Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 30

Some of the most important discoveries in genetics have come from studies with viruses that infect bacteria; these viruses are called bacteriophages, or phages for short (see chapter 7). Phages are not alive; instead, they are just genes wrapped in a protective coat of protein and/or membrane, as are all viruses. Because phages are not alive, they cannot multiply outside a bacterial cell. However, if a phage encounters a type of bacterial cell that is sensitive to that phage, the phage, or at least its DNA or RNA, enters the cell and directs it to make more phage.

Phages are usually identified by the holes, or plaques, they form in layers of sensitive bacteria. In fact, the name “phage” (Greek for “eat”) derives from these plaques, which look like eaten-out areas. A plaque can form when a phage is mixed with large numbers of susceptible bacteria and the mixture is placed on an agar plate. As the bacteria multiply, one bacterial cell may be infected by the phage, which multiplies and eventually breaks open, or lyses, the bacterium, releasing more phage. As the surrounding bacteria are infected, the phage spread, even as the bacteria multiply to form an opaque layer called a bacterial lawn. Wherever the original phage infected the first bacterium, the plaque disrupts the lawn, forming a clear spot on the agar. Despite its empty appearance, this spot contains millions of the phage.

Phages offer many of the same advantages for genetics as bacteria. Thousands or even millions of phages can be put on a single plate. Also, like bacterial colonies, each plaque contains millions of genetically identical phage. By analogy to the colony purification of bacterial strains, individual phage mutants or strains can be isolated from other phages through plaque purification.