Overusing plants that have an anti-biotic affect most likely will interfere with our ability to absorb B12, like mentions here (http://www.vegsource.com/talk/raw/messages/11335.html):
As I see it, part of the reason many people have B12 problems, vegan or not, is because they are doing things that interfere with their body's ability to absorb it and/or produce it in the areas it needs to be able to absorb it. Things as common as garlic, mustard greens, seaweeds, excess fat, heated and chilled foods all are potential culprits in adversely affecting our body's natural B12 ways. This doesn't even take into account other lifestyle factors that adversely affect it as well - stress, pollution, dehydration and lack of sleep to name a few.
I'll post more info on B12 and garlic later - anyone else with info on natural anti-biotics and B12?