What on earth are phages? This recurring question, along with a confused expression, is the overwhelming response I get when I begin talking about my latest uni project. It seems that phages are not the most popular of conversation topics these days. Whether it is my parents, friends or other science students alike, excluding my knowledgeable classmates of course, phages are living in a world of anonymity. When you finish reading this blog post, you may come to realize just how cool these creatures (if you can call them that?) are. So put on that lab coat and those googly lab glasses and follow me into the geeky world of phages.
Technically speaking, the proper name for a phage is a bacteriophage. Bacterio- stands for bacteria, while –phage comes from the ancient Greek word meaning to eat or devour. So a bacteriophage is something that consumes bacteria, or more precisely, phages are viruses! Like most viruses, bacteriophages are made up of a protein coat, which encapsulates the RNA or DNA genome inside. This electron microscope image shows bacteriophage attached to E.coli, complete with landing gear, which the virus uses to attach itself to the bacteria so that it can inject its genome inside.
But how does this help the phage to kill the bacterium? Once the bacterium is infected with the viral DNA, the virus enters either the lytic or the lysogenic life cycle (Reece et al., 2011). In the lytic cycle, the virus utilises the bacterial replication machinery in order to replicate many copies of the viral DNA. Viral genes are expressed to create protein coats and an army of new phages assembles inside the bacterial cell. Enzymes lyse (burst) the cell and the phages go on the attack, conquering and destroying other bacteria. The lysogenic cycle, on the other hand, is a more subtle form of warfare. The injected viral DNA is incorporated into the bacterial chromosome forming a prophage, where it bides its time, allowing the unsuspecting bacteria to replicate and divide as normal. Each round of bacterial replication includes the prophage so that bacteria are marked as ‘friendly’ to the lysogenic cause, like the creation of alliances before declaring outright war on the enemy. When the prophage exits the bacterial chromosome, the lytic cycle is induced and viral armies proceed to ravage the bacteria, and so the cycle continues. This diagram may help you to understand the intricacies of viral infection.
The lytic and lysogenic cycles of phages (Reece et al., 2011, pg. 432)
Phages are the ultimate secret spy agents. They travel incognito through our world hunting down their specific targets, the unwitting bacteria. In fact, they are so good at staying undercover that you probably don’t realise there are actually an estimated 1031 phages on earth. In real numbers that is: 1000000000000000000000000000000 bacteriophages. In comparison, the latest estimate for the human population is around 7162000000. As phage hunters, our task is to expose the viruses. To identify, characterise, and even name them if we are lucky.
The next question, that can be guaranteed to follow the first, is: why bother? Why are phages so important anyway? The answer is phage therapy. Pathogenic bacteria that cause disease in humans, plants or animals can be targeted specifically by lytic phages and destroyed. Phage therapy is very specific thus, unlike disinfectants and antibiotics, only the target bacterial cells are destroyed and other surrounding living tissue is unaffected. For every strain of bacteria, there is at least 1 type of phage that can specifically destroy it, which is more than can be said about antibiotics. Furthermore, phages evolve in parallel with the bacteria so that phages to treat a new bacterial strain can be developed in only 15-30 days. Phage therapy has been around since the 1920s, but currently only Russia and Georgia use this technology. Western countries have focused on developing antibiotics to treat bacterial infections yet with increasing bacterial resistance and the long timeframe to bring a new antibiotic to market, phage therapy is potentially a more effective alternative. For more easy reading on this subject follow this link: http://www.globalyzbiotech.com/phage-facts/ and browse the site.
So this concludes your introduction into the fierce world of phages and their mortal combat with bacteria. If someone initiates a conversation with you on the subject of phages, this time you won’t be so ignorant about one of the most abundant organisms on earth. Don’t be too quick to put aside your lab coat and lab glasses, there still is much to discover and much to learn.
Reece, J., Urry, L., Cain, M., Wasserman, S., Minorsky, P., & Jackson, R. (2011). Campbell Biology (9th ed.). San Franciso, United States of America: Pearson Benjamin Cummings.