What are plasmids?

Plasmids are small, circular pieces of DNA found within bacterial cells, distinct from the bacterial chromosome. They are separate, autonomous units of DNA that can replicate independently of chromosomal DNA. One of the most significant features of plasmids is their ability to carry genes that can provide bacteria with advantageous traits, such as antibiotic resistance, the capability to metabolize certain compounds, or the ability to produce toxins.

Because plasmids can easily be transferred between bacteria through processes like conjugation, they play a critical role in horizontal gene transfer, contributing to the rapid spread of these advantageous traits within microbial populations. This mechanism is particularly important in the context of increasing antibiotic resistance, where plasmids can confer resistance genes to susceptible strains, leading to the emergence of multi-resistant bacteria.

The other options do not accurately describe plasmids: large chromosomal DNA refers to the main genetic material in a bacterium, viruses that infect bacteria are called bacteriophages, and proteins that protect bacterial structures, while important, do not pertain to the definition or function of plasmids themselves.