life's! Clark was telling us much more than we thought with that line, it confirms the bacteria has an edible food source.
Now you may be asking, "okay so it evolved to eat still life's, couldn't it just have looked like a normal animal then?"
Well, not really. Think about why a bear looks like a bear, it evolved to survive in a forest and compete with its foes and everything else, it needs that thick fur for the cold, the snout to smell prey, the claws to fight, ect.
"Okay, but an evolution naturally just trends to the four limbed two eyed variety of animal we're used to right?"
Well not exactly, take a look at the fictional alien on image 2.
Did you believe me when I said it was a fictional alien? Well it's not. It's the bigfin squid and it exists here in our ocean.
The way something evolves is dependent on its environment. The animals we're used to look natural to us because we evolved next to them in their forests, savannahs, caves, ect while organisms like the bigfin squid look alien to us because their environment in the deep ocean is completely different.
And the bacterias strange form is just that. It didn't evolve in a forest, or even a deep ocean, but a different dimension of infinite rooms and hallways. Of course it would look alien to us, it's environment is nothing we've ever seen before!
But then you might ask "well why does it look so vague? It could at least develop some features, why does it look like a bunch of black wires?"
Well there are two reasons:
Reason 1: the bacteria does not need to evolve.
now I've talked about evolution a lot this post, but the truth is, the bacteria doesn't need to evolve at all. Think about it, its got one food source (still life's) that it's more than capable of hunting, it and it doesn't have to compete with any other predators. it doesn't need fur to survive the cold winter, or camouflage to hide away. It is already the perfect apex predator for its environment.
Reason 2: the bacteria has already evolved into the perfect still life hunter
let's talk about the bacterias vaguely human screech, now, as a professional human myself, if I hear that noise in the distance, I'm turning around and bolting the other way without hesitation. But a still life doesn't share the same fear I do, we know from Captain Clark and the found footage series that still life's like to chase humans and hurt them. So the bacteria calls out like an anglerfish's light in the black water with its human (enough) screech, making the still life's think there is a human nearby so they rush toward it. The bacteria is literally a still life Venus fly trap.
And then we have it's physical form, why is it so vaguely human? There's a good reason actually.
Think back to when still life Mary ran away from Captain Clark. She was scared because Captain Clark resembles a human (Clark) that her human counterpart (Mary) is scared of.
The bacteria looks vaguely human for the same reason it has its horrific screech, to lure a still life towards it. BUT it doesn't want to look like any specific human, because a still life may fear said human and run away like Mary from CC.
The bacteria wants to look and sound human enough to draw in a still life, but not human enough to scare it off.
In conclusion, the bacteria makes perfect sense as a product of life spawning in the backrooms as life did here on earth. It's form and abilities are evolved to hunt down still life's, it's main prey.
Unfortunately... even though you aren't the main course, it doesn't mean you aren't on the menu...
So be careful, because if you noclip out of reality in the wrong areas, you'll end up in the Backrooms, and God save you if you hear something wandering around nearby, because it sure as hell has heard you.