Autoinducer 2 (AI-2) is widely recognized as a signal molecule for intra- and interspecies communication in Gram-negative bacteria, but its signaling function in Gram-positive bacteria, especially in Staphylococcus aureus , remains obscure. Here we reveal the role of LuxS in the regulation of capsular polysaccharide synthesis in S. aureus NCTC8325 and show that AI-2 can regulate gene expression and is involved in some physiological activities in S. aureus as a signaling molecule. Inactivation of luxS in S. aureus NCTC8325 resulted in higher levels of transcription of capsular polysaccharide synthesis genes. The survival rate of the luxS mutant was higher than that of the wild type in both human blood and U937 macrophages. In comparison to the luxS mutant, a culture supplemented with chemically synthesized 4,5-dihydroxy-2,3-pentanedione (DPD), the AI-2 precursor molecule, restored all the parental phenotypes, suggesting that AI-2 has a signaling function in S. aureus . Furthermore, we demonstrated that the LuxS/AI-2 signaling system regulates capsular polysaccharide production via a two-component system, KdpDE, whose function has not yet been clarified in S. aureus. This regulation occurred via the phosphorylation of KdpE binding to the cap promoter.

Background: Staphylococcus aureus is an important pathogen that causes biofilm-associated infection in humans. Autoinducer 2 (AI-2), a quorum-sensing (QS) signal for interspecies communication, has a wide range of regulatory functions in both Gram-positive and Gram-negative bacteria, but its exact role in biofilm formation in S. aureus remains unclear. Results: Here we demonstrate that mutation of the AI-2 synthase gene luxS in S. aureus RN6390B results in increased biofilm formation compared with the wild-type (WT) strain under static, flowing and anaerobic conditions and in a mouse model. Addition of the chemically synthesized AI-2 precursor in the luxS mutation strain (ΔluxS) restored the WT phenotype. Real-time RT-PCR analysis showed that AI-2 activated the transcription of icaR, a repressor of the ica operon, and subsequently a decreased level of icaA transcription, which was presumably the main reason why luxS mutation influences biofilm formation. Furthermore, we compared the roles of the agr-mediated QS system and the LuxS/AI-2 QS system in the regulation of biofilm formation using the ΔluxS strain, RN6911 and the Δagr ΔluxS strain. Our data indicate a cumulative effect of the two QS systems on the regulation of biofilm formation in S. aureus. Conclusion: These findings demonstrate that AI-2 can decrease biofilm formation in S. aureus via an icaR-activation pathway. This study may provide clues for therapy in S. aureus biofilm-associated infection.

Staphylococcus aureus, a Gram-positive bacterium causes a number of devastating human diseases, such as infective endocarditis, osteomyelitis, septic arthritis and sepsis. S. aureus SraP, a surface-exposed serine-rich repeat glycoprotein (SRRP), is required for the pathogenesis of human infective endocarditis via its ligand-binding region (BR) adhering to human platelets. It remains unclear how SraP interacts with human host. Here we report the 2.05 Å crystal structure of the BR of SraP, revealing an extended rod-like architecture of four discrete modules. The N-terminal legume lectin-like module specifically binds to N-acetylneuraminic acid. The second module adopts a β-grasp fold similar to Ig-binding proteins, whereas the last two tandem repetitive modules resemble eukaryotic cadherins but differ in calcium coordination pattern. Under the conditions tested, small-angle X-ray scattering and molecular dynamic simulation indicated that the three C-terminal modules function as a relatively rigid stem to extend the N-terminal lectin module outwards. Structure-guided mutagenesis analyses, in addition to a recently identified trisaccharide ligand of SraP, enabled us to elucidate that SraP binding to sialylated receptors promotes S. aureus adhesion to and invasion into host epithelial cells. Our findings have thus provided novel structural and functional insights into the SraP-mediated host-pathogen interaction of S. aureus. Staphylococcus aureus is an important pathogen that causes a range of human diseases, such as infective endocarditis, osteomyelitis, septic arthritis and sepsis. The increasing resistance of S. aureus to most of the current antibiotics emphasizes the need to develop new approaches to control staphylococcal infections. As a surface-exposed serine-rich repeat glycoprotein (SRRP), S. aureus SraP is involved in the pathogenesis of infective endocarditis via its ligand-binding region (BR) adhering to human platelets. However, little is known about how SraP interacts with its host receptor(s). Through structural and functional analyses of the BR domain, we have discovered a specific binding of SraP to N-acetylneuraminic acid (Neu5Ac), in agreement with a recent report of the trisaccharide ligand of SraP. Further mutagenesis analysis showed that SraP binding to Neu5Ac and the trisaccharide promotes S. aureus adhesion to and invasion into host epithelial cells. These findings increase our knowledge of surface protein mediated interaction of S. aureus with host epithelial cells.

The Kdp system is widely distributed among bacteria. In Escherichia coli , the Kdp-ATPase is a high-affinity K ⁺ uptake system and its expression is activated by the KdpDE two-component system in response to K ⁺ limitation or salt stress. However, information about the role of this system in many bacteria still remains obscure. Here we demonstrate that KdpFABC in Staphylococcus aureus is not a major K ⁺ transporter and that the main function of KdpDE is not associated with K ⁺ transport but that instead it regulates transcription for a series of virulence factors through sensing external K ⁺ concentrations, indicating that this bacterium might modulate its infectious status through sensing specific external K ⁺ stimuli in different environments. Our results further reveal that S. aureus KdpDE is upregulated by the Agr/RNAIII system, which suggests that KdpDE may be an important virulence regulator coordinating the external K ⁺ sensing and Agr signaling during pathogenesis in this bacterium.

Background: Escherichia coliis an important opportunistic pathogen that could cause inflammation of the udder in dairy cows resulting in reduced milk production and changes in milk composition and quality, and even death of dairy cows. Therefore, mastitis is the main health issue which leads to major economic losses on dairy farms. Antibiotics are routinely used for the treatment of bovine mastitis. The ability to form biofilm increases the antibiotic resistance ofE. coli. Nanoparticles (NPs), a nanosized, safe, and highly cost-effective antibacterial agent, are potential biomedical tools. Given their antibacterial activities, silver nanoparticles (Ag NPs) have a broad range of applications.Methods: In this study, we performed antibacterial activity assays, biofilm formation assays, scanning electron microscopy (SEM) experiments, and real-time reverse transcription PCR (RT-PCR) experiments to investigate the antibacterial and anti-biofilm effect of quercetin, Ag NPs, and Silver-nanoparticle-decorated quercetin nanoparticles (QA NPs) inE. colistrain ECDCM1.Results: In this study, QA NPs, a composite material combining Ag NPs and the plant-derived drug component quercetin, exhibited stronger antibacterial and anti-biofilm properties in a multi-drug resistantE. colistrain isolated from a dairy cow with mastitis, compared to Ag NPs and Qe.Discussion: This study provides evidence that QA NPs possess high antibacterial and anti-biofilm activities. They proved to be more effective than Ag NPs and Qe against the biofilm formation of a multi-drug resistantE. coliisolated from cows with mastitis. This suggests that QA NPs might be used as a potential antimicrobial agent in the treatment of bovine mastitis caused byE. coli.