A VIRUS which invades a bacterium (see BACTERIA). Containing either single-stranded or double-stranded DNA or RNA, a particular phage generally may infect one or a limited number of bacterial strains or species. After infection, once phage nucleic acid has entered the host cell, a cycle may result whereby the bacteria are programmed to produce viral components, which are assembled into virus particles and released on bacterial lysis (disintegration). Other (temperate) phages induce a non-lytic, or lysogenic, state, in which phage nucleic acid integrates stably into and replicates with the bacterial chromosome. The relationship can revert to a lytic cycle and production of new phages. In the process the phage may carry small amounts of donor bacterial DNA to a new host: the production of diphtheria toxin by Corynebacterium diphtheriae and of erythrogenic toxin by Streptococcus pyogenes are well-known examples of this eﬀect.
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