| Bacteriophages ("eaters of bacteria", often known simply as "phages") are, naturally occurring viruses that infect bacteria. |
| It is estimated that the total number of bacteriophages on Earth exceeds ten thousand billion billion billion (10³¹). making them the most numerous form of life. A millilitre of coastal seawater may contain a million bacteriophages, while a millilitre of some fresh water sources, may contain up to a billion. |
| Humans and all other forms of life are continually exposed to a rich and ever-changing bacteriophage ecosystem, but because they only infect bacteria we simply do not see this happening around us. |
| Even among their bacterial hosts, bacteriophages are highly specific, with most infecting only a single species of bacteria. In many cases, only specific strains within that species are infected. |
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More than 90% of bacteriophages have a head containing the double-stranded DNA genome, extending from which is a tail through which the DNA can flow into the cell.
This tail can be long or short, flexible or stiff, and is a key characteristic of the different families of bacteriophages.
Many resemble the classic "lunar lander" image.
Fibres at the end of the tail find specific receptors on the surface of target bacteria.
The virus then binds and its DNA enters into the host bacterial cell. For bacteriophages like the one pictured, this process is a form of hypodermic injection, but for most bacteriophages the precise method by which the DNA enters the cell remains unknown.
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| Once the bacteriophage has infected the host cell, the resultant infection may be either lytic, reprogramming and soon destroying the infected cell or lysogenic, where the phage genome is integrated into the bacterial genome and passed on to the next generations of bacteria. |
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| Lytic bacteriophages are the type used for most therapeutic approaches. A typical lytic bacteriophage will produce 100-300 new bacteriophages from an infected bacterial cell in a matter of hours. These can then go on to infect and kill a whole new generation of the target bacteria. |
| This exponential replication only stops when the bacteriophages run out of target bacteria. Then, they cannot grow any more. Without their target, they are simply small lumps of protein, to be removed by the normal clearance processes of the body. |