Hey Sulma, I've been torn on the Mag issue as well.
Most of the population is not deficient in mag and whether a bacteria feeds off of it, I'm not really sure, but if you are worried about taking mag, sups, maybe up your intake of foods rich in mag like pumpkin seeds, spinach etc.
There are also certain forms of mag that absorb much better. Most of the commercial forms of mag are not even absorbed by the body.
And like was mentioned above, Epson salt baths are a GREAT way to get mag in the body.