ngin - Norfolk Genetic Information Network

17 January 2002


New E. coli study could shed light on movement of GM bacteria by Robert Schubert, CropChoice editor, and E. Ann Clark, professor of plant agriculture at the University of Guelph

(Jan. 16, 2002 - CropChoice news) -- A recent study published in the January 2002 edition of Applied and Environmental Microbiology documents the transmission of a potentially pathogenic strain of E. coli from manure-contaminated soil and water into the roots and leaves of lettuce plants.  Officials at the USDA funded the study by microbiologist Karl Matthews and his colleagues at Rutgers University out of concern about the microbial quality of soil and irrigation water.

Should the results of this study call into question the safety of using manure and compost for fertilization of crops?  Not according to Matthews, senior author, who says it’s not a concern.  Others, including E. Ann Clark, co-author of this article, point out that the Rutgers study may shed light on the pathways of movement of genetically engineered bacteria amongst soil, plants and insects.

After growing lettuce in composted manure, each gram of which they had inoculated at a rate of 100 million E. coli 0157:H7 organisms, the Rutgers research team documented a buildup of the pathogenic bacterium within plant leaves.  Because the accumulation was within the leaves, not on them, simply washing the leaves in water would not cleanse or remove them.

One point deserves emphasis. The amounts of the E. coli bacterium that his team applied in the laboratory were much, much greater than would occur in nature, where they would be "below a level of detection," Matthews says.  The pathogen was also applied post-composting, whereas in the real world, the heat generated by the composting process is intended to kill it.

"Composting, a key component of organic farming, is a pathogen reduction process," writes biologist David G. Patriquin in his report, "Reducing Risks from E. coli 0157 on the Organic Farm."
"Under experimental conditions, most bacterial pathogens are killed by exposure to temperatures of 55-60oC for a few hours or less (8).  Such temperatures are achieved and last for days to weeks in the thermophilic stage of composting. Canadian commercial compost standards require that under windrow composting a temperature of 55 degrees Celsius or greater is maintained for at least 15 days during the composting period and that during this period, the compost is turned at least five times (20). For industrial composting systems in which the composting process is conducted indoors, is closely monitored and controlled, and for which different sets of equipment are used at earlier and later stages of processing to avoid reintroduction of pathogens, a high degree of consistency in the elimination of pathogens can be achieved."

Nonetheless, the next logical step from this important new study would be to determine the likelihood of detecting pathogenic E. coli in farm-grown vegetables.

Some critics of organic and other sustainable methods of growing food crops have argued, and might use this study to press their point, that the use of manure is dangerous.

"The vast majority of manure used in this country is supplied to chemical (non-organic) farms," says Brian Leahy, president of the California Certified Organic Farmers.  Yet, the USDA does not regulate the use of manure on chemical farms, whereas it does in organic agriculture.

Organic farmers do use composted plant and animal (manure) material and aged manure to fertilize their crops.  The California Certified Organic Farmers, a 1,000-member, non-profit organic trade association that includes a certification program, surveyed its member farmers in 1998 about their fertilization methods.  According to the results:

71 percent used cover crops,
62 percent used compost, of which 21 percent produced the compost on their farm,
30 percent used aged manure,
47 percent used gypsum, lime or other materials,
22 percent used blended fertilizers,
8 percent used sodium nitrate,
and 5 percent used raw manure.

Leahy notes the use of raw manure in organic agriculture is declining rapidly.

The following protocol must be followed to ensure the thermal destruction of most organisms in composting plant and animal (manure) materials:

 For vessel or static-aerated pile system, the temperature must remain between 131 degrees and 170 degrees Fahrenheit for three consecutive days. In a windrow system, the temperature range is the same, but the duration is 15 consecutive days.

The rule on any non-composted manure, including aged and raw, is that growers may not apply it to plants whose edible portions have direct contact with the soil any sooner than 120 days before harvest.  For orchard crops and others who edible parts don’t touch the soil, the rule
is 90 days.

The standards from Organic Crop Producers and Processors/Pro-CERT, one of the largest organic certifiers in Canada, discourage raw manure.  If applied, the standards impose strict time limits on when manured crops can be harvested (4 months) and a prohibition on using uncomposted manure directly on edible crops.  See

Also important to remember is that there are thousands of strains of E. coli bacteria, many of which aren’t pathogenic; some of those are sitting in our stomachs right now.

What worries Leahy is the creation of new strains of bacteria from what he terms the "indiscriminate use of antibiotics in feed."  Note, organic agriculture standards disallow the use of antibiotics and hormones.

Indeed, antibiotic use has skyrocketed in confinement feeding systems, particularly for hogs and chickens (Mellon et al., 2001). 1  Fully 80 percent of the antibiotics used in contemporary farming are prophylactic in nature or growth promoters - not to cure animal illness.  The US livestock industry annually consumes 24.6 million pounds of antimicrobials, such as tetracycline, penicillin, and erythromycin for non-therapeutic uses (in the absence of disease).  Of these, hogs and poultry are the most dependent upon prophylactic antibiotics (in excess of 10 million pounds each), compared to cattle at 3.7 million pounds. Pretty et al. (2000) cites evidence from the WHO (1997) that resistant strains of Salmonella, Campylobacter, Enterococci, and E. coli resulted from use of these antimicrobials in farm animals. 2

So, should consumers of green leafy vegetables be concerned about health risks because of this study?  In the view of Matthews, lead author of the Rutgers study, and based on USDA rules for use of compost and manure in organic agriculture, the answer is no.

Nonetheless, the Rutgers study makes an important contribution to understanding potential pathways of transmission of potentially harmful soil microbes to the human food chain.

E. Ann Clark, a professor of Plant Agriculture at the University of Guelph in Canada, has been following the issue of microbial movement into plants via their roots, but for a different reason.

Elaine Ingham, a well-known soil microbiologist, conducted lab research involving klebsiella planticola, a common soil microbe.  Scientists had genetically engineered this organism to convert crop residues into alcohol.  Ingham showed that contrary to expectation, when wheat plants were grown in soil amended with the genetically modified klebsiella, they died within a few days.  As a result, companies did not commercialize the experimental microbe.

However, Clark wonders, had the microbe been allowed to proceed to field trials, what would have prevented its transmission from the test plot into surrounding vegetation, given other unrelated work she’d recently read from a group at Clemson University. (REFS 4, 5, and 6).

They demonstrated that a different (and transgenic) microbe could enter through the rhizosphere (root system) and then move up to the leaves of all 16 tested monocot and dicot species (including corn, wheat, oats, broccoli and beans) via the phloem (vascular system).  Once there, she says, the authors had demonstrated that biting and sucking insects could ingest and then transfer the bacteria to other plants.

"The authors identified this as a significant issue for spatial movement of GE rhizospheric bacteria, as it meant that GE bacteria cannot be contained - even though they move very very little in the soil itself," Clark says.

Their study showed that southern corn rootworm (Diabrotica undecimpunctata) and red-legged grasshopper (Melanoplus femurrubrum) could acquire genetically engineered pseudomonas aureofaciens, a common rhizospheric bacterium, by feeding on infected plants.  Grasshoppers ingesting GE Pseudomonas chlororaphis (formerly aureofaciens) were able to infect other plants through subsequent feeding. The authors emphasized the implications of their findings to the risks associated with large scale release of GM rhizospheric organisms," Clark wrote in a chapter that Environment Canada commissioned (3).

The Clemson group also cited a number of studies by other authors who had documented the ability of various microbes to enter into plants through the roots and travel up into the leaves.

In the aggregate, these various studies suggest significant reason to question field-testing of transgenic microbes, because they show that soil microbes are completely uncontainable.  Ingham’s study was purely her own initiative ­ she was not involved in either the development of the transgenic organism or in the process of risk assessment that the EPA oversaw.  What if she had not taken the initiative and blown the whistle in time?  What if her insight and imagination are not represented the next time someone thinks to field test a transgenic organism.

1. Mellon, M., C. Benbrook, and K. Lutz Benbrook. 2001.  Hogging It: Estimates of Antimicrobial Abuse in Livestock.  Union of Concerned Scientists, Washington, D.C.

 2.  Pretty, J.N., C. Brett, D. Gee, R.E. Hine, C.F. Mason, J.I.L. Morison, H. Raven, M.D. Rayment, and G. van der Bijl. 2000. An assessment of the total external costs of UK agriculture.  Agricultural Systems 65:113-136.

3.  Clark, E. Ann.  Genetically modified organisms and their potential impact on biodiversity.  In:  O. Hendrickson et al. (eds)  Ecosystem Globalization: Threat to Canadian Biodiversity Environment Canada and the Canadian Forest Service, Ottawa. (IN PRESS)

4.  Lamb, G.T.; Tonkyn, D.W.; Kluepfel, D.A. 1996. Movement of Pseudomonas aureofaciens from the rhizosphere to aerial plant tissue. Can. J. Microbiol 42:1112-1120.

5.  Snyder, W.E.; Tonkyn, D.W.; Kluepfel, D.A. 1994.  Transmission of a genetically engineered microorganism by a common crop insect. Bull. Ecol.Soc. Am. 75:215-216.
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