When a species of bacteria is referred to by its strain, are they a clone of single founder or is a certain amount variation allowed?
In theory they're clones, but depending on the age of the strain(some strains are surprisingly old: ~40 years) there's variation inside strains.
The reverse is also true. Bacteria from a single species are isolated twice and named different things by different labs and the mistake can take years to even find, much less correct.
Bacteria effectively clone themselves so theoretically all clones are identical. However like every organism they're subject to Darwinian evolution so there's always a chance a random mutation happens. Since bacteria usually reproduce fast the rate of these mutations can happen fast resulting in the strain evolving to a (slightly) different strain.