Bacteriophages are viruses that infect bacteria and multiply in the process. For each bacterium, there is also a specific bacteriophage. Bacteriophages are used in medicine and genetic engineering.
What are bacteriophages?
Bacteriophages represent a group of viruses that infect bacteria and archaea (primordial bacteria). In doing so, they continue to replicate while destroying the bacterium. For each bacterium there is a specific bacteriophage according to the lock and key principle. Bacteriophages, according to traditional definition, like all viruses, are not living organisms. They depend on a host for their reproduction. No biochemical processes take place outside the host. Phages use the enzymes of their host for this purpose. Bacteriophages consist only of DNA or RNA, which are surrounded by a protein coat. However, the vast majority of phages possess DNA as their genetic material. Phages were first described in 1917 by the Canadian biologist Félix Hubert d’Hérelle. The structure of the various phages differs. Basically, bacteriophages consist of several components. Mainly the structure of the so-called T-phages was investigated, which among other things also infect the bacterium Escherichia coli. The T phages consist of a polyhedral head, which is connected via a neck to an elongated injection channel (injection apparatus). Under the injection apparatus is the base plate with tail fiber and spikes. The head is a capsid, which contains nucleic acid. Capsid, injection channel and base plate are made of protein. Tail fiber and spikes serve to anchor the phage to the cell wall of the bacterium.
Occurrence, distribution, and characteristics
Bacteriophages are universally distributed. Approximately 10 to the power of 30 bacteriophages are present in seawater. For each bacterium, there is also a corresponding phage. The propagation of bacteriophages can be divided into five phases. First, the phage is absorbed at a specific cell wall receptor. The ends of the tail filaments attach to the cell surface. In the next step, the phage injects its DNA or RNA into the bacterium. This leaves the empty protein envelopes on the bacterial surface. The third phase is called the latency phase, during which no phage can be detected. During the latency phase, which lasts several hours, translation into viral mRNA and replication into viral nucleic acid begins. In the so-called production phase, the viral proteins are produced. Then, in the subsequent maturation phase, the individual viral components are assembled. After viral production is complete, a lysozyme produced by the remodeled bacterial cell dissolves the bacterium and releases the produced phages.
Significance and function
Today, bacteriophages are already widely used in many fields. Particular areas of application are opening up in medicine, biology or agricultural engineering. In medicine, bacteriophages are used to detect bacterial strains due to their specificity to certain bacteria. This field of application is called lysotypy. Currently, intensive research is taking place to combat bacteria using bacteriophages in infections. Especially under the impression of the ever increasing number of antibiotic resistant bacterial strains, this research area is gaining increasing importance. However, the low stability of phages in the body is problematic. They are immediately eliminated by the body’s own phagocytes. In principle, this possible application was already considered by Félix Hubert d’Hérelle. However, after the discovery and introduction of antibiotics, the research results on this possible application were completely forgotten. However, the Eliava Institute for Phage Research, founded by D’Hérelle in 1934, still exists today in Tbilisi, Georgia. Together with the Ludwik Hirszfeld Institute for Immunology and Experimental Therapy in Wroclaw, Poland, research is being carried out there today into alternative ways of combating antibiotic-resistant bacteria using phages. Phages are also used in a variety of ways in the food industry. For example, various sprays of phages are used in the packaging of cheese or sausages to protect them from bacteria. Genetic engineering is also a major area of application. Today, for example, phages are used as vectors for certain genes and introduced into bacteria.With the aid of this method, insulin-producing bacterial strains of Escherichia coli could be generated. These vectors are also becoming increasingly important for the production of other active substances. Furthermore, they can be used in part to combat genetic defects.
Diseases and disorders
However, bacteriophages do not only show positive effects. Some serious infectious diseases break out only with the involvement of phages. For example, diphtheria is caused by the bacterium Corynebacterium diphtheriae. However, the disease can only break out if Corynebacterium diphtheriae is also simultaneously infected with bacteriophages. After infection with phages, these bacteria produce the characteristic toxin that causes the severe, sometimes life-threatening symptoms. The disease begins with signs such as difficulty swallowing, lassitude, nausea and abdominal pain. A white coating appears on the tonsils, which has a foul, sweet odor. Complications such as pneumonia or myocarditis are common and can be fatal. Another disease caused by bacteriophages is scarlet fever. Scarlet fever is caused by streptococci infected with bacteriophages. Through this infection, the bacteria produce a particularly insidious toxin. Severe signs of illness occur with chills, fever, vomiting and pharyngitis. Initially, the tongue is coated white, but turns raspberry red after a few days. A skin rash also occurs. If the bacterium were not infected with bacteriophages, it would only lead to a harmless tonsillitis. Cholera is also caused by a bacterium infected with bacteriophages called Vibrio cholerae.
