by Gina Cora

There has been much debate over whether using antibiotic resistance marker genes in genetically engineered foods results in antibiotic resistance of stomach bacteria. Antibiotic resistance genes are incorporated into nearly every genetically engineered organism as markers to indicate that an organism has been successfully engineered. Both sides of the issue agree that antibiotic resistance marker genes and their products are present in foods derived from organisms developed using the markers. The point that they disagree on is whether these genes and their products promote antibiotic resistance in stomach bacteria, a potentially dangerous occurrence for humans. If harmful bacteria in humans become resistant to antibiotics through consuming genetically engineered food, then when people try to treat the bacteria with the antibiotics there will be no result. Many people could get very sick and/or die.

The FDA believes that these genes and their enzyme products do not add to existing levels of resistance in bacterial populations in any significant way, but the agency does express some uncertainty on the issue. The FDA acknowledged that marker gene-coded enzymes that inactivate certain clinically useful antibiotics, when present in food, theoretically might reduce the therapeutic efficacy of oral antibiotics. However, the FDA does not believe that this would be very likely to occur (www.vm.cfsan.fda.gov).

One point that the FDA could use to support the view that it is unlikely for stomach bacteria to become antibiotic resistance is that DNA is degraded when food is processed. This means that the when genetically engineered food goes through humans’ digestive tract the antibiotic resistance marker genes would be destroyed, and thus stomach bacteria would be unable to become antibiotic resistant through horizontal gene transfer. Another test done using highly purified microbial (plasmid) DNA and specialized bacterial cells designed for "test-tube" experiments showed that stomach acids effectively destroyed DNA, even when at high concentrations (www.biotechknowledge.com). This would have the same meaning as the previous point.

Other scientists oppose the FDA’s position; they believe that the risk is too great to outweigh the benefits of genetically engineered food. The antibiotic resistance genes most commonly used as markers are genes conferring resistance against the aminoglykoside antibiotics and a gene conferring resistance to ampicillin. Aminoglykoside antibiotics belong to a group of "new generation" antibiotics that are of great value in treating serious infections. There would be very serious effects if these antibiotics were rendered ineffective against pathogenic bacteria.

Even though here are no studies proving that transfer of resistance genes from genetically engineered food to bacteria in the stomach, there are a number of observations that indicate that this might occur. Interestingly, a research team lead by Walter Doerfler in Cologne, Germany has come up with the exact opposite results regarding gene degradation as the FDA’s side. This research team has repeatedly demonstrated that pieces of DNA, as large as genes, can survive digestion in the gastrointestinal tract. These pieces were absorbed and found in the blood. There are other studies that showing that especially stressed and starving bacteria are prone to take up pieces of isolated DNA. There is also evidence indicating that genes, including antibiotic markers, may be transferred between bacteria. This includes "horizontal gene transfer" between gut bacteria in small animals.

These observations indicate that genes may survive digestion, that isolated pieces of DNA (like genes) may be taken up by bacteria, and that antibiotic marker genes may be transferred between bacteria. Some scientists say that these indications make it impossible to dismiss the possibility that genes from the food may be taken up by normal bacteria that inhabit the gastrointestinal channel and may be transferred from them to pathogenic bacteria that happen to enter the gut. If so, these scientists say that the result might be an increase in the prevalence of pathogenic bacteria that are resistant to valuable antibiotics, a dangerous side effect already discussed (www.psrast.org/antibiot.htm).