[Fatalis]

They are all descended from a single cell. In the absence of mutation, they are all either resistant or non resistant. If the original cell was non-reistant, and a descendent is resistant, then the descendant became resistant.

Question for edification: how does one determine the first cell is not resistant without killing it?

[Right Wing Professor]

Question for edification: how does one determine the first cell is not resistant without killing it?

Pick it randomly from a culture; show that at least some very high percentage of the other cells in the culture are not resistant (say >99.9%). If the 99.9% or whatever isn't a satifactory degree of certainty, repeat the experiment a few times.