ISIS Report 25/05/04
Bio-remediation Without Caution
A bacterium living inside plants could be improved for cleaning up
environmental pollutants without genetic modification.
Prof. Joe Cummins and
Dr. Mae-Wan Ho reveal that this
seemingly beneficial development is beset with danger, as the bacterium
concerned is a known pathogen.
Sources
for this report are available in the ISIS members site.
Full details here
Water soluble and highly volatile organic environmental pollutants, such
as benzene, toluene, ethylbenzene and xylene compounds, chlorinated solvents
and nitrotoluene ammunition wastes, are being cleaned up using plants in
combination with microorganisms that naturally live inside the plants
(endophytes).
Endophyte bacteria live within the tissue of the plant without harming
it. They are found in most plant species, and many can colonize the vascular
system. The highest densities of bacteria are usually found in the roots, less
in the stem, and least in the leaves. The plants take up the pollutants through
their roots, and the bacteria break these down within the roots or in other
parts of the plant.
This natural process is inefficient because the compounds tend to get
transported up the plant faster than the bacteria can break them down. Once
transported up, the plants metabolize the contaminants, and some of the
metabolites as well as the contaminant can be toxic. For example,
trichloroethane is metabolized into trichloroacetic acid, both of which are
toxic. Worse still, plants tend to release volatile pollutants and their
metabolites into the atmosphere via evaporation from the leaves, which turns
bio-remediation into bio-pollution.
A recent article in Nature Biotechnology reports how this clean
up process could be greatly improved by engineering an endophyte bacterium
Burkholderia cepacia, a natural resident of the yellow lupine.
Researchers from Linburgs University in Diepenbeek, Belgium and Brookhaven
National Laboratory in New York, USA, created a strain of B. cepacia
that has enhanced ability to degrade toluene within the plant, enabling the
plant to tolerate high levels of toluene, and also substantially reduced the
amount of toluene released into the atmosphere.
The engineered strain of bacterium carries marker genes for kanamycin
resistance and nickel resistance and is derived from the natural endophyte. By
adding to this endophyte strain a toluene-degrading plasmid from another strain
of B. cepacia that normally lives in the soil through natural
conjugation (bacterial reproduction) between the strains, a new endophyte
strain is created that can live in the plant and degrade toluene taken up by
the plant.
Plants inoculated with the engineered bacterium grew much better than
plants that were not inoculated; or else inoculated either with the control
strain lacking the plasmid, or with the strain that normally lives in the soil.
More impressively, the plants inoculated with the engineered bacterium reduced
toluene evaporation into the atmosphere to about 50% of the control. This looks
very promising, as the researchers point out, the experiment could have been
done without any genetic modification. The plasmid containing all the
toluene degrading enzymes belonged to a natural soil bacterium, and an
endophyte host without the marker genes could easily have been used to receive
the plasmid by conjugation.
A non-GM bacterial endophytic strain created in this way may well be the
very first really useful and beneficial product from the industry. So
what’s wrong?
The research paper did not deal with safety. What metabolites of toluene
are generated in the plant, and will they be toxic? How will the plants be
disposed of? There are three lupine species cultivated for fodder - blue, white
and yellow - and there are a also a number of wild species. The wild species
contain alkaloid chemicals that are very toxic to cattle and sheep while the
cultivated species are edible for farm animals, provided care is taken to treat
the seeds in such a way as to remove the toxins. Lupines thrive on poor soil
and provide ground cover and green manure as well as fodder for animals.
More importantly, the research report failed to mention that B.
cepacia has the ability to cause fatal disease in humans.
The groundwater of Wichita, Kansas was found to be polluted with the
chemical solvents dichloroethylene and trichloroethylene, and was cleaned up
using a natural strain of B. cepacia. But no special public health
measures or follow up seemed to have been implemented after the clean up.
The United States Environmental Protection Agency (EPA) has considered
the problems associated with approval of B. cepacia as a plant
pesticide, for, not only is the bacterium used to fight plant pests but is
itself a pest as it is a disease agent in humans. EPA, through a Scientific
Advisory Panel (SAP), reviewed B. cepacia as a plant pesticide and
acknowledged that it is linked to human disease. The SAP risk assessment
peculiarly noted "Bc [B. cepacia] has been referred to as an opportunistic
human pathogen. However, as might be expected, the strains registered or
proposed for use as biopesticides were isolated from the soil or plant roots,
rather than from human patients".
In reality, the SAP comment offered cold comfort because the B.
cepacia strains isolated from patients proved essentially undistinguishable
from strains isolated from the roots of crops such as corn. The American
Phytopathological Society produced a useful review of the risks from plant
disease or human disease along with the benefits in cleaning up chemical
pollution and fighting some plant diseases. Unfortunately, there has been no
clear and simple way to differentiate between the ‘evil’ and the
beneficial strains of B. cepacia, and no way of preventing the two from
exchanging genes.
B. cepacia has an unusual genetic makeup; it has a relatively
large amount of DNA (about twice that of E. coli) and unlike most
bacteria, which usually have a single chromosome, B. cepacia strains
have as many as five large replicons (chromosomes) and the different
chromosomes are rich in insertion sequences that allow for extensive gene
exchange between different strains, and insertion of disease related genes from
other bacterial species. B. cepacia is a prominent cause of death among
cystic fibrosis patients, the bacterium frequently reaches epidemic proportion
among such patients and an epidemic related strain was identified in soil
samples in USA. It is believed to be a complex species made up of seven
distinct genomic subspecies all of which are capable of infecting humans; and
all of the disease-related subspecies were isolated from maize rhizosphere
(root zone). The disease is difficult to contain because disease bacteria may
be replenished continually from the soil and plant material.
Hospital acquired B. cepacia epidemics appeared among patients
with diabetes, malignancy, heart failure and chronic obstructive pulmonary
disease. One such B. cepacia outbreak appeared in an intensive pediatric
care unit, and B. cepacia infection was common among renal transplant
patients. Different B. cepacia clones showed different infectivity among
cystic fibrosis patients and patients with different complaints. Antibiotic
resistant B. cepacia infection was the most common cause of death among
lung transplants for cystic fibrosis patients. B. cepacia causes feared
infections because the strains tend to be antibiotic resistant. Bacteria
isolated from different infections were found to be resistant to all seven
tested antibiotics but were sensitive to treatment with honey.
Do lupines pose a threat to people with compromised immune systems or
cystic fibrosis? Yellow lupines, and perhaps the other commercial species as
well, contain potentially disease-causing B. cepacia endophytes so their
presence in hospitals and homes of compromised people is unwise. The bacteria
may be transferred by direct contact with broken plant stems or petals along
with the dust and debris associated with the plant; a gift of lupines could be
fatal.
There is clearly a large literature on the threat of B. cepacia
infection and its death toll among compromised patients. The existing evidence
indicates that the bacterial infections may pass from the ecosystem to the
hospital ward and there seems no way of ensuring that the B. cepacia
strains used in biotechnology are unable to infect compromised humans.
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