According to a study by researchers at University of Arizona and Loma Linda University, out of 84 reusable bags tested, "bacteria were found in almost all bags and coliform bacteria in half." With the addition of meat juices and incubation in a car trunk for two hours, bacteria increased 10-fold. This means that the potential for contaminating other foods in the bags goes up significantly.
In case you were wondering, the bags "were collected from consumers entering grocery stores from the San Francisco Bay area, the greater Los Angeles area and Tucson, Arizona", with most made of woven polypropylene. People were also surveyed on how they use their bags, and only 25 percent have separate bags for their meats and vegetables and only 3 percent cleaned their bags on a regular basis. Fecal contamination was indicated by the presence of E. coli in 12 percent of the bags, with greater numbers of bacteria present in California bags than in the ones from Arizona. Yum
The good news? When you wash your bags, the bacteria count drops nearly to zero.
Thanks for the heads up from Eliza, over at the Bad Astronomy blog.
A heads up about the above study. It was funded by the American Chemistry Council, which represents some plastic bag makers. The article can be found here.