Plasmids

What are plasmids?

Plasmids are small pieces of circular extrachromosomal DNA in bacteria. They are able to copy themselves inside bacterial cells independently from the chromosome (self-replicating) and exist in multiple copies. Some can integrate into the chromosome. They can transfer from one cell to another via cell-to-cell contact, a process called conjugation. Plasmids are important because they are mobile and can move between different bacterial cells, including strains of the same species of even between different bacterial species. Plasmids carry multiple genes, in some cases disease-causing and antibiotic resistance genes.

How plasmids are maintained in bacterial populations

A bacterial cell can carry or acquire a plasmid encoding a protein that provides resistance to an antibiotic. This might be present in all the cells in a bacterial population or in just a few.

Figure 1. A: Population of bacteria, with a highlighted individual carrying a plasmid. B: Antibiotic kills all the sensitive bacteria, indicated by a skull and crossbones. The highlighted individual from A is resistant. C: Population of resistant, plasmid-bearing bacteria unaffected by antibiotic. D: Transfer of plasmid from resistant to sensitive bacteria, which survive in the presence of antibiotic

In the presence of an antibiotic, all bacterial cells except for the ones carrying the plasmid will be killed (Figure 1B). In this context, the plasmid confers an adaptive advantage and antibiotic-resistant cells are able to grow and multiply. Over time, the new bacterial population will be made up of mostly (or entirely) antibiotic-resistant plasmid-carrying cells (Figure 1C). The plasmid (and thus antibiotic resistance), that has now increased in prevalence, may be transferred to other bacterial populations via conjugation (Figure 1D).