species as a strictly defined category only exists for eukaryotes which can only breed with a narrow group of other living beings

for bacteria this is not applicable as any bacteria can breed with possibly any other bacteria as long as the environmental circumstances are right (conjugation in bacteria, horizontal gene transfer, ...). yeah yeah somebody will correct me and say "but the different bacteria expect different environmental circumstances, so they cannot interbreed", to which i say, it's complicated.

so instead, for bacteria the concept is a bit more applied spontaneously without much consistency behind it at all. it's kinda used in a "huh, these bacteria look similar and behave similarly (biochemically) so now i call them the same species" thing.