They are good for the environment, but reusable grocery bags are also a breeding ground for bacteria.Many responsible shoppers carefully choose their groceries and put them into the same cloth or plastic bags over and over again on every trip to the store.Did you ever wash your grocery bags? asked Call7 Investigator Theresa Marchetta.Um, no! I never wash my plastic bags or my paper bags," responded a 7NEWS colleague.Marchetta could not find anyone who regularly cleaned their reusable bags.Do you ever think to wash the bags? Marchetta asked another colleague.No. Not really, the other worker replied, laughing.The CALL7 Investigators tested several reusable bags used by 7NEWS colleagues and another from a woman going into a Denver grocery store.Marchetta took the lab results to Dr. Michelle Barron, the infectious disease expert at the University of Colorado Hospital."Wow. Wow. That is pretty impressive," said Barron.Barron examines lab results for a living."Oh my goodness! This is definitely the highest count," Barron commented while looking at the bacteria count numbers.She admitted she was shocked at what was found at the bottom of the bags."We're talking in the million range of bacteria," she said.Marchetta used swabs provided by a local lab to test several grocery bags for bacteria, mold and yeast.Three of the samples had relatively low bacteria counts, posing little risk of causing illness.Two were in the moderate range, posing some risk, according to Barron.Two other bags had extremely high counts -- 330,000 to nearly 1 million colonies of bacteria.Four of the samples also had relatively high levels of yeast and mold."It would be a level of concern getting on your food, on your hands, too," said Barron. "Digging in there, you touch, rub your eyes ...all that good stuff. Um, yeah, that's gross. Good to know, said a 7News employee whose bags were tested.It is not only gross, but also painful if you get sick."You can have a terrible diarrhea, stomach ache, vomiting. Not a fun thing to have," said Barron.To demonstrate the risk, Marchetta dusted grocery bags with a substance that glows in the dark to see how harmful germs can travel.With the lights off, it was clear the Glo-Germ had not only stuck to our groceries, it was also on Marchettas hands, the counter top, and in the cupboard and refrigerator.They like porous surfaces and live longer on plastic, said Barron, about the bacteria. Fortunately, it is a problem that is easy to fix.Wash reusable bags or wipe them out with a bleach wipe after each use."We're trying to be environmental. I fully support that. But not at the cost of your health," said Barron.Another suggestion -- designate one bag for each type of food to prevent germs from spreading.