MEDICINE: The Killers All Around

(6 of 9)

Microbes' extraordinary ability to adapt, observes Harvard microbiologist Fields, "is a fact of life. It's written into evolution." Indeed, the end run that many organisms are making around modern antibiotics is a textbook case of Darwin's theory in action (anti-evolutionists, take note). In its simplest form, the theory states that new traits will spontaneously appear in individual members of a given species -- in modern terms, mutations will arise in the organisms' genetic material. Usually the traits will be either useless or debilitating, but once in a while they'll confer a survival advantage, allowing the individual to live longer and bear more offspring. Over time, the new survival trait -- camouflage stripes on a zebra, antibiotic resistance in a bacterium -- will become more and more common in the population until it's universal.

The big difference between animals and bacteria is that a new generation comes along every few years in large beasts -- but as often as every 20 minutes in microbes. That speeds up the evolutionary process considerably. Germs have a second advantage as well: they're a lot more promiscuous than people are. Even though bacteria can reproduce asexually by splitting in two, they often link up with other microbes of the same species or even a different species. In those cases, the bacteria often swap bits of genetic material (their DNA) before reproducing.

They have many other ways of picking up genes as well. The DNA can come from viruses, which have acquired it while infecting other microbes. Some types of pneumococcus, which causes a form of pneumonia, even indulge in a microbial version of necrophilia by soaking up DNA that spills out of dead or dying bacteria. This versatility means bacteria can acquire useful traits without having to wait for mutations in the immediate family.

The process is even faster with antibiotic resistance than it is for other traits because the drugs wipe out the resistant bacterium's competition. Microbes that would ordinarily have to fight their fellows for space and nourishment suddenly find the way clear to multiply. Says Dr. George Curlin of the National Institute of Allergy and Infectious Diseases: "The more you use antibiotics, the more rapidly Mother Nature adapts to them."

Human behavior just makes the situation worse. Patients frequently stop taking antibiotics when their symptoms go away but before an infection is entirely cleared up. That suppresses susceptible microbes but allows partially resistant ones to flourish. People with viral infections sometimes demand antibiotics, even though the drugs are useless against viruses. This, too, weeds out whatever susceptible bacteria are lurking in their bodies and promotes the growth of their hardier brethren. In many countries, antibiotics are available over the counter, which lets patients diagnose and dose themselves, often inappropriately. And high-tech farmers have learned that mixing low doses of antibiotics into cattle feed makes the animals grow larger. (Reason: energy they would otherwise put into fighting infections goes | into gaining weight instead.) Bacteria in the cattle become resistant to the drugs, and when people drink milk or eat meat, this immunity may be transferred to human bacteria.