New Clues To Microbial Contamination Of Leafy Greens

Farms in California’s Salinas Valley produce much of the world’s lettuce and other leafy greens, earning the area the nickname “the salad bowl of the world.” But the region has made headlines for more troubling reasons lately: Several recent outbreaks of pathogenic bacteria in food have been traced to produce grown there. Now scientists have identified two factors in the area’s soil that might help explain why the bacteria flourish there (Environ. Sci. Technol., DOI: 10.1021/es302738z).

In 2006, 200 people in 26 states became ill and three people died after eating spinach grown in the Salinas Valley. The culprit was Escherichia coli O157:H7, a pathogenic strain of the bacteria. Scientists traced the microbes to cow manure from fields adjacent to the spinach ones.

Once E. coli reaches farm fields’ soil, the microbes can survive for up to three months, says microbiologist Mark Ibekwe of the Department of Agriculture’s Agricultural Research Service in Riverside, Calif. He and his colleagues wanted to understand which soil conditions help the bacteria survive.

They collected 32 soil samples from farms in the Salinas Valley and from two other major growing regions: the Imperial Valley in California and fields near Yuma, Ariz. In the lab, the researchers added E. coli O157:H7 to the samples and monitored how well the bacteria grew.

E. coli survived about 10 days longer in soils from Salinas than from the Imperial Valley and Yuma, Ibekwe says. The scientists analyzed more than a dozen properties of the soil, including pH and moisture content, and found three that correlated with high survival rates.

The bacteria did not fare well in soils with higher salinity, the researchers found. Farmers in the Salinas Valley irrigate their land using groundwater, which has a lower salinity than the water from the Colorado River used in the Imperial Valley and in Yuma.

Another factor that the researchers linked to E. coli survival was the amount of assimilable organic carbon in the soil. This type of carbon is in a form that bacteria readily can take up and use. The researchers found that higher levels correlated with longer E. coli survival times. “The Salinas area is also very rich in this source of carbon,” Ibekwe says.

Although the team found a correlation between E. coli survival rates and total nitrogen content in the soil samples, nitrogen levels didn’t differ significantly between the three regions, Ibekwe says. So, he says, nitrogen content doesn’t explain the difference in bacterial survival rates between soils in Salinas Valley and those in Imperial Valley and Yuma.

Michael Doyle, director of the Center for Food Safety at the University of Georgia, Griffin, says the study adds three new factors to those already known by scientists to affect survival of pathogens in soil, such as temperature, pH, and moisture content. He says the new factors could help scientists develop better ways to prevent microbial contamination.

But Doyle points out that crops such as spinach and lettuce cannot tolerate high salinity, so increasing salinity would not be a good way to reduce E. coli soil contamination. Ibekwe says the best method to avoid contamination is still to prevent bacterial sources such as animal waste from reaching the soils in the first place.