Dear @suesa, I finished reading the article and I feel there's a need to underscore something essential, when you talk about selecting for the bacteria having incorporated the right plasmid.

Bacteria are lean and mean energy optimizing machines - they compete to make the most of the resources available in their environment. When two bacterias try to reproduce, one with your plasmid and the anti-ampicilin gene inside and the other without, what happens:

• if in their medium there is ampicilin, the latter will simply die and the former will reproduce
• but absent ampicilin from the medium, the former will replicate slower than the latter, because it will need more energy and time to also replicate your plasmid ... for no immediate benefit.

After a few generation, the proportion of bacteria without the plasmid (without the anti-ampicilin gene) will be much higher. Those carrying the anti-ampicilin gene will be adversely selected (when there's no ampicilin in the medium). They'll start shedding the plasmid in order to compete on a level playing field with their brethren.

This is an important mechanism that needs to be understood by people throwing a fit over Genetically Modified Organisms. When the modification concerns the addition of a gene that confers a benefit under a selection agent then the risk of that modification spreading in the wild population is much much smaller than what the anti-GMO groups would have people believe.

Because absent that selection agent ( a proprietary pesticide for instance that the evil Monsanto wants to sell) having that genetic modification is actually a handicap for the organism.

And that is precisely Monsanto's business model: they do not want farmers to be able to maintain the modified varieties without also purchasing their pricy pesticide. So those modified varieties are usually quite fragile and impaired when let in a field without the pesticide .

This is to say that one of the most touted risks of GMO, that they might spread and "contaminate" the general population is, in many cases at least, the result of poor scientific reasoning: those genetic modification are ONLY beneficial in conjunction with some artificial selection conditions and are DETRIMENTAL absent those conditions.

In the wild, most GMO cannot compete with the non-GMO versions (if you don't also buy and spray the Monsanto poison)

Coming back to your post, this effect is seen in the fact that ampicilin is still used as an antibiotic despite generations of biology students unknowingly smearing anti-ampicilin bearing plasmids all over the place: the gene conferring ampicilin resistance is pretty big and costily to replicate and when there's no ampicilin around for it to be useful, bacterias quickly dump the resistance gene because it's slowing them down too much.

Students are certainly being super-careful but there's so many of them all over the world and mistakes happen from time to time. What "saves" us here is rather natural selection mechanisms.