By interacting with components of the human host, including extracellular matrix (ECM) proteins, Streptococcus pneumoniae has evolved various strategies for colonization. Here, we characterized the interaction of pneumococci with the adhesive glycoprotein vitronectin and the contribution of this protein to pneumococcal uptake by host cells in an integrin-dependent manner. Specific interaction of S. pneumoniae with the heparin-binding sites of purified multimeric vitronectin was demonstrated by flow cytometry analysis. Host-cell-bound vitronectin promoted pneumococcal adherence to and invasion into human epithelial and endothelial cells. Pneumococci were trapped by microspike-like structures, which were induced upon contact of pneumococci with host-cell-bound vitronectin. αvβ3 integrin was identified as the major cellular receptor for vitronectin-mediated adherence and uptake of pneumococci. Ingestion of pneumococci by host cells via vitronectin required a dynamic actin cytoskeleton and was dependent on integrin-linked kinase (ILK), phosphatidylinositol 3-kinase (PI3K), and protein kinase B (Akt), as demonstrated by gene silencing or in inhibition experiments. In conclusion, pneumococci exploit the vitronectin–αvβ3-integrin complex as a cellular receptor for invasion and this integrin-mediated internalization requires the cooperation between the host signalling molecules ILK, PI3K and Akt.

Thrombospondin-1 (TSP1) is a matricellular glycoprotein that has key roles in interactions between human cells and components of the extracellular matrix. Here we report a novel role for the lectin TSP1 in pathogen-host interactions. Binding assays and flow cytometric analysis demonstrate that Streptococcus pneumoniae and other gram-positive pathogens including S. pyogenes, Staphylococcus aureus, and Listeria monocytogenes interact specifically with human TSP1. We also show for the first time that host cell-bound TSP1 promotes adherence of gram-positive pathogens to human epithelial and endothelial cell lines. Pretreatment of bacteria with sodium periodate but not Pronase E substantially reduced TSP1-mediated bacterial adherence to host cells, suggesting that a glycoconjugate on the bacterial cell surface functions as the receptor for TSP1. Lipoteichoic acids did not affect TSP1-mediated adherence of S. pneumoniae to host cells. In contrast, attachment of S. pneumoniae and other gram-positive pathogens to host cells via TSP1 was blocked by soluble peptidoglycan, indicating recognition of bacterial peptidoglycan by TSP1. In conclusion, our results demonstrate that recognition of gram-positive pathogens by TSP1 promotes bacterial colonization of host tissue cells. In this scenario, peptidoglycan functions as adhesin and TSP1 acts as a molecular bridge linking gram-positive bacteria with receptors on the host cell.

Infertility in men and women is frequently associated with genital contamination by various commensal or uropathogenic microbes. Since many microorganisms are known to release quorum-sensing signals in substantial amounts, we raised the question whether such molecules can directly affect human spermatozoa. Here we show that farnesol and 3-oxododecanoyl-L-homoserine lactone, employed by the opportunistic pathogenic yeast Candida albicans and the gram-negative bacterium Pseudomonas aeruginosa, respectively, induce multiple damage in spermatozoa. A reduction in the motility of spermatozoa coincided in a dose-dependent manner with apoptosis and necrosis at concentrations which were nondeleterious for dendritic cell-like immune cells. Moreover, sublethal doses of both signaling molecules induced premature loss of the acrosome, a cap-like structure of the sperm head which is essential for fertilization. Addressing their mechanism of action, we found that the bacterial molecule, but not the fungal molecule, actively induced the acrosome reaction via a calciumdependent mechanism. This work uncovers a new facet in the interaction of microorganisms with human gametes and suggests a putative link between microbial communication systems and host infertility.

The immunomodulatory power of seminal plasma may help spermatozoa to circumvent the attack of DCs of the female reproductive tract, thereby supporting successful fertilization. At the same time, however, such protective effects of seminal plasma may also modulate DC action during host-pathogen interactions.

Platelets and coagulation are involved in bacterial colonisation of the host. Streptocococcus pneumoniae (pneumococcus) are important etiologic agents of respiratory tract infections in humans. The formation of pneumococci-platelet associations may facilitate haematogenous dissemination of pneumococci by providing an adhesive surface on damaged endothelium. However, the formation of platelet-pneumococci associations and the factors involved in this process have not been described so far. The formation of platelet-pneumococci associates was analysed and quantified using flow cytometry. Binding of pneumococci to platelets was significantly increased after activation of platelets with thrombin, while platelet activation by ADP or collagen did not promote formation of platelet-pneumococci associates. In addition to be a platelet agonist, thrombin cleaves fibrinogen, which results in the generation of fibrin. The simultaneous formation of fibrin and activation of platelets was shown to be a prerequisite for a high number of platelet-pneumococci associates. Moreover, exogenously added human thrombospondin-1 (TSP-1) significantly enhanced the association of pneumococci with activated platelets. Soluble fibrin and TSP-1 are key co-factors of platelet-pneumococci-association. Similar results were recently demonstrated for S. aureus-platelet adhesion. Consequently, we hypothesise that the described mechanism of platelet-bacteria-association might represent a general and important strategy of Gram-positive bacteria during development of invasive diseases.