factors. For instance, if there are several other infecting phages (or if there is a high multiplicity), it is likely that the phage will use the lysogenic cycle. This may be useful in helping reduce the overall phage-to-host ratio and therefore preventing the phages from killing their hosts, also thereby increasing the phage’s potential for survival, making this a form of natural selection. A phage may decide to exit the chromosome and enter the lytic cycle if it is exposed to DNA-damaging agents, such as UV radiation and chemicals. Other factors with the potential to induce temperate phage release include temperature, pH, osmotic pressure, and low nutrient concentration. However, phages may also re-enter the lytic cycle spontaneously. In 80-90% of single-cell infections, phages enter the lysogenic cycle. In the other 10-20%, phages enter the lytic cycle. Evidence of Lysogeny It is sometimes possible to detect which cycle a phage enters by looking at the plaque morphology in bacterial plate culture. Since phages that enter the lytic cycle kill the host bacterial cells, plaques will appear clear (photo A). The plaques may also appear to have a halo-like ring around the edge, indicating that these cells were not fully lysed. In contrast, infecting phages that enter the lysogenic cycle will produce cloudy or turbid plaques, as the cells containing the lysogenic phage are not lysed and can continue growing (photo B). However, exceptions to this rule are also known to exist, where non-temperate phages still exhibit cloudy plaques, and temperate phage mutants can generate clear plaques as a result of loss of lysogen formation ability. See a comparison of clear and turbid plaques, formed by lytic and lysogenic phages, respectively, in the Phage Discovery Guide. Detection methods of phages released from the lysogenic cycle include electron microscopy, DNA extraction, or propagation on sensitive strains. Via the lysogenic cycle, the bacteriophage's genome is not expressed and is instead integrated into the bacteria's genome to form the prophage. In its inactive form, a prophage gets passed on each time the host cell divides. If prophages become active, they can exit the bacterial chromosome and enter the lytic cycle, where they undergo DNA copying, protein synthesis, phage assembly, and lysis. Since the bacteriophage's genetic information is incorporated into the bacteria's genetic information as a prophage, the bacteriophage replicates passively as the bacterium divides to form daughter bacteria cells. In this scenario, the daughter bacteria