Do you know what germs are lurking in your reusable grocery bag? If you don't wash them regularly you could be loading your groceries into a bag full of potentially harmful bacteria.

Reusable bags can harbour potentially dangerous germs, like E-coli and salmonella. So consumer reporter Lynda Steele brought an assortment of reusable grocery bags to Dr. Karen Bartlett, a microbiologist at UBC’s School of Population and Public Health, to test what was lurking inside the bags.

On some of the bags you could already see there was grime.

"There is some residue on this plastic surface. Could be dried blood, but it could also just be fruit juice," said Dr. Bartlett, "In some cases for example, if it were campolabactor, it's actually kind of a nasty gastroenteritis, lots of vomiting and diarrhea."

Dr. Bartlett swabbed the bags for general purpose bacteria, one that is specific to skin microflora and one that's associated with raw meat. Then she tucked the sample inside the lab refrigerator.

48 hours later, we checked back to see what grew. Even to the untrained eye, you can see our reusable grocery bags harboured all kinds of stomach turning bacteria. But it actually wasn't as bad as it looks.

"The good news is I didn't find any bacteria that would likely be associated with food poisoning," she said.

But do you really want any bacteria growing inside the bag you carry your groceries with? After showing our little science experiment to Vancouver shoppers many were rethinking the way they use and store their reusable bags.

So how often do your bags need to be cleaned?

"How can I put it delicately?  We would never wear underwear for a whole week and not wash it. So washing your grocery bags is a good thing," explained Dr. Bartlett. 

Most bags can be thrown in the washing machine. If the fabric isn't machine washable, hand washing in soap and water will do the trick. Just make sure the bags are completely dry before you store them. 

If you're buying meat ask the cashier to put it in a plastic bag first and make sure the meat never goes into the reusable bag you use to carry fruits and vegetables.

One of the best ways to avoid cross contamination is  to wash your hands and sing happy birthday while you do it.  When the song is done your hands should be basically bacteria free.