How bacteria get into plants.

All things green.
Oct. 31 2006 1:10 PM

Root Causes

How do bacteria that make us sick get into plants?


I just sent off my seed order for next summer's vegetable garden. Reviewing the list—bush beans, peas, eggplant, cherry tomatoes, red lettuce—I realize I didn't order spinach seeds.

Chalk this up to the irrational response to the outbreak of E. coli infection last month that was traced to bagged spinach grown in California. Spinach grown in a home garden, untainted by fresh animal manure, is unlikely to make anyone sick.


How did the bacteria get into the commercially grown vegetables in California? A lot of Web posts have surmised that the spinach plants took up the bug through their roots. I like to stick up for plants, which are not gifted at verbal communication. So, I planned to explain to you that these spinach plants were innocent bystanders, and their roots did not contribute to the E. coli disaster. I hoped to say that the Web chatters were spreading misinformation. But the particular villain involved in the poisonings in August and September—E. coli O157-H7, known to researchers as O157—has some disturbing special talents, which make it hard to be sure.

The most likely cause of the E. coli infection was animal manure that got splashed onto the plants' leaf surfaces. Roots have evolved to be very selective about what gets into the rest of the plant. Water and nutrients come in through the root hairs, threadlike, thin-walled vessels similar to our capillaries. These hairs take up nitrates, potassium, and other substances in ion form. These are little atom groups. A one-celled bacterium, by contrast, is generally too big to be absorbed by roots. That's pretty crucial to the food chain, because soil is alive with bacteria; if the plants we eat took up the critters regularly through the roots, they'd be chock-full of pathogens and we'd be sick all the time.

Think about it: A relevant experiment has been going on for centuries in China, where the fields were (and may still be) fertilized with human waste. The leaf surfaces in those Chinese fields may be contaminated. But the plants' internal systems are not, according to the experts I consulted. There are a few bacteria that can make a plant sick. They have devised ways to degrade the cell walls of the roots so they can invade. But these plant pathogens don't make a connection to the human gut.

The clever O157, however, seems to have developed some frightening qualities that set it apart from other bacteria and other E. coli strains. It can tolerate heat, dryness, and acid conditions better than its germ siblings. This is distressing news for human beings. And O157 may also be bad news for my plant-innocence argument. A 2002 article by Department of Agriculture researchers in the Journal of Food Protection showed that the O157 strain of E. coli can get into the root system of leaf lettuce—which may mean it can also invade the roots of a spinach plant. My argument for the impenetrable defenses of spinach roots is weakened—rats. The ingenious bacterium O157 may be slipping in through a back door. Still, even if the DOA researchers are correct and bacteria can infect roots directly, no one has shown that the germs get up to the leafy part that we eat this way. I'm sticking, so to speak, with splashed manure.

Either way, we've got to figure out a way to keep manure away from crops to outwit, or out-evolve, these clever bacteria. It seems logical: Food and crap ought to be far apart. Exactly how O157 got to the plants in the field is still a mystery. Here is a clue, though. (And now I am harkening back to my California life, when I worked for the San Jose Mercury News.) The E. coli contamination of the California spinach leaves occurred in fields south of San Jose that are downstream from the Gabilan Mountains, where feral pigs root among the oak trees and beef cattle, dairy cows, sheep, and horses graze. (It's beautiful country, celebrated most notably by John Steinbeck. In his story "The Red Pony," the ill-fated pony is named Gabilan.)

The Food and Drug Administration guidelines for vegetable growers are voluntary. There are no federal regulations requiring a vegetable field to be a minimum distance from a pasture or uphill and upstream from grazing animals. So, consumers have to look out for themselves. If you're going to eat raw leafy vegetables, wash them well, and eat them promptly. Better yet, cook them, because that kills even O157. The Chinese figured out a way to avoid getting sick from their fertilizing practices. They came up with the wok.

Constance Casey is a former New York City Department of Parks gardener and writes the monthly "Species" column for Landscape Architecture Magazine.