Here's why:
* Lack of cell wall: Mycoplasmas are bacteria that lack a rigid cell wall, making them exceptionally small and flexible.
* Oxygen-independent: Some mycoplasmas are facultative anaerobes, meaning they can survive with or without oxygen. However, there are also obligate anaerobes, which cannot survive in the presence of oxygen.
* Size: Mycoplasmas are among the smallest known self-replicating organisms, with some species being only 0.1-0.2 micrometers in diameter.
While there might be other very small, cell-wall-less organisms that can survive without oxygen, mycoplasmas are a well-studied example and likely the most common.
