Evolution of Plasmid Host Range, NHR and BHR

Plasmids are known to be able to move from one bacterial strain to another. However, plasmids cannot move to and replicate in all strains.

narrow-host-range - One of the patterns that we see most in nature is a narrow-host-range (NHR) plasmid, a "specialist". Endosymbionts such as intracellular parasites tend to specialize to specific hosts i.e., they become increasingly better at invading and replicating in that host. Similarly, many genetic elements seem to have specialized to a narrow range of hosts. It is thought that some NHR plasmids have developed in certain hosts to be second chromosomes, probably because they were very stable in a particular strain, successfully increasing their size by acquiring genes from the host's chromosome.

broad-host-range - In contrast, a broad-host-range (BHR) plasmid, a "generalist", has adapted to transfer and replicate in many different and frequently distantly related bacterial species. This generalist characteristic of BHR plasmids can cause problems in human society, such as the spread of multiple antibiotic resistance among pathogenic bacteria. It thus appears that NHR plasmids are functioning as genetic reservoirs for a closely related group of bacteria while BHR plasmids function as especially active gene transporters across a diverse range of species. Even with all the knowledge about plasmids that we've gained in the last 50 years, and the usefulness they've proved in the laboratory, relatively little is known about their origin.

It has recently become clear that through small genetic changes such as single point mutations, both types of plasmid could drastically improve their ability to stably replicate in strains wherein they used to be lost easily. This suggests the possibility that plasmids can evolve to expand or at least shift their host range, which is undesirable for public health in the case of drug resistance or virulence plasmids. Scientists, including Dr. Eva Top, are now trying to determine if and how the host ranges of several plasmids can evolve, i.e., how they expand, contract, or shift over time. By learning more about how and at what rates the host ranges of plasmids evolve, we may eventually be able to limit the rapid spread of drug resistance to important human pathogens.