A recent study that continues to get attention suggests that reusable cloth shopping bags are a perfect breeding ground for dangerous bacteria. But some experts are less than convinced.

Researchers at the University of Arizona and Loma Linda University say they tested 84 reusable cloth shopping bags and found bacteria in all but one bag.

Some of the germs were coliform bacteria, a category that includes the dreaded E. coli -- suggesting the bacteria came from raw meat or uncooked vegetables. They even found E. coli in 12 per cent of the bags.

The researchers then went out and asked shoppers heading into grocery stores in California and Arizona if they routinely washed their cloth bags. They found that 97 per cent of shoppers said they didn't.

As well, three-fourths acknowledged that they don't use separate bags for raw meats and for vegetables, and about a third said they used their cloth bags for everything from storing gym cloths to carrying raw chicken.

The researchers conclude that reusable bags "can play a significant role" in the cross contamination of foods.

"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," co-author Charles Gerba, a University of Arizona professor of soil, water and environmental science said in a news release.

"Furthermore, consumers are alarmingly unaware of these risks and the critical need to sanitize their bags on a weekly basis."

The study found that a simple run through a washing machine with soap can eliminate most of the bacteria in the bags.

Andre Jean, a scientific evaluator at Health Canada, said health concerns arise when shoppers put raw meat in their reusable bags.

"If you put raw meat without protecting those packages, you could have meat juice that may leak out of the package, and by doing that you would find the juice in the bottom of your bag," Jean told CTV News Channel Monday afternoon. "So if you don't clean those bags, chances are you'll cause cross-contamination whenever you use the bag again for your next shopping trip."

But some are not convinced that cloth shopping bags pose a health hazard.

Prof. Rick Holley from the department of Food Sciences at the University of Manitoba notes that the study was not published and therefore not peer-reviewed.

As well, the researchers sampled only a "very small sample" of bags to reach their conclusions.

"I think that the authors have overstated the significance of the issue in order to dramatize and draw attention to a risk that may be there," he told CTV News.

Holley also notes that the mere presence of bacteria on shopping bags doesn't mean there's a risk to human health; most bacteria most don't cause illness in humans.

Even most strains of E. coli are harmless, though the public often hears about the most deadly strain: E. coli 0157:H7.

"What this study tells us is that there is risk, but it did not look at the ability of the organisms to be transferred from the material that was contaminated to food. That is a whole other issue," Holley says.

It's worth noting that the study was funded by the American Chemistry Council, which represents some plastic bag makers. The study was also released just as California considers a state-wide ban on plastic bags.

Still, Holley does think the study is a reminder that consumers should separate meats and poultry, which are often loaded with bacteria, from fruits and veggies and other items.

As well, if they use reusable bags, they probably should try to throw them in the wash once in a while, particularly if a food product has leaked in the bag.

Health Canada estimates there are more than 11 million cases of food-related illnesses in Canada every year; many of these illnesses could have been prevented by following proper food handling techniques.

With a report from CTV Winnipeg's Eleanor Coopsammy

Here are some more tips from Health Canada on reusable shopping bags:

  • When you are using cloth bags, make sure to wash them frequently, especially after carrying fresh produce, meat, poultry or fish.
  • Some reusable grocery bags may not be machine washable. If you are using this type of grocery bag, you should make sure to wash them by hand frequently with hot soapy water. Plastic bins should be washed using hot soapy water on a regular basis as well.
  • If you notice that juices from your food have leaked into the bag or bin, make sure you wash them before using them again.
  • You should place fresh or frozen raw meat, poultry and fish in separate bins or bags from fresh produce and other ready to eat foods.
  • You can also put your meat, poultry or fish in plastic bags, such as the clear bags you can find in the produce and some meat sections. This will help prevent the juices from leaking out and contaminating your reusable bags and bins and also other foods. Fresh produce should also always be placed in plastic bags to protect them from contamination.
  • If you are using your grocery bags or bins to store or transport non-food items, they should be thoroughly washed before using them for groceries.
  • Always follow proper safe food handling and preparation techniques in your kitchen, whether or not you are using reusable grocery bags. The four key steps are: clean, separate, cook and chill.
  • After putting your groceries away, clean the areas where you placed your bags or bins while unbagging your food, especially the kitchen counter and the kitchen table.