How do bacteria transmit antibiotic resistance to other bacteria?

All bacteria contain a single principal chromosome with anywhere from 1,000 to 10,000 genes in it. It’s a circular piece of DNA that contains all of their genes. As the cells grow and before they divide into two daughter cells, that chromosome is copied. So each daughter cell receives an exact copy of the original mother cell chromosome. Many bacteria contain in addition accessory small accessory chromosomes, called plasmids, which are also circular pieces of DNA but which contain anywhere from 5 to 50 or 100 genes– much, much smaller than the bacterial chromosome. These plasmids exist in nature. They are found in most bacteria, anywhere from 0 to 10 of them per bacterium.

And they provide the bacteria that carry them special genetic properties, special genetic abilities. Such plasmids are a principal mechanism by which antibiotic resistance is found. And the interesting and dangerous property of many of these plasmids is that they contain in addition genes that provide a mechanism to transfer the plasmid from the cell in which it resides to another nearby cell that doesn’t have it. These are called conjugative plasmids. And the process is called conjugation. And as you can see this makes drug resistance a wildly transmissible trait, just as viral infection in people is a wildly transmissible trait. Many of these plasmas exist. Many mechanisms of conjugative transfer exist.

And the sole purpose of those conjugative genes is spreading the plasmid from one cell to another. So this is a principal mechanism by which bacteria acquire or are given to new genes. There is a second mechanism that the students in my lab happen to study in which some bacteria have a set of 20 to 30 genes whose sole purpose, as far as we know, is to allow the cell that carries them to kill another bacterium on contact and grab DNA that’s released from that cell and bring it into the living attacker’s cell where genes in that DNA can be inserted into the attacker cell’s chromosome. This has been called natural genetic transformation.

Bacteria that have it have this– have it widely within a single species. And that species then becomes a kind of a cooperative gene library where every cell is potentially a source of new genes to any other cell that has been evolving independently. So both of those mechanisms depend on a set of a dozen or a score or more genes that have the specific function of allowing genes to be transferred from one cell to another. In the case of transformation, those genes are in the cell’s own large chromosome. In the case of plasmids, the genes are carried on the plasma that can thereby transfer itself.

So the important– for a biologist the important difference is that the genes for conjugation are on the plasmid that is transferred by conjugation. The genes for natural transformation are scattered around the chromosome of every cell of the species, making every cell in the species potentially a genetic aggressor that can kill another cell of the same or a different species and obtain DNA from the dead cell. It’s a very aggressive mechanism of gene exchange. But it is successful and widespread in many but not all bacteria.

Well, they don’t do it every minute. But they do it at a significant rate. And if you apply an antibiotic to a bacterial population, the ones that have resistance survive or the ones that got resistance by one of these mechanisms survive and the others don’t. So even transfer at a modest rate under the selection provided by therapeutics, the rate becomes highly significant. The takeaway here is that bacteria have natural ways of moving resistance genes around. So if you select for them to move around, you will have more and more resistance genes. Minimising selection is a way to minimise spread of resistance genes.