While the attention of the world is focused on genetically modified (GM) crops, GM microbes have been released for at least six years with little or no public awareness or debate. Prof. Joe Cummins reports.
A number of GM microbes are being widely deployed since their first release six years ago.
Sinorhizobium meliloti is a bacterium added to soil or inoculated into seeds to enhance nodule formation and nitrogen fixation in the roots of legumes. It was released for commercial production in 1997.
The other commercial GM microbes are designated as bio-pesticides. These include GM Agrobacterium radiobacter k1026, used to prevent crown gall disease in fruit and vegetable plants, and Pseudomonas fluorescens modified with a number of different Cry delta-endotoxin genes from different subspecies of Bacillus thruingiensis (Bt). The modified P. fluorescens cultures are killed by heat pasteurization and provides a persistent biopesticide preparation that degrades much slower in sunlight than Bt.
Neither the people selling nor those using the preparations are necessarily aware that the microbes are genetically modified, however. Even organic farmers may be using them inadvertently.
The legume symbiont, Sinorhizobium meliloti, is tremendously important for fixing nitrogen from the air into plant roots and the soil. Legumes signal to the bacterium by exuding flavonoids from their roots, activating the expression of nodulation genes in the bacterium, resulting in the production of Nod factors that regulate the formation of nitrogen fixing root nodules [1]. The S. meliloti genome has been fully sequenced. It is unusual in containing three chromosomes (or a chromosome and two very large plasmids), all of them contributing to the symbiosis with the plant roots [2]. The genetically modified commercial strain (RMBPC-2) has genes added that regulate nitrogenase enzyme (for nitrogen fixation) along with genes that increase the organic acid delivered from the plant to the nodule bacterium. It also has the antibiotic resistance marker genes for streptomycin and spectinomycin [3]. The commercial release was permitted in spite of concerns about the impact of the GM microbe on the environment.
Evidence supporting the initial concerns has accumulated but that has not dampened the use of the GM microbe. For example, a recent review reports that GM S. meliloti strains persisted in the soil for six years, even in the absence of the legume hosts. Horizontal gene transfer to other soil bacteria and microevolution of plasmids was observed [4]. Other studies showed that a soil micro arthropod ingested GM S. meliloti, and a GM E. coli in the arthropod gut facilitated gene transfer to a range of bacteria [5].
There is little doubt that the antibiotic resistance markers for streptomycin and spectinomycin will be transferred to soil bacteria and to a range of animal pathogens. For example, the resistance genes for streptomycin could be observed to transfer from their insertion as transgenes in plant chloroplast to infecting bacterium Actinobacter sp. [6] when homologous gene sequences were present.
The antibiotics spectinomycin and streptomycin are used extensively in human and animal medicine. Spectinomycin is used to treat human gonorrhea [7] and bovine pneumonia [8]. Streptomycin is used to treat human tuberculosis [9] and Meniere’s disease [10] and as a pesticide on fruits and vegetables [11]. Thus, the commercial release of GM Sinorhizobium meliloti has resulted in the establishment of the GM microbe in the soil in millions of acres of cropland, where it can spread antibiotic resistance genes for antibiotics that are extensively in use in medicine and agriculture.
Agrobacterium radiobacter k1026 [12] is a bio-pesticide derived from A. radiobacter k84, a natural bacterium used to control the crown gall disease of fruits and ornamental trees and shrubs. Crown gall disease is due to the bacterium Agrobacterium tumefaciens that causes tumors to form on the plant stems, and is the most common vector employed in plant genetic engineering.
GM Agrobacterium radiobacter releases a chemical warfare agent bacteriocin (agrocin) against A. tumefaciens. Bacteriocin is a novel nucleic acid derivative that prevents the crown gall tumors from forming in the infected plants. The GM A. radiobacter has an engineered deletion in the genes controlling plasmid transfer so that the ‘male’ bacterium cannot transfer its plasmid, but it can act as a ‘female’ to receive a plasmid transfer. However, recent research suggests that retrotransfer of genetic material can occur from ‘female’ recipient to ‘male’ donor bacterium [13].
Pseudomonas flourescens strains modified with Cry delta endotoxin genes from Bacillus thuringiensis are killed before being marketed [14]. The killed GM bacteria are more persistent than are the conventionall B. thuringiensis sprays. The main fallacy in the approval of these biopesticides is to suppose that bacteria cannot enjoy sex (conjugation) after death, they do.
Soil bacteria are also easily transformed with cell lysates (squashed dead cells) and function in their genetically modified form in soil microcosms [15]. P. fluorescens and A. tumefacians are both transformed in soil [16]. Soil Pseudomonas and Actinobacter can also take up genes from transgenic plants [17]. So, the combination of transgenic crops and GM biopesticides can create genetic combinations capable of devastating the soil microflora and microfauna.
In conclusion, GM microbes have begun to be ubiquitous invaders of the North America ecosystem. This massive invasion took place with little or no public awareness and input, and with very little monitoring of the impact of the invasion. The environmental risk assessments of the commercial microbes were rudimentary and frequently erroneous. We may have a bio-weapons equivalent of a time bomb on our hands.
Article first published 24/04/03
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