A new study done by the University of Arizona and Loma Linda University (California) found that 97% of people do not wash their reusable bags. Unfortunately I am one of them and really I should know better. It is kind of a no brainer I suppose, but yet I never even thought about it.
This study looked at 84 bags and more than half of them contained coliform and heterotrophic bacteria and E. coli was present in 12% of the bags. Don't worry too much though, the strains that they found aren't likely to make anyone sick.
So obviously we all need to make a point to wash our bags. Make sure you check the manufacturer's washing instructions before you wash so that you don't ruin any bags. I think at a minimum you should wash your bags once a month. My goal is going to be every other week.