Impact of Lactate on Innate Immunity Interactions with the most common Respiratory Organism, Streptococcus

The lower airway microbiome comprises various pathogenic and non-pathogenic bacteria, as well as fungi and viruses. Among the bacterial constituents, Streptococcus species represent a significant proportion. While many species in the genus may be harmless commensals, others like Streptococcus pneumoniae are pathogens and contribute to the progression of chronic lung diseases.Lactate plays a role as a key signaling molecule affecting immune cell function and contributing to inflammation and tissue responses in disease contexts. Studies have shown elevated lactate level in chronic lung diseases especially in COPD patients, particularly during exacerbations, and may reflect underlying metabolic or inflammatory disturbances.Streptococci produce lactate through fermentation and it is not clear how exogenous lactate may influence growth or pathogenicity of Streptococci in lung disease or interaction between macrophage and bacteria in the lung environment, representing a major knowledge gap in understanding microbial dynamics in chronic lung disease. Methods: A multi-bacterial strain model of macrophage/microbiome interaction was established using clinical isolates of Streptococcus including S. pneumoniae, S. salivarius, S. oralis and S. infantis. Streptococci spp. were grown in Brain Heart Infusion Broth (BHI) in the presence of L-lactate or D-lactate with varying concentrations from 10 mM to 30 mM, and growth was monitored using the Cerillo Stratus Reader at 37 °C with 5 % CO₂. Monocytes collected from peripheral blood of healthy and COPD donors were cultured with GM-CSF to produce monocyte-derived macrophages (MDMs). Phagocytosis assays were performed to assess macrophage discrimination between commensal and pathogenic strains.Results: We have established growth curves and abundances of clinical Streptococci species under different culture conditions with 10 mM, 20 mM and 30 mM concentrations of both L- and D- stereoisomers of lactate, and the same concentrations of lactate were introduced to MDMs to assess cytokine release. In complex media, physiological concentrations of lactate showed no significant effect on bacterial growth among the four Streptococcus species tested.Conclusion: Different concentrations of L- and D-lactate did not show any significant effect on bacterial growth. Macrophages from COPD patients exhibit impaired phagocytic capacity and show selective uptake of certain pathobionts, contributing to altered bacterial diversity in the COPD lung environment. Future experiments will involve MDM co-culture with mixed Streptococcus species and in defined media to further analyze macrophage selectivity under different metabolic conditions.