Why don't all bacteria have F-plasmids by now?

Question

Some bacteria can undergo gene transfer by conjugation. Conjugation is a form of horizontal gene transfer, meaning from one (unrelated) bacterium to another (in contrast to vertical gene transfer, which is from predecessor bacterial cell to successor (daughter) cells). Conjugation is initiated when one bacterium has an F-plasmid (Fertility plasmid) and forms an F-pilus, connecting the F+ bacterium with an F- bacterium, initiating transfer of the F-plasmid. Why is it that during evolution not all bacteria have acquired this F-plasmid?

Answer 1

Evolution favors fitness, so it's not beneficial for a bacterial cell to maintain and express genes for conjugation if no F⁻ cell is nearby to receive the plasmid. Producing conjugation machinery requires energy, so unless the environment creates a selective advantage for gene transfer, such as stress or rapid change, the cost outweighs the benefit. Even if no conjugation occurs, maintaining the F-plasmid requires resources for replication and gene expression. This slows growth and gives an advantage to plasmid-free cells in stable environments.

Acquiring new genes is also not always helpful; some may be neutral or even harmful. Gene uptake can introduce harmful or incompatible genes (e.g., toxic products, regulatory disruptions), so bacteria don't "want" to take up DNA blindly. Host defense systems, like restriction enzymes or CRISPR-Cas, can block plasmid uptake. The CRISPR-Cas system, for example, targets incoming plasmid DNA if it contains sequences matching the system's stored spacers. In the absence of selection pressure, plasmids can be naturally lost through cell division, especially when they don't provide an advantage.