A Journal of Infectious Diseases paper co-authored by Bill Keene about an investigation into a norovirus outbreak that hit 13 members of a soccer team has created a media furor and a press release competition. It could have been a gym bag, or a plastic bag – but the implicated vector happened to be a reusable grocery bag that was left in a hotel room that contained a bunch of food.
Cue the witch hunt on reusable bags.
The Academy of Nutrition and Dietetics recycled a recent press release saying that reusable bags need to be washed regularly by users as pathogens grow well and cross-contamination is likely.
From the release:
Reusable grocery totes are a popular, eco-friendly choice to transport groceries, but only 15 percent of Americans regularly wash their bags, creating a breeding zone for harmful bacteria, according to a survey by the Home Food Safety program, a collaboration between the Academy of Nutrition and Dietetics (formerly the American Dietetic Association) and ConAgra Foods.
“Cross-contamination occurs when juices from raw meats or germs from unclean objects come in contact with cooked or ready-to-eat foods like breads or produce,” says registered dietitian and Academy spokesperson Ruth Frechman. “Unwashed grocery bags are lingering with bacteria which can easily contaminate your foods.”
I don’t dispute that washing bags is a good idea, I’m just not sure there’s data that supports their statements.
Williams and colleagues (2011) have published the only peer-reviewed study on the microbial safety of reusable bags and tested growth of Salmonella in 2 batches. They spiked the bags with 10^6 cfu and let them sit in the trunk of a car for 2 hours. One of the batches, where the temperature reached 47C/117F, showed a one-log increase in the Salmonella. The other batch, where the temperature reached 53C/124F, there was a one-log reduction. That data doesn’t show just a breeding zone – it shows they can be a killing zone too (and I’m not sure how realistic a 10^6 contamination really is).
The part of the press releases that is the least rooted in science is that pathogen-containing bags "easily contaminate your foods." The same Williams study showed generic E. coli is floating around in bags, recoverable in 12 % (n=58) of those tested but can it be (or is it likely) to be transferred to any ready-to-eat foods, or somehow to food contact surfaces in the home?
Just because the bacteria might be there, doesn’t mean it can contaminate a ready-to-eat food. No one has presented data to support that. Maybe the Academy of Nutrition and Dietetics – if so, they should share it.
In a cross-contamination event there is a dilution effect when it comes to transfer. 1000 cfus of Campylobacter on the outside of the package of raw chicken might become 100 cfus when transferred to the bag, and then only 10 cfus when transferred to ready-to-eat apples.
Washing bags frequently (as the Academy of Nutrition and Dietetics suggests) is probably a good idea (like washing hands in warm water) and probably won’t increase risk, but I wonder how much it decreases the probability of cross-contamination when compared to doing nothing.
Aron Hall from CDC took a much more realistic approach to the outbreak focusing on norovirus (as opposed to the vector) calling the virus "the perfect human pathogen." I couldn’t agree more, as I cleaned up puke over the past couple of days.