German wikipedia defines a biological species as a group where individuals can reproduce offspring with other members of the group, but not with individuals outside of the group.
First of all, to the best of my knowledge, proper sexual reproduction only happens with Eukaryotes. Then this means that no bacteria ever reproduce offspring with other individuals, and therefore each bacterium is its own species.
But that is a meaningless definition. If each bacterium is its own species, then the categorization into species becomes meaningless.
On top of that, bacteria have “pseudosexual” horizontal gene transfer (HGT) which allows them to exchange genetic material with any other bacterium (if the circumstances are right; if i understand this correctly). So all bacteria are in a single species if you look at it that way.
I understand that bacteria normally don’t undergo HGT with all other bacteria because some might only open up at hot temperatures while others only open up in cold temperatures - thus creating a natural barrier. But it is also my understanding that while such barriers exist, they’re not permanent and can be overcome in nature (without human intervention) for example due to certain virus infections and similar circumstances.
Long story short:
Wouldn’t it make more sense to just consider that the concept of “species” only apples to eukaryotes and not to bacteria at all? Wouldn’t that save all of us a headache? Maybe we should consider bacterial species to be less strict that eukaryotic species. Maybe we should describe bacteria by their individual features and give that group a name, instead of expecting that diverging lines of evolution cannot ever come together again.
We can’t even give a solid definition of what life is, let alone how to divide that into species in a reliable and consistent way. Biology is a really messy science and strict definitions just don’t cut it. There are always exceptions and edge cases that violate the rules we come up with. So yes, this applies to all life, not just eukaryotes. Bonus points for those who can name situations where we’ve tried to classify complex things and failed to come up with anything that works perfectly.
Biology is just far too complex for simple rules like that. Whatever appealing definitions you come up with are always rough and inaccurate. If you expect a biology definition to always work in every situation, you’re going to be disappointed. This isn’t the kind of simple matter where simple definitions are possible. Spoiler: there are many.