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as growth promoters in cows and chickens favored the emergence of resistant microbial pathogens, rendering the antibiotics useless against human infections. Plant biotechnology raises similar concerns. In creating new plant varieties, agricultural biotechnologists link genes for antibiotic resistance to the genes they want to transfer into plants; these genes act as selection markers to identify the rare plants that actually accept the new genes. This selection system works because the plants that take up the antibiotic-resistance marker genes are the only ones to survive when grown in a broth containing the antibiotics (see appendix). Given the importance of antibiotic resistance as a public health problem, it makes sense to ask whether genetically engineered foods contribute to that problem. Answering that question requires a brief discussion of how antibiotics work.

Molds and bacteria naturally produce chemicals—antibiotics—that interfere with the growth or reproduction of other bacteria but are not nearly so toxic to animals or humans. Antibiotics act by blocking specific steps in the synthesis of structures or in metabolic processes unique to bacteria: cell walls (penicillin), cell membranes (polymyxin B), proteins (streptomycin, chloramphenicol, tetracycline), nucleic acids (rifampin), or folic acid vitamins (sulfonamide, trimethoprim). When animals or humans take antibiotics appropriately—in the right dose for the right length of time—the drugs suppress the growth of all sensitive bacteria. Bacteria, however, are exceedingly small, and the normal digestive tract contains hundreds of billions of them. Among this multitude, some are likely to lack the target structure; these grow in the presence of the antibiotic. Penicillin, for example, has no effect on bacteria that lack cell walls. Bacteria can acquire antibiotic resistance by mutations that change the structure of DNA and favor survival or produce enzymes that destroy the antibiotics or pump them out. The use of low-dose antibiotics “selects” for such bacteria; the drugs kill off most competing bacteria and allow the resistant ones to proliferate.21

The use of marker genes for antibiotic resistance in plant biotechnology raises additional concerns. Perhaps the genes for such characteristics will jump to other bacteria, and the bacteria will become resistant to multiple antibiotics. Scientists transfer new genes into plants by using special pieces of bacterial DNA called plasmids. Plasmids often contain three kinds of genes relevant to this discussion: (1) genes that enable them to “infect” and transfer selected genes into plants, (2) genes for antibiotic resistance, and (3) genes that enable them to infect many different kinds of bacteria (see appendix). Plasmid-containing bacteria in the intestines of animals or people could transmit antibiotic resistance to other bacteria, some of which might be pathogenic. This possibility is not just theoretical. Some pathogenic bacteria once easily controlled by penicillin are now thoroughly resistant to that drug, and others in ground meat have been found to resist treatment by as many as 12 antibiotics.22

Such findings explain why the continued use of low-dose antibiotics in farm animals elicits so much concern. They also explain why health officials want food biotechnologists to stop using clinically important antibiotics as selection markers. They want to avoid any chance—no matter how improbable—that transgenic plants might “lose” their recombinant antibiotic-resistance markers and transmit them to soil bacteria, to animals, or to people. In the worst-case scenario, a plant gene might recombine with the DNA of bacteria living in the intestines of animals or people and pass the trait for antibiotic resistance along to disease-causing bacteria. The antibiotic used in the selection process would then be ineffective as a treatment option. Alternatively, the antibiotic might be useless if people taking it were eating foods containing genes for resistance to that drug.

Perhaps because most scientists believe that such possibilities