Lysogeny in probiotic lactobacilli

Abstract

Bacteriophages show several life cycles: lytic (as a virulent phage), lysogenic (as a temperate phage or prophage) and persistent infections (pseudolysogeny, phage-carrier state). The lysogenic decision of a temperate phage to either follow a lytic cycle or insert its genome into host chromosome and continue to replicate in a dormant stage (prophage) is made just after infection, and depends upon host density and environmental factors. Bacterial and phage genomes are not completely independent entities; they underwent a co-evolution which favored both. During evolution, bacteria tend to eliminate unessential prophage DNA giving place to the following sequence: inducible prophages - defective prophages (with deletions, insertions and/or rearrangements) - prophage remnants (with massive loss of the original phage genome) - isolated prophage genes. The latter, called lysogenic conversion genes (LCG) confer a competitive advantage to the host cell. Hence, lysogeny greatly contributed in the generation of diversity, being responsible for almost all genetic differences found among strains of the same species. Lysogeny is more the rule than the exception and many bacteria harbour more than one prophage (poly-lysogeny). Prophages and LCG were first described on pathogenic bacteria, where they are known to encode for diverse virulence factors, but later they were postulated to help in the adaptation of any bacterial strain to their ecological niche. This obviously applies to gut commensal bacteria, which are constantly under selection pressure and include probiotic strains. Among lactic acid bacteria (LAB), most lactococcal strains are lysogenic and poly lysogenic, but lysogeny was also found to be highly widespread in lactobacilli, which encompasses most probiotic strains currently used by dairy industries. In particular, phage infections in probiotic lactobacilli are especially important due to singular and unique characteristics ascribed to each strain. In addition, lysogeny could potentially affect strain functionality, either by improving or counteracting its probioticity. Finally, it is known that lactobacilli play an active role in the maintaining of mucosal health. Several studies have correlated the alteration of this equilibrium with the presence of virulent phages or furthermore, with prophages that are induced under certain circumstances, causing lactobacilli depletion.