The Abi-domain protein Abx1 interacts with the CovS histidine kinase to control virulence gene expression in group B Streptococcus

Group B Streptococcus (GBS), a common commensal of the female genital tract, is the leading cause of invasive infections in neonates. Expression of major GBS virulence factors, such as the hemolysin operon cyl, is regulated directly at the transcriptional level by the CovSR two-component system. Using a random genetic approach, we identified a multi-spanning transmembrane protein, Abx1, essential for the production of the GBS hemolysin. Despite its similarity to eukaryotic CaaX proteases, the Abx1 function is not involved in a post-translational modification of the GBS hemolysin. Instead, we demonstrate that Abx1 regulates transcription of several virulence genes, including those comprising the hemolysin operon, by a CovSR-dependent mechanism. By combining genetic analyses, transcriptome profiling, and site-directed mutagenesis, we showed that Abx1 is a regulator of the histidine kinase CovS. Overexpression of Abx1 is sufficient to activate virulence gene expression through CovS, overcoming the need for an additional signal. Conversely, the absence of Abx1 has the opposite effect on virulence gene expression consistent with CovS locked in a kinase-competent state. Using a bacterial two-hybrid system, direct interaction between Abx1 and CovS was mapped specifically to CovS domains involved in signal processing. We demonstrate that the CovSR two-component system is the core of a signaling pathway integrating the regulation of CovS by Abx1 in addition to the regulation of CovR by the serine/threonine kinase Stk1. In conclusion, our study reports a regulatory function for Abx1, a member of a large protein family with a characteristic Abi-domain, which forms a signaling complex with the histidine kinase CovS in GBS.

The gram-positive Streptococcus genus includes three major human pathogens that are members of the normal microflora: Streptococcus pneumoniae (also known as the pneumococcus), Streptococcus pyogenes (Group A Streptococcus), and Streptococcus agalactiae (Group B Streptococcus). Their carriage in the population is highly dynamic and mostly asymptomatic. However, each of these species can cause a wide spectrum of diseases, from local infections to systemic and fatal infections including septicemia and meningitis. Expression of streptococcal virulence-associated genes is tightly regulated at the transcriptional level. However, the signal(s) and the precise molecular events controlling the switch from commensalism to virulence are not yet understood. In this study, we identified and characterized a bacterial protein essential for virulence gene expression in Group B Streptococcus, the main pathogen of neonates. We show that this transmembrane protein, named Abx1, interacts with the histidine kinase CovS to modulate the activity of the major regulator of virulence CovR. We define how a core set of four proteins, Abx1, CovS, CovR, and the serine/threonine kinase Stk1, interact to control the expression of virulence genes in S. agalactiae. We propose that Abx1-like proteins, that are widespread in bacteria, might be part of a conserved mechanism of two-component system regulation.

An expanded protein-protein interaction network in Bacillus subtilis reveals a group of hubs: Exploration by an integrative approach

We have generated a protein-protein interaction network in Bacillus subtilis focused on several essential cellular processes such as cell division, cell responses to various stresses, the bacterial cytoskeleton, DNA replication and chromosome maintenance by careful application of the yeast two-hybrid approach. This network, composed of 793 interactions linking 287 proteins with an average connectivity of five interactions per protein, represents a valuable resource for future functional analyses. A striking feature of the network is a group of highly connected hubs (GoH) linking many different cellular processes. Most of the proteins of the GoH have unknown functions and are associated to the membrane. By the integration of available knowledge, in particular of transcriptome data sets, the GoH was decomposed into subgroups of party hubs corresponding to protein complexes or regulatory pathways expressed under different conditions. At a global level, the GoH might function as a very robust group of date hubs having partially redundant functions to integrate information from the different cellular pathways. Our analyses also provide a rational way to study the highly redundant functions of the GoH by a genetic approach.

Enterococcal leucine-rich repeat-containing protein involved in virulence and host inflammatory response

Enterococcus faecalis is an important nosocomial pathogen associated with high morbidity and mortality for patients who are immunocompromised or who have severe underlying diseases. The E. faecalis genome encodes numerous surface-exposed proteins that may be involved in virulence. This work describes the characterization of the first internalin-like protein in E. faecalis, ElrA, belonging to the recently identified WxL family of surface proteins. ElrA contains an N-terminal signal peptide for export, a leucine-rich repeat domain that may interact with host cells, and a C-terminal WxL domain that interacts with the peptidoglycan. Disruption of the elrA gene significantly attenuates bacterial virulence in a mouse peritonitis model. The elrA deletion mutant also displays a defect in infection of host macrophages and a decreased interleukin-6 response in vivo. Finally, elrA expression is induced in vivo. Altogether, these results demonstrate a role for ElrA in the E. faecalis infectious process in vivo and suggest that this surface protein may contribute to E. faecalis virulence by stimulating the host inflammatory response.

Comparative genomic hybridization analysis of Enterococcus faecalis: identification of genes absent from food strains

Enterococcus faecalis, a member of the natural microbiota of animal and human intestinal tracts, is also present as a natural contaminant in a variety of fermented foods. Over the last decade, E. faecalis has emerged as a major cause of nosocomial infections. We investigated the genetic diversity in 30 clinical and food isolates, including strains V583 and MMH594, in order to determine whether clinical and food isolates could be distinguished. Data were obtained using comparative genomic hybridization and specific PCR with a total of 202 probes of E. faecalis, selected using the available V583 genome sequence and part of the MMH594 pathogenicity island. The cognate genes encoded mainly exported proteins. Hybridization data were analyzed by a two-component mixture model that estimates the probability of any given gene to be either present or absent in the strains. A total of 78 genes were found to be variable, as they were absent in at least one isolate. Most of the variable genes were clustered in regions that, in the published V583 sequence, related to prophages or mobile genetic elements. The variable genes were distributed in three main groups: (i) genes equally distributed between clinical and dairy food isolates, (ii) genes absent from dairy food-related isolates, and (iii) genes present in MMH594 and V583 strains only. Further analysis of the distribution of the last gene group in 70 other isolates confirmed that six of the probed genes were always absent in dairy food-related isolates, whereas they were detected in clinical and/or commensal isolates. Two of them corresponded to prophages that were not detected in the cognate isolates, thus possibly extending the number of genes absent from dairy food isolates. Genes specifically detected in clinical isolates may prove valuable for the development of new risk assessment markers for food safety studies and for identification of new factors that may contribute to host colonization or infection.

C-terminal WxL domain mediates cell wall binding in Enterococcus faecalis and other gram-positive bacteria

Analysis of the genome sequence of Enterococcus faecalis clinical isolate V583 revealed novel genes encoding surface proteins. Twenty-seven of these proteins, annotated as having unknown functions, possess a putative N-terminal signal peptide and a conserved C-terminal region characterized by a novel conserved domain designated WxL. Proteins having similar characteristics were also detected in other low-G+C-content gram-positive bacteria. We hypothesized that the WxL region might be a determinant of bacterial cell location. This hypothesis was tested by generating protein fusions between the C-terminal regions of two WxL proteins in E. faecalis and a nuclease reporter protein. We demonstrated that the C-terminal regions of both proteins conferred a cell surface localization to the reporter fusions in E. faecalis. This localization was eliminated by introducing specific deletions into the domains. Interestingly, exogenously added protein fusions displayed binding to whole cells of various gram-positive bacteria. We also showed that the peptidoglycan was a binding ligand for WxL domain attachment to the cell surface and that neither proteins nor carbohydrates were necessary for binding. Based on our findings, we propose that the WxL region is a novel cell wall binding domain in E. faecalis and other gram-positive bacteria.