Could you gut bacteria be making you fat?

It always makes me laugh when I see an advertisement for antibacterial hand wash and house hold disinfectants to help sterilise your home. Considering that the number of microorganisms in your intestines alone is 10 times more than the total amount of cells in your body (up to approximately 100 trillion little critters), I think it’s clear to see that bacteria is here to stay. So why all the fuss to kill the “Nasty bacteria”?