The existence conditions for bacterial plasmids: Theory and reality |
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Authors: | Lone Simonsen |
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Institution: | (1) Department of Zoology, University of Massachusetts, 01003 Amherst, Massachusetts, USA;(2) Present address: Department of Microbiology, Technical University of Denmark, Building 221, DK-2800 Lyngby, Denmark |
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Abstract: | Bacteria abound with conjugative and nonconjugative plasmids that often carry genes determining a number of environmental
adaptations. Plasmids may also encode genes that enable them to transmit themselves infectiously to new host cells, by conjugation
or mobilization. The question of whether plasmids can be maintained in a bacterial community as parasitic DNA, that is, while
conferring a selective disadvantage to their host, serves as a basic hypothesis in theoretical studies of the population biology
of plasmids. The conditions necessary for the establishment and maintenance of plasmids have been determined analytically
for the simplest possible models. Based on these a priori conditions, on some reconsiderations and extensions of these models,
and on recent estimates of transfer rates of liquid and surface bacterial populations, it will be argued that within a bacterial
population, a parasitic lifestyle is unlikely for most naturally occurring plasmids. This result raises anew the problem of
how cryptic plasmids are maintained and why plasmids encode costly and elaborate genes for horizontal transfer. |
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