I was just re-reading the recent study by Dr. Zhang of Johns Hopkins in which he tested 165 FDA approved drugs against Bb. Metronidazole performed extremely poorly in his study. According to Table 4 in this report, after 5 days of treatment, 81% of the Bb were still alive after Metronidazole.
I know goes against the grain, but it is food for thought. i just watched a presentation by Dr. Burrascano and mentioned that you need to treat with an 'azole' - tinidazole, metrinidazole, for 14 days, so maybe that is the difference.
From PLOSone
An Optimized SYBR Green I/PI Assay for Rapid Viability Assessment and Antibiotic Susceptibility Testing for Borrelia burgdorferi
The link doesn't seem to be working
www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0111809