Above: Illustration of bacteriophages attacking bacteria. Image © ktsimage, iStockphoto.com
Scientists believe that all organisms on Earth can be infected by viruses. That includes bacteria. Viruses that can only infect bacteria are called bacteriophages, or phages for short. And just like humans, who can be infected by viruses through sexual contact, bacteria are vulnerable to viral infection during conjugation, a process that is sometimes called “bacterial sex”.
There are obviously many differences between human and bacteria. But the fact they can both fall prey to viral infections shows how eukaryotes (more complex organisms, including humans) and prokaryotes (simpler organisms, including bacteria) are not so different after all. In fact, these similarities make it possible for scientists to manipulate phages for humans benefit. Examples include the development of tools for decontaminating medical equipment and for fighting drug-resistant bacteria.
Did you know? Some scientists argue that viruses are not living things, because they cannot reproduce without infecting a host.
Bacterial conjugation and male-specific phages
Bacteria usually reproduce asexually, creating offspring that are each genetically identical to their parent. However, bacteria can also use conjugation to transfer plasmids (bits of DNA) between each other. In fact, conjugation is one of the few ways for bacteria to exchange and integrate DNA within a population. This can increase genetic diversity, making it possible for the bacteria to evolve more quickly!
Within a population of bacteria, cells that contain a self-transmissible plasmid are potential donors. Cells that don’t have one are potential recipients. Self-transmissible plasmids encode the proteins needed to prepare the plasmid DNA for transfer. They also encode the proteins needed to create the conjugation pilus--also called the sex pilus--that extends from the donor cell and attaches to a recipient cell. The sex pilus holds the two cells tightly together and provides a channel for the plasmid DNA to pass through on its way to the recipient cell.
Did you know? There are plant and bacterial equivalents to human sexually transmitted infections.
Plasmids are double-stranded pieces of DNA, but only one of the strands is sent to the recipient cell during conjugation. This allows both donor and recipient cells to recreate a double strand through DNA synthesis. As a result, both cells end up with a working double-stranded plasmid, making them both potential donors for another transfer. (5)
Phages that infect bacteria during conjugation, such as the M13 and f1 phages, are called male-specific phages. They enter a donor (or “male”) cell by attaching themselves to the end of the the sex pilus. When a specific protein on the phage makes contact with the cell’s sex pilus, the pilus retracts. This draws the phage to the cell surface and allows it to enter the cell.
However, some viruses can only infect certain types of bacteria. This is because the receptors on the outside of the virus and the cell membrane need to be compatible.
Did you know? Viruses are made up entirely of nucleic acid (DNA or RNA), surrounded by a protective membrane.
Implications for human medicine
The equivalent human disease to the M13 and f1 phage infections of bacteria would be a sexually transmitted infection (STI) that only infects men. Of course, human viruses do not selectively infect only half the population in this way. So when it comes to human STIs, everyone is at risk! So what is the significance of male-specific phages for human health?
Well, the fact that phages can only infect certain bacterial cells means they could be an important tool for fighting antibiotic resistant bacteria. Certain types of phages could be used to attack and kill only those microbes that cause disease, leaving human cells and good bacteria untouched. A phage could be engineered with receptors compatible with a specific antibiotic-resistant bacteria. The phage could then be applied to a person with a bacterial infection, bind to the cells of the targeted bacteria, and inject its DNA into the target cells. The bacteria would be destroyed and the infection would be cured. For example, researchers have engineered an M13 phage that binds specifically to cells of H. pylori, a bacterium that causes ulcers in the stomach and intestines.
Using phages for decontamination
Phages could also be used to help disinfect or sterilize contaminated material, to make it safe to use again. Currently, decontaminating research labs and hospitals involves harsh chemicals that have many disadvantages. They tend to be corrosive, they irritate skin and eyes, they contain ingredients that cause cancer, and they can be environmentally unfriendly.
By replacing harsh chemicals with highly-specific phages, decontamination could become safer and more efficient. Phages could be engineered to target specific biological contaminants without harming other organisms (like people!) or damaging equipment. There has already been some research into using phages to decontaminate bioterrorism sites, as well as produce, and food processing plants.
These are just some of the ways that research on bacteriophages could lead to big advances in healthcare and biotechnology. But before doctors can regularly use phages to treat infections, scientists need to develop better ways of identifying specific disease-causing microbes. And they also need to perfect techniques for culturing and engineering phages capable of destroying just those microbes and not human cells and good bacteria.
Articles and websites with general information on bacteriophages:
Scientific articles about using bacteriophages in medical treatments:
Scientific and magazine articles about using bacteriophages in decontamination: