Prophage Excision Activates Listeria Competence Genes that Promote Phagosomal Escape and Virulence

Abstract

The DNA uptake competence (Com) system of the intracellular bacterial pathogen Listeria monocytogenes is considered nonfunctional. There are no known conditions for DNA transformation, and the Com master activator gene, comK, is interrupted by a temperate prophage. Here, we show that the L. monocytogenes Com system is required during infection to promote bacterial escape from macrophage phagosomes in a manner that is independent of DNA uptake. Further, we find that regulation of the Com system relies on the formation of a functional comK gene via prophage excision. Prophage excision is specifically induced during intracellular growth, primarily within phagosomes, yet, in contrast to classic prophage induction, progeny virions are not produced. This study presents the characterization of an active prophage that serves as a genetic switch to modulate the virulence of its bacterial host in the course of infection.

Listeria monocytogenes σHContributes to Expression of Competence Genes and Intracellular Growth

“…Among Gram-positive bacteria, two master regulators control the expression of competence-related genes: the transcription factor ComK (in B. subtilis) (18) and the alternative sigma factor H (in S. pneumoniae [19,20], S. aureus [21], and Lactobacillus sakei [31]). Rabinovich et al reported the presence of a ComK recognition site (the ComK box) upstream of the competence machinery operons (comG, comE, and comF) in L. monocytogenes (16). However, the involvement of H in the expression of these genes has not been explored.…”

“…The L. monocytogenes EGDe strain has a prophage-disrupted comK gene. This prophage has been reported to induce the expression of competence machinery genes by its excision from comK during intracellular growth (16). To eliminate the influence of the comK-disrupting phage, we cured the EGDe strain and used this as the wild-type (wt) strain for subsequent strain constructions and comparisons.…”

“…Another role for the competence genes in L. monocytogenes is to facilitate optimal phagosomal escape (16). To test the role of H during intracellular growth, RAW 264.7 macrophages were infected with sigH and comK single and double deletion mutant strains at a multiplicity of infection of 1.…”

“…L. monocytogenes has both comK and sigH genes in its genome. ComK was recently demonstrated to be the regulator of competence genes (16). In the present study, we aimed to clarify whether the alternative sigma factor H is involved in the expression of competence genes in L. monocytogenes.…”

“…This machinery is required for natural genetic competence (a specific physiological state developed to undergo natural transformation), and it is broadly conserved among species, including noncompetent bacteria (15). A recent study reported a new function for the competence machinery genes during the intracellular growth of L. monocytogenes: the comG operon and comEC gene are necessary for efficient phagosomal escape (16).…”