Patients who rush to the emergency room complaining of food poisoning may be blaming the wrong underlying culprit.
Reusable shopping bags are being used to store and carry anything from leaky meats and milk products, to gym clothes, laundry or books, all of which can be spewing harmful cross-contaminating bacteria that actually grow and infect other foods and products subsequently carried inside these bags.
That's what researchers in Arizona and California discovered when they randomly sampled reusable grocery bags—mostly woven polypropylene—that were carried into stores by shoppers in Tucson, Los Angeles, and San Francisco.
"Our findings suggest a serious threat to public health, especially from coliform bacteria including E coli, which were detected in half of the bags sampled," said Charles Gerba, Ph.D., a University of Arizona professor of soil, water and environmental science and co-author of the study. Researchers at Loma Linda University in California assisted with the survey.
"Unfortunately, almost no one interviewed ever washed their reusable bags," Gerba wrote in his report. "Public unawareness of the potential risks seems almost universal. Thus, a sudden or significant increase in use of reusable bags without a major public education campaign...would create the risk of significant adverse public health impacts."
Washing the reusable bags by hand or in a machine with soap reduced the number of bacteria the bags harbored by more than 99.9%.
The report adds another layer of implications to several proposals in California and other states that would prohibit stores from putting products, especially food items, into disposable plastic bags.
"Consumers are alarmingly unaware of these risks and the critical need to sanitize their bags on a weekly basis," Gerba said. When the researchers asked the shoppers how often they washed their bags, only 3% said they did so regularly.
E-coli were identified in 12% of the bags. Also, a wide-range of enteric bacteria, including several opportunistic pathogens were discovered as well.
To create a model of how this transmission might occur, the researchers collected juices from raw chicken and beef in a beaker, spiked it with the bacteria Salmonella typhimurium that was grown in an overnight culture.
The juices were then added to swatches cut from these polypropylene bags and placed them in a Ziploc bag. Some were processed immediately, while some were put in a trunk of a hot car and left overnight to mimic the practice in ordinary use.
The bacteria in those bags grew within two hours of storage, and grew 10-fold when the temperature was 47 degrees C inside of a trunk, Garba said.
In real practice, the bags are often left in the car, ready for the next time the owner needs to go back to the grocery store.
The authors warn that transporting gym clothes or other clothing in those same bags is especially precarious, possibly resulting in cross contamination of bacteria such as methicillin-resistant Staphylococcus aureus.
The authors recommend that all bags carry printed instructions warning owners to wash them in between use and to separate raw foods from other food products.