Because of horizontal gene transfer, it's wrong to talk about bacteria in the same way we regard other kingdoms of life. I propose that all bacterial species that participate in HGT be rebranded as 'eigenspecies' or 'eigengenomes'—as if their genetic material were a vector being applied to a matrix (the environment) to yield a constant result (for humourous reasons, the eigenvalue should be 2, to indicate binary fission.) Of course, no genome is completely static, so they'd be near-eigengenomes. Very rapidly changing species are of course not steady-state at all; they are just some vector getting screwed up by the matrix.

This perhaps can be used to talk about any species, since a stable environment generally means a stable survival rate and negative evolutionary pressure, but I think the transitory nature of the designation is particularly suitable to our little transitory friends.