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I am sorry if this question is too general, and does not have any concrete answer.

I was explaining to my non-biology-background friend about plasmids and how they are picked up by bacteria from the remains of other dead bacteria during stressful conditions. This is when this question arose:

Why would bacteria pick up plasmids released from the dead bacteria, as this means that these plasmids either did not help it survive in those conditions, or could have in fact been the cause of their death? This also lead to the question of whether plasmids can be lethal? (I believe there could be, for example plasmids which are translated into proteins which are endonucleases which would probably kill the bacteria, yet I am not sure).

Moreover, if this sort of plasmids exists, then why could we not engineer them specifically and use them as medicine as an “antibiotic”?

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  • $\begingroup$ You might be interested in this answer, and the other answers to the question. $\endgroup$ – canadianer Jan 11 at 5:38
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The ability of bacteria to take up intact environmental DNA is called natural competence.

One problem with trying to take advantage of this in a therapy is that it is not very efficient. Importantly, natural competence is regulated and tends to be activated when bacteria are already stressed. This is also likely part of the answer as to why a bacterium would take up DNA from the outside: if they are already in bad shape, they don't have much to lose. Bacteria growing in non-stressful conditions are less competent, and possibly negligibly so.


Ween et al 2002 compared competence in two bacterial strains, one with a "normal" regulation of competence. At most bacterial concentrations, the "normal" strain showed less than 1% transformation, and this is still in sort of an ideal laboratory condition, where the culture medium is flooded with plasmids. There wouldn't be a feasible way to even approach delivering that much plasmid DNA into a human.

Ween, O., Teigen, S., Gaustad, P., Kilian, M., & Håvarstein, L. S. (2002). Competence without a competence pheromone in a natural isolate of Streptococcus infantis. Journal of bacteriology, 184(13), 3426-3432.


The other big problem is you are working directly against selection pressures. In the case of a plasmid that improves survival (say, one containing a gene that confers antibiotic resistance in the presence of that antibiotic), then the population will more quickly increase the expression of that plasmid, as bacteria with it grow and reproduce while other bacteria die. However, if you have a lethal plasmid, all the bacteria you are able to transform simply die. They don't spread the plasmid by reproducing, and as they die the non-transformed bacteria will continue to reproduce.

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This also lead to the question of whether plasmids can be lethal?

Yes, genes in plasmids could be beneficial, neutral or even lethal, although lethal plasmids may have trouble surviving for long since they depend on a live cell for replication.

Moreover, if this sort of plasmids exists, then why could we not engineer them specifically and use them as medicine as an “antibiotic”?

Efficacy aside, they're relatively large, highly immunogenic molecules. Not great properties in an antibiotic.

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