Ontario Agri-Food Technologies – FAQs

When one attempts to make transgenic plants, only a small number
actually have the gene incorporated into their genome. To find these
cells, selection systems have been developed using what are termed
selectable markers. Some antibiotics are toxic to plants and are used as
selectable markers. Genes are known in bacteria that produce enzymes that
inactivate the selectable marker e.g. the antibiotic. If this second gene
is linked to the gene we want to insert into the plant, we can simply
treat all the cells with antibiotic and those that survive will have not
only the selectable marker gene but also the gene of interest to us. It is
a quick, and often only way, of finding our transformed plants.

The antibiotic most often used is kanamycin. This antibiotic belongs to
a group of antibiotics known as aminoglycosides. Kanamycin was used as an
antibiotic extensively in the 1960’s and early 1970’s but it was then
phased out because of the advent of more modern antibiotics, although it
is still used occasionally for a small number of diseases. Kanamycin has
serious side effects such as causing deafness. Bacteria had also become
resistant to it. A gene was isolated from these bacteria that produces an
enzyme called neomycin phosphotransferase that inactivates the antibiotic.
It is this gene that is used to make plants resistant to the antibiotic.

It has been suggested that antibiotic resistance in transgenic plants
could be transferred to bacteria in the intestine. Even if this is
theoretically possible, the effect would be trivial compared with the
resistance to antibiotics that has developed through the widespread use of
antibiotics in human and veterinary medicine as well as in animal feeds.

Could the antibiotic resistance gene be transferred to bacteria in the
intestine? It is extremely unlikely. The DNA one eats is very rapidly
broken down by enzymes secreted in the duodenum just after the stomach.
For the gene to be transferred, it would have to be precisely cut from the
large amount of DNA in the food and without being damaged cross the wall
of the bacterial cell and be incorporated intact into its DNA. Not only
that, it would have to be inserted into the bacterial DNA of the cell in a
position where the gene was active. It would be extremely unlikely for all
these events to happen. It has been calculated that we eat several
trillion genes in our food each meal. If gene transfer were too common,
the bacteria in our guts would have a major problem surviving and we would
have found many plant genes in bacteria. This has not been the case.

There are other methods of selecting transgenic plants and these are
now becoming popular and will no doubt replace antibiotics in the near
future.

Mechanisms now exist for the removal of selectable markers and it is
unlikely, in the future, that any crop developed by biotechnology will
have a marker enzyme such as antibiotic resistance.

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