Fertilizer May Help Bacteria Slip Into Groundwater

Modified from USDA

PATHOGENIC MICROBE Escherichia coli 0157:H7 can be a deadly contaminant in untreated well water.

For people who drink untreated well water, sometimes the only thing that stands between them and pathogenic bacteria is soil. New research shows that high concentrations of phosphate, a key ingredient in fertilizer, help a deadly strain of Escherichia coli to slip through the dirt (Environ. Sci. Technol., DOI: 10.1021/es201132s). The result is a potentially greater risk of groundwater contamination.

On rural fields, cow manure is a major source of E. coli 0157:H7, a strain that can trigger internal bleeding and kidney damage when people ingest it. Each year, this pathogen causes about 73,000 illnesses and 60 deaths in the U.S., says Jin Li, an environmental engineer at the University of Wisconsin, Milwaukee. Contaminated drinking water is the culprit in about 15% of those infections.

Previous research has shown that bacteria on fields can drain through the soil and eventually reach groundwater. In the laboratory, scientists study how bacteria move through soil by pouring a slurry of bacteria onto a column of sand and simply monitoring what comes out at the bottom, Li says.

One day, Li and her colleagues were studying E. coli 0157:H7 and found that the time the bacteria took to travel through the sand column depended on the type of solution they were suspended in. She suspected that the solutions' differing concentrations of phosphate caused the time differences.

So Li and her colleagues ran a series of experiments with varying concentrations of phosphate, from no phosphate up to 1 mM, a concentration commonly found in agricultural soils. They found that without any phosphate, 45% of the cells escaped the column of sand after an hour. But when suspended in 1 mM phosphate, 77% of the cells broke through.

Li's team then used a computer model to calculate the attractive and repulsive forces between the E. coli cells and sand particles, with and without phosphate. In the model, phosphate molecules increased the repulsive forces between the cells and sand grains. Li thinks that the phosphates do this by changing the conformation of molecules on the cells' surface.

Researchers will now have to consider phosphate when studying bacteria's movement in soils, says Derick Brown, an environmental biotechnologist at Lehigh University. He points out that researchers used to think that a solution's ionic strength was the most important driver of cell movement. "The new study demonstrates that solution chemistry is more complex than previously thought," he says.

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