Pro biotic live flora can replace and promote growth of your own flora. However, it also promotes the growth of flora that antibiotics themselves allow to run riot. Such as Clostridium Difficile (or C.diff. to give it its more usual moniker.) This is a huge killer of frail people, and sadly, can be contracted easily when in the vicinity of somebody suffering. (We all have it, it is needed, but it can get out of control with too much antibiotic therapy.)

That said, the Royal College of Microbiologists here in the UK are of the opinion that the doses in these health foods are well below the therapeutic level, and if they were above it, could create more problems than they solve. Also, the term pro biotic is a marketing not a technical term. All biology is pro something.....

Your body naturally recovers levels, and if it can't then you have a problem that can be solved, but by doctors and prescriptions rather than supermarkets and adverts.

For my sins, (of which there a couple) I have spent the last three years involved in infection control for a UK health regulator and whilst not a microbiologist myself, my comments above are more or less paraphrased from the advice the Royal College gave to the Advertising Standards Authority recently.