Bacteria are pretty awesome. (Here's a bunch of cool things they can do, as written by me)
They can live pretty much anywhere. So... why don't they live everywhere? Why isn't the whole world covered in a deep layer of bacterial ooze? They can grow exponentially, so why don't they? Why can I freely type this blog post without having to shovel a path to my computer through a sludgey oozing mass?
I am a big blob of cells. They form interesting shapes and combine into exciting organs, but I'm still a big blob of cells.If you took my volume, whatever that may be, and filled it with bacterial cells, they wouldn't be as happy as my cells are now. But why?
We have organs for a reason . Tissues too. Look at blood vessels; without capillaries stretching throughout our entire bodies, our cells wouldn't have enough oxygen or nutrients or whatever and they would die. That's why we die when our heart stops or the blood flow is blocked or whatever. In a big gooey mass of bacteria, the deeper cells would see similar problems. They might not need oxygen, but the nutrients at the surface wouldn't all make it down to the ones in the depths. You don't even need a human sized glob of bacteria for this to be true either; on agar plates you see all sorts of strange looking structures being formed as bacteria grow on top of their starving brothers and sisters. Old plates forgotten in the back of the incubator end up looking really different to when they have fresh healthy growth, for example.
Some bacterial structures, like certain biofilms, have complex morphologies to improve nutrient flow, but it's not as efficient or specialised as blood vessels so it still has limitations.
"But wait!" I hear you cry. "Don't you get those massive tanks full of bacteria for industrial scale production of biochemicals? How do they stay nutriented?"
Good question! Firstly, nutriented isn't a word, but I know what you mean. Those big tanks stir the bacteria constantly, and pump nutrients in all the time, so there's much better distribution than in natural stationary formations, allowing them to get much bigger. The huge tanks run into another problem, however; heat.
If you fit a thousand people into a small sports hall or something, it gets pretty warm. All those bodies giving off heat in one space heat up the surroundings. Incidentally that's one of the reasons battery farming is used; the animals warm each other up so can spend more energy on growing meat and laying eggs. But imagine there are billions of people, all in the same space, with little to no room in between them! That will warm up pretty quickly. Enough body heat in one place can raise the temperature enough to kill; that's actually a defence mechanism used by Japanese honey bees to kill invading hornets; they bundle onto the hornet and vibrate furiously, cooking the invader with their body heat. Now, humans are pretty useless in that respect, with really limited temperature tolerances compared to many bacteria, so in the above example even a hundred people piled in a big ball would probably start cooking in the middle like a reverse steak.
This is all due to physics and Newton's laws of thermodynamics, but basically more metabolic activity produces more heat, so the metabolic activity of a hundred organisms is going to produce more heat. Also in a big blob, the surface area to volume ration isn't conducive to heat loss. (Interestingly we use this to keep warm; our metabolic rate increases when we get cold! Especially in the liver, where there's lots of mitochondria, hence being cold sobering you up quicker.)
Even with the better heat tolerances compared to humans, bacteria in giant fermentation tanks get really hot. Have you ever gone near a compost heap in winter and seen steam rising off them? It's the same thing. So with those massive tanks, one problem with keeping them all alive is keeping them cool, the opposite of me sticking my small bacterial colonies in the incubator to keep them warm. Without complex cooling systems and adequate stirring, the big fermentors would soon be full of dead bacteria, which aren't great at producing biochemicals for industry.
Another issue is that these big fermentors are usually full of one type of bacteria, whereas in the wild there are loads of different ones competing and killing each other and stuff, which keeps the population down. Also also, the bacteria still need to eat stuff at the end of the day, and a neck deep layer of bacteria would use up so much resources from the planet that all kinds of bad things would happen! It would lock in a lot of the carbon from the atmosphere though so would mitigate some of the horrible things we humans are doing to the planet. And make driving cars a lot harder, come to think about it, so our emissions would go down too...
I'm getting off track here, so will wrap up. I have barely scratched the surface of this question, there are so many other factors limiting the exponential growth of bacteria that I don't have time to mention today. But you can trust me when I say; there isn't a neck deep wall of goop covering the entire planet. I promise.
WAIT... Isn't that what soil is? Bacteria chilling out eating decaying plants and stuff? In a thick layer covering much of the globe? Are we not neck deep in bacteria... because we're walking on top of them?! Now I don't know what to think...