Although there are exceptions to everything in life,I believe your understanding is correct and that is the norm.

If I remember right,Leo and Menon said that "K" worked against every cancer that they tested against in the lab.If that holds true in real life,would it eventually become an exception?
Same question applies to "B" with gram + bacteria.

Did penicillin get tested against every bacteria it was ultimately used for ? (It was prescribed for everything from "hangnails to tax audits"!)
I don't know.

Kelt