What if you could pick up bits of DNA and change your traits? In the animal kingdom, organisms are born with their lifetime total of DNA. In the bacterial world, cells can add to their genome by acquiring plasmids.
Imagine you have blonde hair and wake up one morning hating the color. Instead of buying hair dye, you find a person with the color you want and nip a little circle of that person’s DNA, stick it in your cells, and start expressing the new hair genes. Happy with your new chestnut brown hair, you decide to have breakfast – except all the restaurant is serving is oatmeal, and you are deathly allergic to oatmeal. But your neighboring diners are having no trouble at all. As before, you grab another small circle of DNA and problem solved. You too are now immune to the toxic effects of oatmeal.
Does all this sound ridiculous? To a human, it is. To a bacterium, it is simply another typical day of survival.
Those little circles of DNA your bacterial self was pilfering are called plasmids. Specifically, plasmids are nonessential, extrachromosomal pieces of DNA. What exactly does that mean? A plasmid is a short, usually circular, and double-stranded segment of DNA that is found in the cytoplasm separate from the main bacterial chromosome.
Plasmids usually contain between 5 and 100 genes that are not required for the survival of the bacteria. Genes for normal growth, metabolism, and cell structure are located on the main bacterial chromosome. As long as the bacterium is thriving in a low-stress environment, removing all the plasmids would not affect the ability of the bacterium to survive.
If bacteria are able to survive without plasmids, what purpose do they serve? In reality, the possibilities are endless. As long as the genes that the plasmid codes for don’t interfere with the cell’s survival or the plasmid’s ability to copy itself, any gene is a possible plasmid-based gene.
Plasmids have the ability to replicate, or copy, themselves. Generally, bacteria replicate by binary fission. A single bacterial cell, called a mother cell, copies the chromosome, then the cell splits in half, giving each half of the cell a copy of the chromosome. The two new identical daughter cells are essentially clones of the mother cell. But what about the plasmids? Plasmids carry genes that direct their own replication and additional factors that ensure that the copies get separated into the new daughter cells. This ensures that the plasmids are not lost from the cells during binary fission.
There is another way that plasmids ensure their proliferation in bacterial populations. Many plasmids are capable of transferring from one cell to another in a process called conjugation. These transfer plasmids carry genes called transfer genes or tra genes that are responsible for this process. During conjugation, a bacterium that has a transfer plasmid expresses the tra genes to construct a long, thin tube called a pilus. The pilus is hollow and attaches to a neighboring cell, linking the cytoplasm of the two cells together. The plasmid then copies itself and transfers one copy through the pilus into the other cell. Not all plasmids carry these transfer genes, but many do. F plasmids are plasmids that carry factors that allow for the transfer of genetic material from one cell to another via conjugation.
Some plasmids carry resistance factors and are called R plasmids. The genes on R plasmids confer resistance to antibiotics or other bacterial growth inhibitors. A bacterium with an R plasmid for penicillin resistance is able to survive treatment by that antibiotic. R plasmids can also carry the tra genes that allow the plasmid to be spread from cell to cell. The spread of R plasmids poses a very real threat to our current ability to use antibiotics. Since the resistance genes are found on highly mobile plasmids as opposed to the more stable chromosomes, antibiotic resistance is able to spread rapidly through a bacterial population. Plasmids are also not limited to specific species, so the antibiotic resistance can spread between species, creating bacterial strains that are resistant to many common antibiotics.
Bacteriocins and Toxins
If R plasmids are considered a bacterial defense mechanism, bacteriocins and toxins could be considered bacterial offense.
Bacteriocins are proteins produced by bacteria that inhibit or kill other bacteria. Producing bacteriocins can be tricky. How could a cell produce a toxic protein without harming itself? The plasmids that carry the genes for many bacteriocins also carry additional genes that provide resistance to those bacteriocins. This allows the cell to wage war on its neighbors while protecting itself.
Many bacteria carry plasmids that encode toxins or other factors that allow them to cause disease. Clostridium tetani, the causative agent of tetanus, has a plasmid that encodes a potent neurotoxin that causes rigid paralysis. Toxins and proteins that subvert the immune system are often found on plasmids. Removal of these plasmids is often enough to render the pathogen harmless.
Bacteria have naturally evolved a great system for storing genes on plasmids that can be transferred from cell to cell. It was only a matter of time before scientists figured out how to exploit plasmids to engineer bacteria. If a scientist wants to design and express a gene, they can put it on a plasmid, insert the plasmid into a bacterial cell, and then coax the cell into expressing the gene. This allows scientists the opportunity to create an endless variety of artificial plasmids for an infinite possibility of uses. Remember those bacteriocins we discussed? Well, some of them are so effective at killing bacteria and so safe for humans that the food industry has begun using them as preservatives. In order to do this, scientists have to produce large quantities of the bacteriocins. They can use plasmids to express the bacteriocin genes at high levels, producing enough to use commercially.
Let’s review. A plasmid is a small, extrachromosomal, and nonessential piece of DNA. Bacteria utilize plasmids to adapt to stressful environments but generally could survive without them during favorable growth conditions.
There are countless plasmid functions with even more countless varieties of individual plasmids. F plasmids enable conjugation, allowing for the direct passage of DNA between cells. R plasmids code for products that provide resistance to antibiotics or growth inhibitors. Some plasmids have bacteriocin genes that code for products that kill or inhibit other bacteria. And finally, pathogenic bacteria can house their toxin genes on plasmids.
Maybe next time you dye your hair or can’t eat a yummy food because of an allergy, you’ll look back on your time as a plasmid-toting bacterium with longing. If only changing our DNA could be that easy;
After seeing this video, you should be able to:
- Define what plasmids are
- Describe the function of plasmids
- Understand how F plasmids, R plasmids and bacteriocin affect bacteria