Natural Bacteria Killers: How Bacteriophages Find and Eliminate Their Hosts

Bacteriophages, also called phages, are viruses that kill bacteria. They do not kill humans, animals, or plants. Phages only kill one or a few types of bacteria. Therefore, we can use phages that only kill disease-causing bacteria as medicines. Using phages ensures that the helpful bacteria stay alive. How do phages kill some bacteria and not others? They recognize specific parts of a bacterial cell. In this article, we describe how phages find their target bacteria even when other microbes are around.

Phages (like all viruses) cannot reproduce by themselves nor make new phage copies. Viruses always need a cell to infect, called a host cell, to HOST A cell used by a virus to reproduce. reproduce themselves. Bacteriophages take advantage of bacteria for this purpose. These microscopic pirates begin their attack by sticking to a bacterial cell. Then, the phage enters the cell and makes more copies of itself. How many copies are made? Hundreds! For every bacterial cell that is attacked, hundreds of new phages are created. These copies are freed as the bacterial cell explodes ( Figure B) [ , , ]. All of this begins with one phage. This process is happening in the soil, oceans, and even in your own body! Humans are using these natural enemies of bacteria for the greater good. For example, phages can be used as medicines to kill disease-causing bacteria that are antibiotic-resistant, meaning that

ANTIBIOTIC-RESISTANT
A term used to describe bacteria that are no longer sensitive to the medicines normally used to kill them or stop their growth. normal drug treatments used to kill them are no longer e ective. In other cases, phages are applied to cooking surfaces, meat, or produce. This prevents bad bacteria that can cause fever and diarrhea, like Salmonella and Listeria monocytogenes, from getting on our food [ , ]. As you can see, phages are the good viruses. They help us stay healthy and protected from disease-causing bacteria.

Figure
Phages find their specific hosts with a microbial version of Where is Waldo ® ? Phages use receptors on the surface of bacterial cells to recognize their hosts, just like players of Where is Waldo ® recognize Waldo by looking for his outfit. Can you find the Waldo bacteria?

PHAGES RECOGNIZE SOME BACTERIA AND LEAVE THE REST ALONE
Even though there are many types of bacteria everywhere, phages only attack some of them. How do phages know which bacteria to kill? It turns out that phages are picky! Each phage is only able to use certain types of bacteria as hosts. Phages search for their hosts by sifting through all the microbes around them. This is like playing the Where is Waldo ® puzzle game. To find Waldo, you search for a character with glasses, brown hair, a red-and-white striped beanie with a matching shirt, and blue jeans. There may be others in the picture who are wearing similar outfits or parts of Waldo's outfit, but only Waldo is wearing the complete, correctly colored outfit. Phages play a microbe version of this game with bacteria ( Figure ).
To find their hosts, phages look for special parts of the bacterial cell. These parts recognized by the phage are called receptors [ ].
RECEPTOR A unique feature of the host cell that is recognized by a virus.
The receptors serve the same purpose that Waldo's outfit does in Where is Waldo ® . They make the hosts stand out from other microbes around them. Therefore, bacteria without the right receptors are safe from phage attack. The receptors are also the areas where the phage attaches, or sticks, to the bacterial cell at the beginning of the attack.

RECEPTORS USED BY PHAGES ARE IMPORTANT FOR BACTERIA
What kind of bacterial cell parts are used by phages as receptors? It depends on the phage! Some phages stick to flagella (pronounced fluh-jeh-lah), which are long, wavy threads used by bacteria to move in a swimming motion ( Figure A). Other phages stick to sugars found on the surface of the bacterial cell that normally protects bacteria from toxic substances ( Figure B). Protein pumps found in the bacteria's cell membrane are also used as receptors. The cell membrane is the barrier around the cell that helps it keep its shape, and the protein pumps normally get rid of things that can harm or kill the cell ( Figure C)

WHY SHOULD YOU CARE ABOUT PHAGES?
Even though viruses are often thought of as bad, phages are helpful viruses that we want to have around. These natural killers of bacteria are extremely good at what they do! So, we can use them to kill the bad bacteria that cause diseases and hard-to-treat infections. In fact, phages have already made news headlines for saving lives. Bacteriophages have cured humans from bacterial infections of the heart, brain, and urinary tract. These infections are caused by problematic bacteria, such as Pseudomonas aeruginosa or Acetinobacter baumannii [ ], which quickly become resistant to many antibiotics. Some problematic bacteria also form sticky layers called biofilms that are hard to break apart. For these reasons, antibiotics are not always an e ective option for treatment. Therefore, phages may be the only way to get rid of these dangerous infections.
By using phages that only kill these troublemakers, we can make sure that the good bacteria in our bodies survive. We need to study phages in more detail, just like we study other medicines we use. These studies will help us choose the best phages to use in treatments. In the meantime, you can help spread the word about these incredible viruses. Next time you hear someone talk about viruses, be sure to tell them what you know about phages-the good viruses that can help treat infections!

ACKNOWLEDGMENTS
We thank Connor Caswell and Nathalia Barajas Gonzalez for providing us with invaluable feedback on an early draft of this manuscript. . Understanding and exploiting phage-host interactions. Viruses :
. Bacteria-phage coevolution as a driver of ecological and evolutionary processes in microbial communities.