Actually I have elsewhere shown how we can infer that all cats, from tabby to tiger, are part of the one kind; and this is based on the fact that different species and genera of cats can and do interbreed. Perhaps we are actually on firmer biological ground talking about kinds rather than species.
But not all the "cats" kind can interbreed or do. So some of them in there are not of one kind. Or do I misunderstand you. Which are the two (or more) kinds of cat that are included in "from tabby to tiger"?
I've been thinking a bit about the definition of species. As others have pointed out species can have fuzzy boundaries some of the time.
Perhaps it would be useful to construct a definition that puts that fuzziness front and center.
A population of individuals may consist of one or many groups that have gene flow between them to a greater or lessor extent.
What we think of as a "species" in most cases is a population that has effective gene flow through out the entire population and zero gene flow with any other population.
However, there are many cases where there is a bit of gene flow with other groups and not 100 % uniform flow within.
So a group can be 30 % speciated, 80 %, or more or less. It is a mistake to focus on cases and think they have to be at the extremes only.
In fact, that case might be the lessor occurring example.
So looking at gene flow between all different dog breeds and wolves we might decide that there are a number of sub populations that are almost zero % separated from wolves (feral huskies) or effectively 100 % separated from wolves (pomeranians).
Remember, in the case I think is being discussed. The bacteria all came from asexual budding of one bacterium. So they should all have the exact same genome. And they were not resistant. Therefore the resistance had to have come about from mutations.