They reported that while Doxy wiped out 95% of spirochetes in the test period, and penicillin killed 75%, Flagyl killed just under half of them. Looks like Flagyl
does kill some spirochetes. (I wonder what would happen if Doxy & Flagyl were combined. Too bad they didn't test that.)
But of the persister cells, Doxy left nearly 80% alive, penicillin left about 85%, and Flagyl left over 90% alive. So in the lab, Flagyl does a poor job of killing these cyst forms.