Streptococcus gallolyticus subsp. gallolyticus is the identified causative pathogen in about 10 % of infective endocarditis (IE) cases. Notably, S. gallolyticus subsp. gallolyticus-caused IE is often associated with colon carcinoma. Based on this knowledge it was postulated that S. gallolyticus subsp. gallolyticus adheres to the extracellular matrix of malignant epithelium of the colon and translocates through a paracellular pathway through from the epithelium to the bloodstream. Furthermore, the bacterium adheres to the extracellular matrix of the damaged endocardium and induces IE. To date, the pathomechanisms have not been solved.<br />

Macrophages play a key role in the endocardium during IE. Therefore, pathogens develop mechanisms which protect against degradation in phagosomes. It has been shown that S. gallolyticus subsp. gallolyticus phagocytic uptake by THP-1 macrophages, and their survival within these phagocytes was strain-dependent. In this study, it was shown that S. gallolyticus subsp. gallolyticus stimulates cytokine gene expression and lysis of macrophages just marginally with equivalent stimulation of reactive oxygen species, regardless of which strain. Whole genome microarrays were used to examine the reaction of S. gallolyticus subsp. gallolyticus to the phagocytic uptake by THP-1 macrophages. Increased gene expression of a NADH oxidase indicates reaction towards the oxidative burst in macrophages. Furthermore, transcriptomic analysis revealed that Dlt proteins could effect resistance against cationic peptides through D-alanylation of teichoic acids, and that the bacterial cells change their nutrition and react to anaerobic conditions in phagosomes. Beside the phagocytosis, survival or growth of S. gallolyticus subsp. gallolyticus was observed in medium with hydrogen peroxide, lysozyme or acid pH-value, important factors for degradation of bacteria in phagosomes. This revealed that S. gallolyticus subsp. gallolyticus is very resistant to these factors. The strains DSM 16831 and LMG 17956, which were more rapidly killed in THP-1 macrophages, were more sensitive to lysozyme or hydrogen peroxide, respectively, than other tested strains. Furthermore, it was observed that lysozyme triggers biofilm formation of S. gallolyticus subsp. gallolyticus on polystyrene. Therefore, the transcriptome of S. gallolyticus subsp. gallolyticus was analyzed in the presence of lysozyme. It again revealed higher expression of genes of the dlt operon whose products provoke D-alanylation of teichoic acids. This leads to resistance to the cationic antimicrobial function of lysozyme. Additionally, an increase in the expression of genes of the protein biosynthesis, genes for aerobic growth as well as genes of carbohydrate, lipid and amino acid metabolism was observed. <br />

Adhesion to collagen is the initial step of biofilm formation in the endocardium. The transcriptome analysis of S. gallolyticus subsp. gallolyticus adhering to collagen compared to planktonic cells showed strain-dependent regulation of gene expression. The regulation of gene expression of the strain UCN 34 indicate a change in nutritional intake. In the low-adherent strain DSM 16831 the expression of genes associated with a transposon and phage indicate horizontal gene transfer.<br />

The results of this study provide insights about potential pathomechanisms of S. gallolyticus subsp. gallolyticus which show high resistance to antimicrobial compounds and biofilm formation as a survival mechanism against lysozyme.