E. coli lasts longer in Salinas soil, study says

11/14/2012
Mike Hornick

New research finds the pathogen E. coli O157:H7 lives about 30 days in soils from California’s Salinas Valley — 10 days more than in the state’s Imperial Valley or Yuma, Ariz.

Lower salinity in Salinas irrigation water is the main cause of the difference, said Mark Ibekwe, a microbiologist with the U.S. Department of Agriculture’s Agricultural Research Service in Riverside, Calif.

Ibekwe and three colleagues published their findings, “Persistence of Escherichia coli O157:H7 in Major Leafy Green Producing Soils,” in the journal Environmental Science & Technology in October.

The results were based on laboratory-tested soil samples. Field studies of E. coli are typically limited to nonpathogenic varieties.

Increasing salinity in Salinas water would not be realistic or beneficial for leafy greens growers there, Ibekwe said. Nevertheless, the research underscores the importance of keeping new pathogens from entering the fields.

“You don’t want to introduce another variable into the farming environment that will ultimately cause adverse effects on the crops and result in lower yield,” Ibekwe said. “Because of how salinity will react with other factors there, we are not suggesting that.

“What we’re saying is that because we know there’s a longer survival in the Salinas area, we should be very, very careful in introducing pathogens from manure, poorly composted materials or any source at all into the farming environment,” he said.

Imperial and Yuma irrigation water has higher salinity because it’s drawn from rivers, Ibekwe said, whereas Salinas depends on groundwater.

To a lesser extent, the research also links longer survival to higher levels of organic carbon in Salinas soils. Total nitrogen is another contributor, the scientists said, but that shed little light in this case because the three regions have similar levels.

These factors add to a list of others previously identified such as temperature, pH level and moisture content.

The research was funded by the USDA’s National Institute of Food and Agriculture and the Agricultural Research Service.

It’s not the only E. coli research tied to the Salinas Valley.

In 2011, Steven Koike, plant pathology farm advisor at University of California Cooperative Extension, Monterey County, began a two-year study of survival on lettuce residue plowed back into soil. That project was funded for $118,000 by the Center for Produce Safety.

Comments (4) Leave a comment 

This article grossly underestimates the persistence of E. coli in the Salinas region and is inconsistent with other studies and investigations. Islam M, Doyle MP, Phatak SC, Millner P, Jiang X. published a peer reviewed report in 2004 which can be found in the June 2004 Journal of Food Protection. It is stated in the report that "E. coli O157:H7 persisted for 154 to 217 days in soils amended with contaminated composts and was detected on lettuce and parsley for up to 77 and 177 days, respectively, after seedlings were planted." The only factor I can think of off hand that could explain such a variance is soil condition. Soils within the Salinas region are highly variable. A reduction in survival of E. coli in dry, desert, soil conditions versus the cool and moist conditions in the Salinas region is however a valid conclusion.

All this leads up to is a conclusion that we need to be careful about our inputs. If we apply finished composts free of pathogens and do our best to keep animal intrusion to a minimum, the duration of any pathogen is a moot point, isn't it?

How much is actually known about those inputs? In the Salinas region there are ongoing studies that show low levels of EHEC can be found in waterways throughout the year. These results are contrary to previous studies which did small samples with less recovery and detection capability. Those populations are being maintained in the environment somehow. Organic amendments have been identified as being capable of supporting populations and regrowth of pathogens especially in a cool and moist environment. Runoff and farm cross contamination is the best explanation to explain persistence in the environment. Quality control regarding these organic amendments is weak at best and a certificate from a composter means little if the material is contaminated when it is put in the field. Poor pathogen reduction leads to better survivability in the fields and the packinghouses.