Louis Pasteur's experiment proves that microorganisms do not grow from non-living matter, he discovered that microbes can cause food spoilage. So, where do microorganisms come from? Do they come from the environment?
So, where do microorganisms come from? Do they come from the environment?
There is no spontaneous generations. Microorganisms (virus, bacteria, unicellular eukaryote, small multicellular eukaryotes) just like macroorganisms (big-enough to be seen by the naked eye multicellular eukaryotes) are not recreated out of nothing. Abiogenesis happened a long time ago and as far as we know, it is not happening today.
This means that if there is a micro-organism somewhere, it must have come from the environment. It did not pop up out of nowhere.
That is the biggest question to be answered in biology, as of today we do not know where microorganisms originated from. What is known is that they outnumber us by combined weight and population. There are some interesting theories of their origin, one is called "panspermia" which proposes that life might have seeded the Earth from other planets. And considering that some bacteria are capable of surviving extreme levels of radiation (D. radiodurans), it is not unreasonable to think they could have achieved interplanetary travel via space rocks.