Lysogenic Cycle - Lysogenic Conversion

In some interactions between lysogenic phages and bacteria, lysogenic conversion may occur. It is when a temperate phage induces a change in the phenotype of the infected bacteria that is not part of a usual phage cycle. Changes can often involve the external membrane of the cell by making it impervious to other phages or even by increasing the pathogenic capability of the bacteria for a host. In this way, temperate bacteriophages also play a role in the spread of virulence factors, such as exotoxins and exoenzymes, amongst bacteria. Strategies to combat certain bacterial infections by blocking prophage induction (the transition from the lysogenic to the lytic cycle) have been proposed.

Examples:

• Corynebacterium diphtheriae produces the toxin of diphtheria only when it is infected by the phage β. In this case, the gene that codes for the toxin is carried by the phage, not the bacteria.
• Vibrio cholerae is a non-toxic strain that can become toxic, producing cholera toxin, when it is infected with the phage CTXφ.
• Shigella dysenteriae, which produces dysentery has toxins that fall into two major groups, Stx1 and Stx2, whose genes are considered to be part of the genome of lambdoid prophages.
• Streptococcus pyogenes, produce a pyrogenic exotoxin, obtained by lysogenic conversion, which causes fever and a scarlet-red rash, Scarlet Fever.
• Certain strains of Clostridium botulinum, which causes botulism, express botulinum toxin from phage-tranduced genes.

Extra genes present in prophage genomes which do not have a phage function but (may) act as fitness factors for the lysogen are termed "morons" (more DNA).