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roundtable: the evolving enemy Watch Show 4:
"The Evolutionary Arms Race"
on PBS
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Select a question:
How concerned should we be about becoming infected with resistant bacteria from foods like chicken, beef, and even aqua-cultured fish that may have been grown on diets treated with antibiotics? Will eating less of these foods, perhaps by switching to "organic" chicken, reduce our risk, and do vegetarians have a reduced risk for this reason?
If the use of antibiotics in a widespread fashion at low doses in agriculture, particularly in the raising of animals, is so common and yet so potentially dangerous from the standpoint of evolution of resistance, why is it still the practice? What are the costs and benefits of this practice?
Several people have submitted questions not about antibiotics, but about all of the liquid soaps that are now for sale in supermarkets that are labeled "antibacterial." Do these work like antibiotics in causing further resistance? If so, how, and if not, how is their action on bacteria different than just plain soap?
Americans now seem to be getting fanatic about sterility, and people are trying to raise their children in an almost "germ-free" environment. We have a question from a woman who says that she has several friends, young mothers, generally, who seem to overuse antibacterial products. She asks: Is it not true that the presence of some "germs" in moderation is actually good for the development of our immune system?
In one of the programs in the Evolution broadcast series, there was the story of the leafcutter ants that culture Streptomyces bacteria on their bodies and seem to use those bacteria to apply antibiotics to keep their fungal farms parasite free. This has been inferred to mean that somehow the Streptomyces and the antibiotics they produce have been evolving with the enemy, and this is a kind of use that appears to have been going on for 15 million years without permanent resistance being developed. Is there anything we can do to put ourselves in that situation versus the more static situation we're in now?
Could you give an explicit description of how evolutionary theory informs integrated pest management? I think the cross-fertilization here between bacteria and antibiotic resistance and agricultural pests and insecticide resistance is very interesting, and I don't think many people make that connection.
Some physicians, who are quite impassioned, say they would like to prescribe fewer antibiotics, but their patients demand them. Since in effect they cannot be absolutely positive there isn't a bacterium involved, when, say, a parent brings in a young child who is ill, and they can't be absolutely certain that there won't be bacterial complications as a result of a viral illness, they would like to know how the public will be educated and who is going to take on the job, so that patients will basically get off the doctors' backs and let their colds run their course.
Another part of that question is that doctors are afraid of being sued if they don't prescribe an antibiotic and a bacterial infection does develop. So from the medical perspective of overuse of antibiotics, how might we address this basic problem in the interface between healthcare providers and the consuming public?


Q: Could you give an explicit description of how evolutionary theory informs integrated pest management? I think the cross-fertilization here between bacteria and antibiotic resistance and agricultural pests and insecticide resistance is very interesting, and I don't think many people make that connection.
Panelist Responses: < back to intro page
Stephen Levy
The way that they connect is that the rate of an evolutionary change is dependent upon a lot of things, but one of them is the strength of selection. If you go into a population of organisms, be they insect pests or bacteria, and you remove one percent of that population with some selected agent, then evolutionary response is going to be relatively minor because you've only taken one percent out.

If you go in and take 70 percent out, and you take it in a selective fashion, then the strength of that selection is much greater, and the evolutionary process can work much more quickly. Integrated pest management essentially takes the mortality you want to impose on a population and splits it up among a bunch of different categories -- weeding or pruning is getting rid of a huge number of the weeds in the field, trapping or baiting might be getting rid of a large number of insects. Only some of them are actually being controlled with the chemical agent, and so the overall selection due to the chemical in that example is a lot lower. That leads, according to the theory, to a slower rate of evolution to resistance.

So integrated pest management is essentially hedging your bets by using lots of different strategies to affect the pest population. The pest population then has to evolve in a variety of different ways to try to combat it -- speaking in an anthropomorphic sense -- and the evolution of any one of those sectors to chemical resistance or baiting or whatever is slower as a result.

I'll mention another feature in the medical world, which I think involves similar concepts and has to do with vaccines. Probably most people feel that vaccines are so well-targeted that they doesn't leave or have an impact except to clear you of a disease problem. I've challenged the virologists to tell me that it does do more than that, because quite honestly I think with antiviral vaccines I don't see an effect other than protecting you against a virus, but I don't know what viruses are out there for, so that's another question.

But in terms of bacteria, I think we've learned a lot. Effective vaccines have been made against two organisms called Haemophilus and Pneumococcus, that are involved in ear infections and meningitis, the inflammation of the linings of the brain. And in fact, the vaccines are effective enough to virtually cut down, to a few percent now, the incidents of ear infections caused by the Haemophilus.

But we then go back and say, Well, what happens to the environments where these organisms usually are housed? They're usually found in the recesses of your mouth, your nasal passages, where most of them don't normally do harm, but some varieties do. And in fact what we've learned is that the Haemophilus strains that were causing diseases have now been replaced with other Haemophilus strains which cause different diseases, only this disease is something like a sinusitis, which isn't life-threatening, just troublesome. We gained a benefit, but we haven't totally eliminated disease.

The same with the Pneumococcus. The vaccines have eliminated most of the kinds that cause ear infections, but what has been found in the nasal passages of the vaccinated people are other strains of the Pneumococcus. That's telling us that there's a lot more to that environment in which these Pneumococcus find a residence, which says, "You get rid of some of our members, it's still the nice, cozy place we want to be," and so they'll get replaced with other strains.

Now, these strains of the Pneumococcus are not the ones that are most involved with infectious diseases, but because we're now seeing them replacing the others, we're on the alert. And I think much of the industry is looking at making vaccines for these, so that we can control them, too. But I think the bottom line is that we're dealing with a much, much larger nature out there, and that any little effect we're making will have a domino effect elsewhere. And we ought to look a little further rather than just narrowly to see what the effect of our development or our methodology had on that particular individual, on that particular environment.

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