ABSTRACT: Monoclonal antibodies are widely used to treat follicular lymphoma (FL) and multiple myeloma (MM), but prolonged exposure may impair immune function and increase infection risk. Thus, probiotics are emerging as a strategy to counteract pathogenic colonization. Among them, Streptococcus salivarius 24SMB emerges as a promising oral probiotic. In this project, its beneficial activity was investigated using the amoeba model system Dictyostelium discoideum, widely used for studying host–bacteria interactions. Dictyostelium can be considered as a primitive macrophage, able to feed on bacteria. It was used to evaluate the effects of S. salivarius and the oral pathogen Streptococcus mutans through growth, viability, chemotaxis and phagocytosis assays, combined with transcriptomic and metabolomic analyses. Both S. salivarius and S. mutans reduced Dictyostelium growth compared to E. coli (control). However, S. salivarius significantly improved viability, particularly in sequential exposure experiments, where pre-culture with S. salivarius followed by S. mutans increased vitality by 30%. Moreover, while S. mutans reduced Dictyostelium chemotactic competence, S. salivarius enhanced it. Pre-culture with S. salivarius also increased phagocytosis of oral pathogens (S. mutans and E. corrodens), confirmed by higher intracellular ROS. Transcriptomic and metabolomic analyses further support obtained results, indicating a more active metabolic and phagocytic-primed state after S. salivarius-exposure. In contrast, S. mutans induced a stressed, energy-depleted profile. In conclusion, S. salivarius primed innate immune functions, enhancing Dictyostelium viability, chemotaxis and phagocytic capacity. These findings support Dictyostelium as a simple, cost-effective model for probiotic screening and pathogen-induced stress response studies.
