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It's a drag. Feeding antibiotics to animals speeds their weight gain, but it also breeds antibiotic resistant bacteria. It turns out that understanding why antibiotics speed weight gain may allow an end run around the dilemma.

Animal scientist Mark Cook says antibiotics increase weight gain by reducing the animal's immune stimulation. Ordinarily, the immune switch is stuck in the "on" position, so the animal can battle the bugs in its environment. But the immune system produced inflammatory molecules that cause wasting of the muscles -- exactly the opposite of what animal producers want.

When low doses of antibiotics kill the microbes, the animals needn't mount the immune response, and they grow nearly as fast as they do in a germ-free environment. That explains why low doses of antibiotics promote growth.

Substitution solution
Cook has found that this inflammatory response disappears if you hold the antibiotics and add conjugated linoleic acid (CLA) to the feed. The animals gain about as much weight as they would with antibiotics, but they can still mount an immune attack on pathogens.

In the absence of antibiotics, however, there's no pressure for antibiotic resistance, so the utility of these critical drugs should be preserved. "It would allow us to remove antibiotics, and have them as an arsenal if pathogens did break out," says Cook.

Although the goal is the same from the farmer's point of view -- faster weight gain -- the strategy is fundamentally changed, he says. Antibiotics "target the microbes. We're targeting how the body reacts to its own immune system."

What's weird in the world of food?

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