The use of antibiotic resistance markers in the development of transgenic crops has raised concerns about whether transgenic crops will transfer their antibiotic resistance genes to soil micro-organisms, thus causing a general increase in the level of antibiotic resistance in the environment.

At several stages of the laboratory process, developers of transgenic crops use DNA that codes for resistance to certain antibiotics, and this DNA often becomes a permanent feature of the final product although it serves no purpose beyond the laboratory stage. Will transgenic foods join cattle feedlots and chicken farms as contributors to antibiotic resistance in the environment?

Many soil organisms have naturally occurring resistance as a defense against other organisms that generate antibiotics. European researchers (Smalla et al., 1993) sampled river water, pig manure slurry, municipal sewage sludge, and soil and reported that nptII, the antibiotic resistance gene used most often in transgenic plants, was present in rivers, manure, and sewage before transgenic plants were widely grown.

Some experts believe that antibiotic resistance is so widespread naturally that any additional contribution from transgenic plants would be insignificant (Smalla et al., 2000). One biotech company, Calgene, has calculated that bacteria acquire resistance to antibiotics through the natural mutation process far more frequently than they acquire it by taking in pieces of foreign DNA.

Recent studies (Osterblad et al., 2001; Routman et al., 1985) on the prevalence of antibiotic resistance in the gut bacteria of wildlife indicate that mammals (moose, deer, voles, baboons) living in areas isolated from humans do not harbor resistant bacteria, while mammals living in human-populated areas do harbor such bacteria.

Moose. Photo: Alaska Division of Community and Business Development

The suggestion that human activities may contribute to the occurrence of antibiotic resistance in nearby organisms needs further study, but this effect appears to occur independently of transgenic crops.

While the risks from antibiotic resistance genes in transgenic plants appear to be low, steps are being taken to reduce the risk and to phase out their use.

• The FDA recommends that developers of transgenic crops use antibiotics that are not commonly used for treatment of diseases in humans. Thus, if horizontal gene transfer does occur, micro-organisms in the body are not likely to have acquired resistance to the antibiotics that a doctor might prescribe to fight infection.
• Scientists are changing their development methods. Other marker genes, such as green fluorescent protein, or mannose (Joersbo et al., 1998), may be able to do the job that antibiotic resistance markers have done.
• Scientists are also experimenting with methods for removing the antibiotic resistance genes before the plants are released for commercial use (Dale and Ow, 1991; Ebinuma et al., 1997; Iamtham and Day, 2000; Zuo et al., 2001), so that these genes can be used during development and then eliminated from the final product
• European scientists have developed a method for inactivating the antibiotic resistance genes in the event that they are transferred to bacteria in the environment (Libiakova et al., 2001). A special DNA sequence inserted into the antibiotic resistance gene will prevent the gene from functioning inside a bacterium. Plants are able to snip out the special sequence to let the gene function correctly.