Plasmids are extra chromosomal DNA of bacteria, but they have numerous functions for the survival and living of bacteria. It is claimed that bacteria lose their plasmids when they live without stressful conditions. Then are they lose all the plasmids? What may be the best conclusion about this question?
I have always learned it like this:
Let's consider a plasmid that encodes some function that give the cell an advantage, e.g. antibiotic resistance. If this cells are grown in media with this antibiotic the cells containing this plasmid will grow while cells that for some reason ends up with no plasmids will die or be growth inhibited. If the cells are grown without any antibiotic and one cells ends up without any plasmid this cell will not have a disadvantage but quite the reverse since this cell do not have to spend energy on reproducing the plasmid (or the genes that are encoded in the plasmid). Thus the cells without plasmid will have a growth advantage and will be dominating the culture.
I have always learned it like this:
Let's consider a plasmid that encodes some function that give the cell an advantage, e.g. antibiotic resistance. If this cells are grown in media with this antibiotic the cells containing this plasmid will grow while cells that for some reason ends up with no plasmids will die or be growth inhibited. If the cells are grown without any antibiotic and one cells ends up without any plasmid this cell will not have a disadvantage but quite the reverse since this cell do not have to spend energy on reproducing the plasmid (or the genes that are encoded in the plasmid). Thus the cells without plasmid will have a growth advantage and will be dominating the culture.
In bacteria, plasmids are mobile genetic elements in addition to chromosomal DNA. Unlike chromosomal DNA, they are not equal divided during cells proliferation. They are mobile elements can enter the cells during the cell growth but also can lost during the cell divisions. With selections, any cells that lost the plasmids will die and all the survived cells have the plasmids. if without selection, all the cells can grow and end up that the cells without plasmids take their growth advantages to dominant the culture as mentioned by Viktor's answer.
Dear Huanting Liu and Viktor would you give me any reference of your speech.
Syed, this link is to a publication that discusses plasmid loss and persistence https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1894622/
Plasmids are extra-chromosomal genetic elements that, by definition, are not required for the survival of the bacterial species under "typical" conditions. Plasmids are known to encode genes that provide a selective advantage under specific conditions, such as the presence of an antibiotic. So, if an antibiotic is present, a plasmid that encodes an antibiotic resistance gene (for that specific antibiotic) provides a selective advantage for those cells that carry the plasmid. In the presence of antibiotic challenge cells with the plasmid live while cells without the plasmid die. But there is more. Plasmids may be transmitted horizontally by conjugation or natural transformation/transduction from cell to cell of the same species, or to different species. Moreover, many antibiotic resistance genes on plasmids reside within transposable elements so that the antibiotic resistance gene can move from the plasmid to the chromosome, or from one plasmid to another. So in the presence of antibiotic challenge it is not simply that cells that contain a plasmid encoding an antibiotic resistance gene live while other cells die. One must also consider that antibiotic challenge also provides selective pressure favoring horizontal transmission of plasmids and transposition. In a laboratory environment where a pure bacterial culture is grown, the absence of an antibiotic challenge can lead to the loss of the plasmid within the population within a relatively few sub-culturing events, at least in some cases, because the retention of the plasmid places a small, but measurable, metabolic burden on the cell.Therefore it is energetically more favorable for cells to lose the plasmid. However the loss or retention of plasmids in nature is a more interesting and more complex question. When there are multiple bacterial species and myriad micro-ecosytems in a natural environment such as soil, then the loss or retention of a plasmid in the absence of antibiotic challenge has not been well characterized and can make for an interesting research topic. Plasmids that encode antibiotic resistance genes are often assumed to provide no selective advantage when antibiotic challenge is not present. But is this really true? Do plasmids provide other contributions beyond antibiotic resistance to microbial populations?
http://journal.frontiersin.org/article/10.3389/fmicb.2017.01547/full?utm_source=F-AAE&utm_medium=EMLF&utm_campaign=MRK_368025_51_Microb_20170822_arts_A
Here is a recent publication that discusses cryptic plasmids demonstrating that there is much about plasmids that we do not yet know.
Here are some more references of simple explanations, http://2015.igem.org/Team:Technion_HS_Israel/PlasmidLoss http://www.ucl.ac.uk/~ucbpafe/sp.pdf
It sounds like you are writing a paper for a class. You need to do some actual reading of about the basics of bacterial growth, selection pressure, and evolution. You can NOT cite a personal conversation about such a common topic. There are no shortcuts to learning something for yourself.
If a given bacterial race is not exposed to any selection pressure,as an antibiotic or a collicin (bactericide protein) producing bacteria ,it will not need to have a antibiotic resistance gene on it's plasmid .
- Badges
- Science topic
- Similar topics
- Biological Science
- Molecular Biology
- Biomolecules
- Nucleic Acids
- DNA