EFSA report on antibiotic-resistance markers

Restricted use

On the 22 April 2004 the scientific panel responsible for safety matters relating to green genetic engineering (GMO Panel) of the European Food Safety Authority (EFSA) adopted a report on antibiotic-resistance marker genes in genetically modified plants. The experts consider that a general ban on these markers is not justified, recommending instead a differentiated and sensible approach: some antibiotic-resistance markers are to have their approval withdrawn, others are to be authorised only with restrictions. Nothing will change as far as the nptII marker gene is concerned – a gene that has been used in most GM plants and confers resistance to the antibiotic kanamycin.

Although the frequency of horizontal gene transfer from a GM plant to micro-organisms is classed as "very low", the GMO Panel of the EFSA assumes when assessing the safety of the various antibiotic‑resistance marker genes (ABR genes) that such an event is possible in principle. The yardstick of the EFSA experts was that a marker gene should not have any harmful impact on the environment or health even in the extremely rare event of horizontal gene transfer taking place.

Several criteria were used to assess the different ABR genes:

The medical significance of the particular antibiotic, its current use in human and animal medicine and its effectiveness in controlling specific infectious diseases.
The natural distribution of the particular occurrences of antibiotic resistance in micro-organisms, how they occur in soil or water and also in the digestive tract of humans and mammals.

The EFSA recommends as before that GM plants should be authorised for release into the environment if they contain ABR marker genes which are already widespread even under natural conditions and only render ineffective those antibiotics which are no longer used in medicine.

Like all EFSA scientific committees, the GMO Panel consists of independent, high-ranking scientists. It is consulted in the case of approvals of GM plants in the EU and gives scientific opinions. The EFSA expert committees only perform an advisory role. Political institutions such as the European Commission, the Council of Ministers and the European Parliament take decisions relating to approvals or other issues.

Not all antibiotic-resistance genes are the same

The GMO Panel of the EFSA classifies the antibiotic‑resistance genes used as markers into three groups.

Group 1: ABR genes which are already widely distributed among naturally occurring micro-organisms. The antibiotics concerned are of no or only very limited relevance for human and animal medicine. This group includes the nptII gene (kanamycin resistance ) and the hph gene (hygromycin).

Example: nptII gene. This marker gene , the most widely used in transgenic plants for several years, comes from a transposon (jumping gene). It confers resistance to several antibiotics including kanamycin and neomycin. However, these are only used occasionally, for instance in patients who are unable to tolerate other antibiotics. Kanamycin can also cause serious side effects.
It is assumed that the use of marker genes in this group in transgenic plants will not change their existing distribution in the environment. The EFSA experts are unable to find any safety grounds to justify the restriction of these ABR genes. They therefore recommend that GM plants with these ABR marker genes should continue to be authorised without restriction for both field trials and commercial cultivation.

Group II: ABR genes which are widely distributed in naturally occurring micro-organisms. However, the associated antibiotics are still prescribed to control certain diseases. This applies to the ampr gene (resistance to ampicillin) , the aadA gene (streptomycin) and the Cmr gene (chloramphenicol).

Example: ampr gene. This gene confers resistance to the antibiotic ampicillin. It comes from E. coli bacteria and is used in approved transgenic plants (Bt176 maize). Although ampicillin is only prescribed occasionally, it remains the agent of choice for certain infections.
It is assumed that the use of marker genes in this group in transgenic plants will have virtually no effect on the existing distribution. If these marker genes were to have an impact on the health of humans or animals, it would be regarded as minimal. The EFSA experts recommend authorising the use of these ABR markers only for deliberate release trials, but not for commercially grown GM plants.

Group III: ABR genes which confer resistance to antibiotics that are highly relevant for human medicine.

Example: nptIII gene. This gene confers resistance to the antibiotic amikacin, an important reserve antibiotic which is effective in controlling various infectious diseases.
Even if there is no proven risk of reducing the effectiveness of this antibiotic as a result of using corresponding marker genes in GM plants, these marker genes should be dispensed with as a precautionary measure. The EFSA panel advises that GM plants containing these marker genes should not be released into the environment either for trials or commercial purposes.

Horizontal gene transfer: an extremely rare occurrence

Although horizontal gene transfer from plants to micro-organisms is possible in principle, it is an extremely rare event. So far horizontal gene transfer has only been proven under optimal conditions in the laboratory.

A number of different factors must converge before horizontal gene transfer can actually take place.

Release of the intact antibiotic-resistance gene from the plant cell.
As a rule, when DNA is released from plants it is broken down into smaller units by the plant’s own enzymes (nucleases). Enzymes like this, which break down DNA, are also found in the human digestive tract and in the rumen of herbivores.
Absorption of the gene by competent bacteria.
So far this ability has only been observed in relatively few bacteria.
Integration can take place through the process of recombination , but this is only possible if certain prerequisites are met. For example the DNA sequence of the resistance gene and of a bacterial gene must correspond closely (homology).
Successful expression of the antibiotic-resistance gene.
The gene must be expressed and converted into a protein which confers resistance to the particular antibiotic. To enable this to happen the gene must have recognition signals (regulation sequences) which can also be "read" by bacteria.