This section is under construction
When you hear the term “species”, you probably think of a specific kind of creature, like a dog or a cat. More generally, among the kinds of plants and animals we encounter in the visible world, the term “species” refers broadly to organisms that can mate with one another to produce offspring of the same kind. Cats and dogs are different species because they can’t mate with each other.
But bacteria don’t mate: they reproduce by dividing themselves in half. So how do we define a species? In fact, even terms like “parent” or “child” aren’t quite appropriate if each new cell is an identical copy of the old one. For very broad categories, like phylum or even genus, the similarities among like cells is high enough that we feel comfortable grouping them together with a common name, but at what point do we reach the lowest, most specific level.
The answer is tricky for another reason, called horizontal gene transfer, a process by which sometimes (in fact, quite often), a microbe will absorb genes from nearby organisms, altering its genome and its corresponding functions, sometimes significantly. Once that happens, the resulting new microbe can itself divide indefinitely, producing more and more copies of itself with the new gene. Although the new microbes still mostly resemble their original ancestor, if the new gene makes a protein that affects your body somehow, it might as well be an entirely different species.