This blog discusses food safety quite a bit because microbial contaminants in one part of the food chain can have a major impact on human health. Since the last post on Chipotle contamination, there’s been a major recall at Starbucks, Wonderful brand Pistachios, and another scare at a Chipotle restaurant.
From 2000-2009, leafy greens (such as spinach and lettuce) was the most frequent food source of contamination outbreaks, which were largely due to Escherichia coli O157:H7 and Salmonella enterica contamination. How do these pathogens, normally associated with animal (or human) colonization, end up in our salads?
One potential source of contamination is proximity between the growing greens and the grazing livestock that may harbor the pathogens. Cattle often harbor zoonotic pathogenic E. coli, and while slaughterhouses may do their best to prevent beef contamination, there’s still a possibility that the bacteria from these animals can pollute the nearby produce. Guidelines are in place to recommend minimum distances between concentrated animal feeding operations and crops, but there is little evidence to support these guidelines. Gathering scientific evidence regarding the best distance was the goal of a recent study published in Applied and Environmental Microbiology.
Current recommendations are that leafy greens be kept at least 120 m (400 feet) away from animal feedlots. A research team led by first author Elaine Berry, working in collaboration with Patricia Millner, systematically planted plots at 60, 120, and 180 m from a cattle feedlot (see schematic, right), and tested the plants for E. coli presence. The study took place over two years, taking into consideration the differences that weather, cattle colonization, and cattle movement may make in results.
The scientists found that even the greatest distance, 180 m, did not prevent E. coli or E.coli O157:H7 from reaching the crops. In 2011, there was no significant difference in the number of E. coli O157:H7-positive samples between the differently distanced plots (1.5% for 60 m; 0.4% for 120 m; 0.8% for 180 m). In 2012, there were significantly more positive samples for the 60 m plot than the 180 m plot (3.5% vs 1.8%). PFGE typing found that most bacteria isolated from the leafy greens matched that of the cattle manure, strongly suggesting the cattle as a source of contamination. Cumulatively, these results suggest there may be a small distance effect in feedlot contamination of produce, but that crops grown at 1.5x the recommended distance are still vulnerable.
How did E. coli, a gram-negative bacterium, travel such far distances? Air sampling failed to capture any E. coli O157:H7 cultures at any distance during any part of either year, but other E. coli strains were constantly detected – with no significant differences between the feedlot itself and any of the plots, at any distance. There was anecdotal observation of increased airborne bacterial counts when the cattle were more active (as previously reported in other studies), but the researchers didn’t systematically measure cattle activity for this study. Similarly, bacterial counts were lower after the pens were cleaned and manure removed, which seems a promising lead toward actions that may decrease contamination. But while conditions influenced airborne bacterial counts, these counts remained constant over all the crop plots.
Understanding how and from where contamination originates in our food sources is an important step in prevention. This study was the first to identify that 120 m may not be sufficient to protect crops from nearby animal feedlots. The more data collected by food scientists, the more accurate the recommendations can be to protect the food chain and prevent some of the outbreaks like those seen recently in the U.S.
-- Julie Wolf