Life cycle of an obligately lytic bacteriophage. As this is a cycle, the “beginning” is arbitrary. A successful infection nonetheless progresses through adsorption, infection, release (here via lysis), and a period of extracellular “search” for new bacteria to infect. Deviations from this life cycle can include inactivation during the extracellular stage, a failure to successfully adsorb, and various forms of phage inactivation that can occur during infection, including as explicitly mediated by bacterial cells [17, 164]. Though lytic phages are released via lysis, other phages exist, most notably filamentous phages such as phage M13, that instead are released from infected bacteria chronically. Generally such nonlytic phages are not used for phage therapy. Another variation on the phage life cycle is lysogenic cycles, which are nonvirion productive extensions of the infection stage. Only temperate, particularly not obligately lytic phages display lysogenic cycles, and temperate phages typically also are not among the first choice for phage therapy purposes [222]. Shown too, in the middle, is reference to pharmacological aspects of phage infections. Particularly these are distribution throughout body tissues that can occur while in the free phage state (a.k.a., phage penetration to target bacteria) along with amplification of phage numbers in situ as can occur as a consequence of phage infection of bacteria, which is a component of what pharmacokinetically is known as metabolism.
