Bacteriophages, also known as phages, are viruses that infect bacteria. These phages also require a bacterial host in order to replicate themselves. Bacterial viruses, as these are also often called, are made up of proteins that coat an inner core of nucleic acid – either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). Phages also vary in structure, ranging from the simple to the more elaborate and complex.
Bacterial viruses are specific to one or a limited number of bacteria; thus, they are named after the bacteria group, strain, or species that they infect. For example, the phages that infect the bacterium Escherichia coli are called coliphages.
Before bacteriophages were officially discovered, several bacteriologists in the 1890s had already observed an unidentified substance that seemed to be responsible for limiting bacterial activity. Although they had noted such a phenomenon, none of them further explored these findings. Thus, it wasn’t until 1915 when Frederick Twort, a bacteriologist from England, observed the same phenomenon (the killing of bacteria by an unknown agent) and advanced the hypothesis that it might be a virus. Unfortunately, lack of funding as well as the start of World War I interrupted Twort’s work. Two years later, a French-Canadian microbiologist named Félix d’Hérelle made a similar observation and succeeded in discovering what he called “an invisible, antagonistic microbe of the dysentery bacillus”. Unlike the previous investigators, d’Hérelle was certain that the unknown substance he had discovered was a virus that was able to parasitize bacteria. He called it a “bacteria-eater” or “bacteriophage”, from the word bacteria and the Greek word “phagein” which means to eat or devour.
Bacteriophages come in different sizes and shapes but most of them have the same basic features: a head or capsid and a tail. A bacteriophage’s head structure, regardless of its size or shape, is made up of one or more proteins which protectively coats the nucleic acid. Though there are some phages that don’t have a tail, most of them do have one attached to its head structure. It is a hollow tube through which the nucleic acid passes through when the bacteriophage infects a host cell. Some of the more complex phages such as T4 have a tail with a base plate as well as one or more tail fibers that aid the phage in attaching itself to a bacterial cell.
How Bacteriophages Work
In order to infect a host cell, the bacteriophage attaches itself to the bacteria’s cell wall, specifically on a receptor found on the bacteria’s surface. Once it becomes tightly bound to the cell, the bacterial virus injects its genetic material (its nucleic acid) into the host cell. Depending on the type of phage, one of two cycles will occur – the lytic or the lysogenic cycle. During a lytic cycle, the phage will make use of the host cell’s chemical energy as well as its biosynthetic machinery in order to produce phage nucleic acids (phage DNA and phage mRNA) and phage proteins. Once the production phase is finished, the phage nucleic acids and structural proteins are then assembled. After a while, certain proteins produced within the cell will cause the cell wall to lyse, allowing the assembled phages within to be released and to infect other bacterial cells.
Viral reproduction can also occur through the lysogenic cycle. The main difference between the two types of cycles is that during lysogeny, the host cell is not destroyed or does not undergo lysis. Once the host cell is infected, the phage DNA integrates or combines with the bacterial chromosome, creating the prophage. When the bacterium reproduces, the prophage is replicated along with the host chromosomes. Thus, the daughter cells also contain the prophage which carries the potential of producing phages. The lysogenic cycle can continue indefinitely (daughter cells with prophage present within continuing to replicate) unless exposed to adverse conditions which can trigger the termination of the lysogenic state and cause the expression of the phage DNA and the start of the lytic cycle. These adverse conditions include exposure to UV or mutagenic chemicals and desiccation.
Bacteriophages have several applications. In some countries such as Russia and other Eastern European nations, phages are used therapeutically for the treatment of pathogenic bacterial infections that are resistant to antibiotics. Also known as phage therapy, this method involves the use of a phage to destroy the infective bacteria such as E. coli or salmonella. Bacteriophage is also used in identifying pathogenic bacteria (also called phage typing) in diagnostic laboratories. One other use for bacteriophages is for killing specific bacteria found in food. For example, LISTEX by Micreos is made up of bacteriophages that can kill the L. monocytogenes bacteria in cheese.