Date Published: January 16, 2014
Publisher: Public Library of Science
Author(s): Camille Danne, Sarah Dubrac, Patrick Trieu-Cuot, Shaynoor Dramsi, Michael R. Wessels.
The molecular triggers leading to virulence of a number of human-adapted commensal bacteria such as Streptococcus gallolyticus are largely unknown. This opportunistic pathogen is responsible for endocarditis in the elderly and associated with colorectal cancer. Colonization of damaged host tissues with exposed collagen, such as cardiac valves and pre-cancerous polyps, is mediated by appendages referred to as Pil1 pili. Populations of S. gallolyticus are heterogeneous with the majority of cells weakly piliated while a smaller fraction is hyper piliated. We provide genetic evidences that heterogeneous pil1 expression depends on a phase variation mechanism involving addition/deletion of GCAGA repeats that modifies the length of an upstream leader peptide. Synthesis of longer leader peptides potentiates the transcription of the pil1 genes through ribosome-induced destabilization of a premature stem-loop transcription terminator. This study describes, at the molecular level, a new regulatory mechanism combining phase variation in a leader peptide-encoding gene and transcription attenuation. This simple and robust mechanism controls a stochastic heterogeneous pilus expression, which is important for evading the host immune system while ensuring optimal tissue colonization.
Streptococcus gallolyticus, formerly known as Streptococcus bovis biotype I, is present asymptomatically in the gastrointestinal tract of 2.5–15% of the human population . However, this commensal bacterium can become a pathogen responsible for infective endocarditis in the elderly. Intriguingly, epidemiological studies pointed out a strong association, up to 65%, between endocarditis due to S. gallolyticus and colorectal malignancies –. Whether S. gallolyticus presence is a cause or a consequence of colon cancer development remains unknown . Genome analysis of S. gallolyticus UCN34, a strain isolated from a patient suffering from infective endocarditis and colon cancer, revealed the existence of three pilus loci named pil1, pil2, and pil3. Pili are long filamentous structures extending from the bacterial surface, composed of covalently linked pilin subunits, which play key roles in adhesion and colonization of host tissues. Each pilus locus encodes two structural LPXTG proteins and one sortase C, an enzyme which covalently links pilin subunits during assembly of the pilus filament. The Pil1 locus of strain UCN34 is composed of three genes encoding a major pilin, PilB (Gallo2178), a collagen-binding adhesin, PilA (Gallo2179), and a sortase C (Gallo2177). In previous studies, PilA was shown to bind to collagen type I, the major component of cardiac valves, and to collagen type IV, enriched in basal lamina of pre-cancerous polyps , . PilA constitutes the major collagen-binding protein in S. gallolyticus, conferring adhesive properties to the pilus, and is involved in the development of infective endocarditis in a rat experimental model .
Several recent publications have identified pili as playing a key role in the virulence of many gram-positive bacteria , . Pili have been implicated in many facets of the infectious process such as adhesion/colonization of host tissues, translocation of epithelial barriers, and modulation of the innate immune responses. Their surface localization and high immunogenicity makes them attractive targets for the development of vaccines against important gram-positive pathogens such as Group B Streptococcus. However, although the expression analysis of these important antigens is of obvious interest, very little is known about the regulation of pilus expression and how it is modulated by different environmental conditions.