Food Australia, who should know better, published an article earlier this year titled "Contaminated reusable grocery bags pose health risks" (63 (5) - May 2011). It was based on an article by Gerba & Sinclair entitled "Assessment of the Potential for Cross Contamination of Food Products by reusable Shopping Bags". You can download the full article here.
The article suggests bacteria, including E. coli, flourish in the green bags.
And the are probably right. The same way bacteria can flourish in pretty much any environment. Because bacteria is IN the environment. If they had done their survey by swabbing the jeans of the shoppers rather than the bags, they would have got similar results.
And as for soaking the shopping bags in meat juices and leaving them in a car boot (trunk) for two hours - well, duh!, of course bacteria levels will increase.
It was all a bit odd and I had a niggling suspicion, which is why I chased down the original article rather than relying on the summary in Food Australia. My suspicion was confirmed in the following line:
"The authors would like to acknowledge and thank the American Chemistry Council for providing funding to support this study."
So, the American Chemistry Council, a body that includes all the plastic (bag) manufacturers of America in its ranks funded the research.
Doesn't mean it's wrong but there are flashing lights all over it!