T phages are viruses that are bacteriophages specialized to infect only Escherichia coli intestinal bacteria (coliphages). There are 7 different known species, designated T1 to T7, of which the even-numbered ones are distinguished from the odd-numbered species by certain common features. In the body, T phages are usually recognized by the immune system; outside the body, they are targeted to produce certain enzymes and for other purposes.
What are T phages?
T phages are among the viruses that specialize in attacking bacteria and are therefore called macrophages or simply phages. Each type of phage is specialized to infect a specific bacterium. The tailed T-phages (the T is derived from the English ‘tail’) are directed at infecting the intestinal bacterium Escherichia coli. The 7 known T phages, designated T1 to T7, belong to the three families siphoviruses (T1, T5), podoviruses (T3, T7), and myoviruses (T2, T4, T6). The even-numbered and odd-numbered T phages are each distinguished by several common features. T phages are characterized by a complex structure. The main components are the base plate, injection apparatus and head. On the base plate there are so-called spikes, with which the phage can cling to the bacterial wall and pierce it. The injection apparatus consists of a contractile tube through which the DNA of the phage is “shot” into the coli bacterium. The double-stranded DNA is located in the head of the T phage and, after docking with the bacterium, is transported through the contractile tube of the injection apparatus into the interior of the coli bacterium. After infection has occurred, the parts of the T phage remaining on the outside of the envelope are no longer needed and detach again from the bacterial wall.
Occurrence, distribution, and characteristics
T phages, like other phages, are usually found where bacteria are also found. In sewage and stagnant waters, where there is normally an enormously large and differentiated association of bacteria, bacteriophages also occur in similarly abundant and differentiated forms. Similar conditions are even found in the extremely clean appearing oceans. In the human body, T phages can be found mainly in places colonized by coli bacteria. In healthy people with an intact immune system, this is primarily the digestive tract. T phages that stray into the bloodstream are recognized by the immune system and trigger an immune response that leads to the destruction of the phages. There is hardly any direct danger of infection with T phages, since they are not disease germs in their own right. With a correspondingly weak immune defense, it is only conceivable that T phages cause a sensitive thinning of the coli bacteria within the intestinal flora. T phages used for therapeutic purposes can be freeze-dried without losing their physiological properties.
Importance and function
T phages, which can exclusively infect and kill coli bacteria, play only a minor role in the human body. However, targeted use against pathogenic coli bacteria outside the digestive tract would be conceivable. Unlike antibiotics, which have a broad-spectrum effect, i.e. they also destroy a large number of beneficial microbes, T phages, like other phages, act absolutely specifically and selectively against certain strains of germs. However, phage therapy is subject to strict regulations in Germany, although it could be an excellent alternative to antibiotics in many cases. The problem of resistance formation is also present with T-phages, but can just as quickly be eliminated by new breeds of modified macrophages. Phage therapy has developed particularly in countries of the former USSR, of which Georgia has been a major player. Some Western research groups are trying to grow phages that are also effective against multidrug-resistant germs. T phages are often cultivated to produce enzymes in coli bacteria in order to obtain larger quantities of enzymes needed for molecular biology purposes. These are enzymes such as T4 DNA ligase, T7 RNA polymerase and several others.The ability of so-called temperate T phages to incorporate their own DNA into bacterial DNA (lysogeny), rather than unbridled replication of their own DNA, can also be used as a gene vector to make targeted genetic manipulations to replace specific defective and disease-causing genes or pieces of genes with intact genes or pieces of DNA.
Diseases and disorders
T phages do not pose direct dangers to humans. Indirectly, T phages could become a problem if they manage to attack and decimate coli bacteria in the intestinal flora unnoticed by the immune system. T-phages and other phages are supposed to support the immune system in order to destroy germs that are difficult to combat and possibly also multi-resistant. Critics of phage therapy say that the therapy can only be used by physicians with the appropriate additional training and that a bacterial culture must first be created for a targeted application in order to be sure that the “right” phage has been selected for the “right” bacterium. In contrast, broad-spectrum antibiotics would be immediately available. However, the main argument against phage therapy expressed by critics is the fear that the phage could change in its genetics through mutations or through uncontrollable gene exchange with the host bacterium in such a way that the phage could lose its phagocytic properties and mutate into an uncontrollable pathogenic virus. During the Cold War, Western medicine relied almost exclusively on antibiotics to ward off infectious germs, while Russia and Soviet Union member states – most notably Georgia – cultivated phage therapy. There are now signs of recognition that both therapies carry specific benefits and risks that must be weighed before use.
