Question

Compare and contrast lytic and lysogenic infections in bacteria and humans.  

Answer 1

Lytic and Lysogenic infection in Bacteria and Human

Lytic cycle is the name itself indicates that it lyses the cells of the host directly to cause the infection or to produce virulent phages, while lysogenic is opposite to the lytic cyle it does not lyse the host cell directly to cause the infection. Both these cycles (lytic and lysogenic) play a major role in or bacterial or viral reproduction. Generally bacterial cells can undergo one of the two types of infection which is termed as lytic (cell lyses) or lysogenic (moderate) infection for eg: lytic infection: E.coli infection is caused by T-phages. In lytic cycle the phage acts like virus were it lyses the host cell and attach. The stages included a) Attachment (phage binds to receptor of the bacterial cell surface) b) Entry (DNA injected to the cytoplasm of bacteria) c) Synthesis (DNA is copied and protein (capsid) are synthesized) d) Assembly (New phage particles produced) e) Lysis (lyse the cell membrane and cell wall-produces hundreds of new phages). IN case of the bacterial cell follows the lysogenic cycle then the phage reproduce witout killing or lysing the host cell.There are only two stages a) Attachment and injection of the genome DNA were it does not get copied or expressed instead of which it combines with specific region of bacterial chromosome as a prophage (genetic material of a phage, incorporated into the genome of a bacteria) b) Cell division in which the cell contaning the prophage divides at a stipulated time. Each time when the host cell divides the prophage is copied along with the host genome. Prophage become active under certain conditions (right time) and triggers the other steps of lytic cycle such as proteins synthesis, phage assembly and lysis. The basic difference between lytic and lysogenic cycle is denoted in Table 1.

Similar phenomenon happens in humans and in other mammals called as endogenous retrovirus (ERVs) which are viral genome elements can be derived from retroviruses. It is noted that about 5-8% of human geneome is made of endogenous retrovirus. Eg) Human immunodeficiency virus (HIV) replicate within the host (human) cell and cause illness (AIDS) to human. These retroviruses do not kill the host instead insert their genome to the host to cause the disease.

Table 1: Comparison and contrast between Lytic and Lysogenic infection/cycle
S.No	Lytic	Lysogenic
1	Lyses the host cell directly	Host cell is not lysed
2	Phages that multiply by lytic cycle called as virulent phages	Phages multiply by both phases (lytic and lysogenic) called as temperate phages
3	No integration of genome to the host	Integration of viral DNA or RNA to host DNA
4	Progeny particles produced	No progeny particle production
5	Lytic cycle cannot be followed by the lysogenic cycle.	Lysogenic cycle is followed by lytic cycle