Microbes communicate with each other by using quorum sensing (QS) systems and modulate their collective ‘behavior’ for in-host colonization and virulence, biofilm formation, and environmental adaptation. The recent increase in genome data availability reveals the presence of several putative QS sensing circuits in microbial pathogens, but many of these have not been functionally characterized yet, despite their possible utility as drug targets. To increase the repertoire of functionally characterized QS systems in bacteria, we studied Rgg144/Shp144 and Rgg939/Shp939, two putative QS systems in the important human pathogen Streptococcus pneumoniae. We find that both of these QS circuits are induced by short hydrophobic peptides (Shp) upon sensing sugars found in the respiratory tract, such as galactose and mannose. Microarray analyses using cultures grown on mannose and galactose revealed that the expression of a large number of genes is controlled by these QS systems, especially those encoding for essential physiological functions and virulence-related genes such as the capsular locus. Moreover, the array data revealed evidence for cross-talk between these systems. Finally, these Rgg systems play a key role in colonization and virulence, as deletion mutants of these QS systems are attenuated in the mouse models of colonization and pneumonia.

In anaerobic environments, mutually beneficial metabolic interactions between microorganisms (syntrophy) are essential for oxidation of organic matter to carbon dioxide and methane. Syntrophic interactions typically involve a microorganism degrading an organic compound to primary fermentation by-products and sources of electrons (i.e., formate, hydrogen, or nanowires) and a partner producing methane or respiring the electrons via alternative electron accepting processes. Using a transposon gene mutant library of the sulfate-reducing Desulfovibrio alaskensis G20, we screened for mutants incapable of serving as the electron-accepting partner of the butyrate-oxidizing bacterium, Syntrophomonas wolfei. A total of 17 gene mutants of D. alaskensis were identified as incapable of serving as the electron-accepting partner. The genes identified predominantly fell into three categories: membrane surface assembly, flagellum-pilus synthesis, and energy metabolism. Among these genes required to serve as the electron-accepting partner, the glycosyltransferase, pilus assembly protein (tadC), and flagellar biosynthesis protein showed reduced biofilm formation, suggesting that each of these components is involved in cell-to-cell interactions. Energy metabolism genes encoded proteins primarily involved in H 2 uptake and electron cycling, including a rhodanese-containing complex that is phylogenetically conserved among sulfate-reducing Deltaproteobacteria. Utilizing an mRNA sequencing approach, analysis of transcript abundance in wild-type axenic and cocultures confirmed that genes identified as important for serving as the electronaccepting partner were more highly expressed under syntrophic conditions. The results imply that sulfate-reducing microorganisms require flagellar and outer membrane components to effectively couple to their syntrophic partners; furthermore, H 2 metabolism is essential for syntrophic growth of D. alaskensis G20.A naerobic oxidation of organic compounds, such as alcohols and fatty acids, is thermodynamically unfavorable when protons or carbon dioxide are used as the electron acceptor, unless the H 2 partial pressure or formate concentration, respectively, can be maintained at extremely low concentrations (1). Thus, complete degradation of organic matter in methanogenic and some other anaerobic systems requires a microbial consortium composed of two or more microbial species to oxidize the carbon and subsequently remove hydrogen (2-5). This synergistic interaction between different microorganisms is defined as syntrophy. The term was first coined in anaerobic, sulfur-oxidizing phototrophic cocultures (6) but was experimentally verified when the methanogenic, archaean Methanobacterium bryantii strain MOH was separated from an ethanol-oxidizing partner, both of which were present in a culture called Methanobacillus omelianskii, which at the time was believed to be a pure culture (7). This work established the general syntrophic model of two microorganisms mutualistically cooperating by transferring electron...

During its progression from the nasopharynx to other sterile and non-sterile niches of its human host, Streptococcus pneumoniae must cope with changes in temperature. We hypothesised that the temperature adaptation is an important facet of pneumococcal survival in the host. Here, we evaluated the effect of temperature on pneumococcus and studied the role of glutamate dehydrogenase (GdhA) in thermal adaptation associated with virulence and survival. Microarray analysis revealed a significant transcriptional response to changes in temperature, affecting the expression of 252 genes in total at 34°C and 40°C relative to at 37°C. One of the differentially regulated genes was gdhA, which is upregulated at 40°C and downregulated at 34°C relative to 37°C. Deletion of gdhA attenuated the growth, cell size, biofilm formation, pH survival, and biosynthesis of proteins associated with virulence in a temperature-dependent manner. Moreover, deletion of gdhA stimulated formate production irrespective of temperature fluctuation. Finally, Δ gdhA grown at 40°C was less virulent compare to other temperatures or than the wild type at the same temperature in a Galleria mellonella infection model, suggesting that GdhA is required for pneumococcal virulence at elevated temperature.

Rggs are a group of transcriptional regulators with diverse roles in metabolism and virulence. Here, we present work on the Rgg1518/SHP1518 quorum sensing system of Streptococcus pneumoniae. The activity of Rgg1518 is induced by its cognate peptide, SHP1518. In vitro analysis showed that the Rgg1518 system is active in conditions rich in galactose and mannose, key nutrients during nasopharyngeal colonization. Rgg1518 expression is highly induced in the presence of these sugars and its isogenic mutant is attenuated in growth on galactose and mannose. When compared with other Rgg systems, Rgg1518 has the largest regulon on galactose. On galactose it controls up‐ or downregulation of a functionally diverse set of genes involved in galactose metabolism, capsule biosynthesis, iron metabolism, protein translation, as well as other metabolic functions, acting mainly as a repressor of gene expression. Rgg1518 is a repressor of capsule biosynthesis, and binds directly to the capsule regulatory region. Comparison with other Rggs revealed inter‐regulatory interactions among Rggs. Finally, the rgg1518 mutant is attenuated in colonization and virulence in a mouse model of colonization and pneumonia. We conclude that Rgg1518 is a virulence determinant that contributes to a regulatory network composed of multiple Rgg systems.

Streptococcus pneumoniae (the pneumococcus) is the major cause of bacterial pneumonia in the US and worldwide. Studies have shown that the differing chemical make-up between serotypes of its most important virulence factor, the capsule, can dictate disease severity. Here we demonstrate that control of capsule synthesis is also critical for infection and facilitated by two broadly conserved transcription factors, SpxR and CpsR, through a distal cis-regulatory element we name the 37-CE. Strikingly, changing only three nucleotides within this sequence is sufficient to render pneumococcus avirulent. Using in vivo and in vitro approaches, we present a model where SpxR interacts as a unique trimeric quaternary structure with the 37-CE to enable capsule repression in the airways. Considering its dramatic effect on infection, variation of the 37-CE between serotypes suggests this molecular switch could be a critical contributing factor to this pathogen’s serotype-specific disease outcomes.