Background: One of the most ecologically complex environments in the human body is the oral cavity, where the balance between health and disease depends on complex interactions between the host and the microbiome. While in general the oral commensal streptococci, including Streptococcus gordonii are innocuous, it is evident that they contribute in the pathogenicity of periodontal disease by interaction with other periodontal pathogens. Oral streptococci are also capable of being causative agents on their own of systemic diseases such as endocarditis. Although phagocytes work as microbial assassins, many bacterial pathogens are able to avoid being killed, with some even being capable of surviving within immune cells. Indeed, our lab has found that S. gordonii is capable of survival within macrophages, with a role for both reactive oxygen species (ROS) resistance and an active ability to damage phagosomes in this survival. We find that S. gordonii DL1 is able to survive and evade macrophage killing once phagocytosed by expressing ROS resistant pathogenicity factors.

Aim: A number of genes with ROS resistance roles are known for S. gordonii. With this project, the aim was to determine the importance of a number of these genes in the ability of S. gordonii to survive within activated (M1) macrophages.

Materials and Methods: RAW264.7 macrophages were activated overnight with 100 microg/ml IFN-gamma followed by stimulation with 0.1 microg/ml lipopolysaccharide (LPS) for 2 hours, producing a macrophage with M1 phenotype characteristics (which produce phagosomes with extended ROS production and concurrent delayed acidification). The S. gordonii uptake and intracellular clearance capability of these macrophages was determined by an antibiotic protection-based assay and S. gordonii gene mutants were tested with a H2O2 killing assay.

Results: S. gordonii DL1 has the capability to survive within macrophage at higher rates than non-pathogenic SK12 and DL1 with mutations in ROS resistant gene products. It was also found that there were no significant differences between S. gordonii DL1 and another pathogenic S. gordonii strains, 38 and CH1 as well as the non-pathogenic strain SK9, with them all surviving in cells surviving within macrophages at similar rates for two hours post phagocytosis as DL1.

Conclusion: These results confirm that ROS resistance genes of S. gordonii are indeed essential to allow for enhanced bacterial survival within activated macrophages. The results also highlight that while ROS resistance leads to increased survival within phagosomes, it is likely not the only factor allowing S. gordonii to act as a pathogen.