By the way, I don't know that the bacteria actually "kills" the yeast, I think it just occupies the same space. The idea is to have a natural population of harmless yeast and healthy bacteria so that there is a right balance of all organisms. Taking Antibiotics throws off the balance. Once that happens, it's tough to get it back to normal again. It's similar to when a plant species overtakes an ecosystem. Other useful species are pushed out,and the balance of the ecosystem is thrown off, affecting everything in it until the offending species is controlled. That's why many folks try to kill the yeasts first, and then repopulate the gut later with probiotics. You can end up with too many bacteria in your system if you're not careful - I have had this problem as a result of probiotic overuse.