I really think this should go without saying, but I'll say it anyway. If you are using cloth bags for grocery shopping (and I am sure everyone reading this is), it is important to wash them on occasion. A report from Canada illustrates that bad things can happen when bags are not washed. However, note that the study was conducted and distributed by plastics manufacturers, so there is some bias (and when you read some of the recommendations, it becomes obvious.)
That being said, the information about contamination of bags is valid and should be a concern.
I wash my bags regularly--usually because the minute I carry them in the house, dog and cat hair jump onto them and they just look like they need washing. But we should wash the bags on a regular basis and especially if anything leaks in them or they get wet in the rain.
A blog at Ecobags.com has some good pointers:
1. Wash your reusable bags or disinfect them frequently. Leaving them in hot cars with food
ick on them is going to breed germs and bacteria...which leads to,
2. Purchase and use better quality bags that are made from canvas or other washable fabrics.
3. If you have meat, fish or veggies, bag them separately.
4. Anything that leaks should be in its own bag and if that bag gets wet, it should be washed or
dried thoroughly before reuse.