Plasmids

Plasmids are extrachromosomal DNA molecules found in bacteria, archaea, and some eukaryotic microbes like yeast. These small, circular DNA structures typically contain fewer than 30 genes, although some may exist linearly. Plasmids vary in their number within a cell, known as copy number. Single-copy plasmids are present in one copy per cell and multi-copy plasmids are present in multiple copies, reaching over 100 copies per cell.

Plasmids usually replicate independently of the chromosomal DNA utilizing the host’s DNA replication machinery. Some plasmids, known as episomes, can integrate into the host chromosome and replicate along with it. A well-known example is the F factor episome, which plays a crucial role in bacterial conjugation. The F factor encodes genes responsible for the formation of sex pili, which facilitate the direct transfer of plasmid DNA between bacterial cells, promoting genetic exchange.

Although plasmids are not essential for basic cellular functions, they provide significant advantages for microbial survival. For instance, some Pseudomonas species carry plasmids that encode enzymes capable of breaking down environmental pollutants, aiding bioremediation. Similarly, plasmids grant ecological benefits, such as those found in Rhizobium bacteria, enabling the formation of nitrogen-fixing root nodules in legumes, which enhance soil fertility.

Plasmids also play a role in bacterial competition and pathogenicity. Specific plasmids encode bacteriocins, proteins that eliminate competing bacteria, giving the host an advantage in resource-limited environments. Some pathogenic E. coli strains harbor plasmids that encode toxins and attachment factors, leading to severe diarrheal diseases in humans.

A particularly significant class of plasmids includes resistance plasmids (R-plasmids), such as R100, which confer resistance to multiple antibiotics. These plasmids pose a significant challenge in medical treatments as they enable bacteria to survive antibiotic exposure, contributing to the growing issue of antibiotic resistance.