Fruits & Vegetables. Image credit: douneika, Flickr

Last week, Jonathan Leff and Noah Fierer, our colleagues at the University of Colorado at Boulder with whom we’re collaborating on the Wild Life of Our Homes project, published new research about the microbial communities living on fresh fruit and vegetables. Their study was the first to assess the diversity of non-pathogenic bacteria living on eleven types of produce, including lettuce, spinach, tomatoes, strawberries, apples, peaches and grapes. Using many of the same genetic techniques we’ll be using in the home microbe study, they assessed what microbes are living on the surfaces of common produce and considered how those microbial communities varied by produce type and farming practices (conventional vs. organic).

We asked lead author Jon Leff a few questions about the research:

We hear stories in the news all the time about outbreaks of dangerous pathogens tracked back to the consumption of certain raw veggies – it seems most research on the bacteria living on produce has focused in on these pathogens. What made you take a whole community approach?

In some respects, it makes a lot of sense to research microorganisms inhabiting raw fruits and vegetables by looking at pathogens. They are known to cause disease under the right circumstances, and the methods used to observe and research them are better developed. We decided to research the much broader bacterial communities inhabiting these foods since we assumed they would include a large diversity of organisms that were under-appreciated. We felt that it would be interesting to look at the bacterial communities on the foods we eat in a more comprehensive way since there is potential for this broad diversity to be relevant to humans.

We know on our own skin, there are “good” bacteria that protect us from “bad” or pathogenic bacteria – is this happening on produce too?

This is certainly a possibility. However, the jury is still out, and further research is needed to determine when increased microbial diversity is helpful or harmful on the foods we eat.

Fruit and veggies often travel hundreds of miles from the farm field or orchard to the grocery store and then to our homes – I realize you just compared communities living on the produce at the point-of-sale, but can you speculate how the microbial community might change over time from farm to table?

Do you think bacteria are getting picked up along the way or perhaps might the reverse happen, species acquired in the fields drop off over time since harvest?

And along those lines, did you pick up human-skin-associated bacteria on produce surfaces?

You’re right, it is important to recognize that the bacterial communities are likely dynamic entities. In fact, some research has shown that the bacterial community composition can change quite dramatically depending on the length of cold storage. This is one potential reason for why we observed differences in the bacterial communities between organic and conventional produce varieties. Likewise, it is possible that differences in the harvesting technique and the amount that different produce items are touched by humans could influence their community composition. However, although we did observe some bacterial taxa that are also commonly found on human skin, overall, the produce bacteria looked dramatically different (and more typical of plants).

A key finding of your study is that the Enterobacteriaceae is the most dominant bacteria across samples — and were much more abundant (64% greater) on the surfaces of conventional labeled spinach, lettuce, tomatoes, and peaches when compared with their organic labeled equivalent. Who are the Enterobacteriaceae and why are they so abundant generally on produce? What do you think accounts for their greater relative abundance on some conventionally labeled produce? Do you think these differences in relative abundance translate to any sort of health consequences on the consumer?

These are very good questions, but their answers may have to wait for future research. The family Enterobacteriaceae is an extremely diverse group of bacteria which includes human pathogens, benign or helpful bacteria, and both common and rare species. There are many potential reasons why these bacteria are relatively abundant on fruits and vegetables. Additionally, they are commonly found on plants in general. It is possible that the differences in relative abundance of these bacteria may have health consequences, but it is too soon to know for sure.

We’re working together on research looking at the microbes that live on surfaces of the home – like the kitchen counter – and your colleague Gilbert Flores did some work looking at the microbial biogeography of kitchens – How will your new results affect our understanding of the microbes we’re finding in our homes?

These microbes living on fruits and vegetables are not only a potential source of bacterial species for your human microbiome but also for those inhabiting your home. By understanding which types of bacteria live on produce items, we can assess whether this is a likely source of bacteria within someone’s home.

Finally, what’s your take when it comes to your own produce hygiene habits: To wash or not to wash? C’mon we’ve all just done the quick shirt wipe with an apple over washing it – how bad is that?

That depends, how clean is your shirt?

Thanks, Jon! And be sure to check out Jon and Noah’s research paper, published 27 March  2013, in PLOS ONE at the link below:

Leff JW, Fierer N (2013) Bacterial Communities Associated with the Surfaces of Fresh Fruits and Vegetables. PLoS ONE 8(3): e59310.