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Borrowing from bacteria: Incorporating an E. coli gene into pigs could reduce the environmental impact of swine production
By Bijal P. Trivedi

Canadian researchers want to make livestock production more environmentally friendly by engineering swine and poultry that excrete lower quantities of phosphorus. In a first step toward this goal, the team has engineered mice that carry the gene for the E. coli enzyme phytase. Mice carrying the gene produce 11 percent less phosphorus in their droppings than their unmodified murine counterparts.

Phosphate is essential for optimal growth of swine and poultry. Because pigs and poultry are unable to metabolize this nutrient directly from plant sources, phosphorus supplements are often added to animal feed. This increases the phosphorus content of manure, which—when spread over fields—tends to seep into the ground water and pollute lakes and streams with drastic environmental consequences.

A less common approach taken by some farmers has been to add the enzyme phytase directly to the feed, which releases the phosphorus compounds from plant matter. While eliminating the need for supplements, phytase-treated food is expensive to produce and the enzyme can lose its potency during storage.

Cecil Forsberg, of the University of Guelph, in Ontario, and colleagues believe that engineering animals with the phytase gene incorporated into their genome overcomes these issues. Forsberg's team engineered mice that secrete the bacterial enzyme phytase in their saliva. The enzyme allows the animals to extract naturally occurring phosphorus from cereal grains, legumes and oilseeds, instead of relying on dietary supplements.

The phytase gene is under the control of a switch that should only be active in the salivary gland. When the scientists shift their focus to swine they will need to investigate whether the gene is active in edible parts and whether it is deleterious to health if consumed.

If the phytase gene is introduced into pigs the authors expect the reduction in fecal phosphorus to be much greater than 11 percent.

Large increases in phosphorus in fresh water can provide ideal conditions for the rapid growth of algae and other microbes. Uncontrolled algal and microbial blooms extract all the oxygen from the water causing the death of fish and other aquatic animals.

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Golovan, S.P. et al. Transgenic mice expressing bacterial phytase as a model for phosphorus pollution control. Nat Biotechnol 19, 429-433 (May 2001).
Ward, K.A. Phosphorus-friendly transgenics. Nat Biotechnol 19, 415-416 (May 2001).

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