Quercetin Attenuates Streptococcus pneumoniae-Induced Oxidative Stress, Inflammatory Mediator Release, and Cytotoxicity in Human Bronchial Epithelial Cells: An In Vitro Study Using an Acapsular Strain

Streptococcus pneumoniae can induce airway epithelial oxidative stress, inflammatory mediator release, and cellular injury. Quercetin has reported antioxidant and anti-inflammatory properties, but its protective effects during pneumococcal epithelial challenge require cautious experimental validation, including direct ROS measurement, dose-response testing, viability confirmation, bacterial-load control, and comparison with a positive-control antioxidant. Human BEAS-2B bronchial epithelial cells were exposed to S. pneumoniae R6x at 106 CFU/mL for 24 h with or without 2 h quercetin pretreatment. The main quercetin concentration was 20 µM, with additional dose-response testing at 5, 10, 20, and 40 µM. Oxidative stress was assessed by catalase activity, SOD activity, GSH/GSSG ratio, DCFH-DA fluorescence, and MitoSOX fluorescence. Inflammatory mediators were quantified by IL-8 ELISA and broader cytokine profiling. Cell injury and viability were assessed by LDH release and MTT assay. Bacterial burden was measured by CFU plating, and N-acetyl-L-cysteine was included as a positive-control antioxidant. S. pneumoniae reduced catalase activity, SOD activity, and GSH/GSSG ratio while increasing IL-8 secretion and LDH release. Quercetin restored catalase activity from 13.98 ± 1.51 to 25.77 ± 2.76 units/mg protein, restored SOD activity from 6.92 ± 1.02 to 12.46 ± 1.22 units/mg protein, and improved the GSH/GSSG ratio from 0.218 ± 0.013 to 0.486 ± 0.013. Quercetin reduced IL-8 from 11,444.49 ± 1476.92 to 3448.57 ± 247.52 pg/mL and LDH cytotoxicity from 44.61 ± 5.03% to 11.97 ± 2.46%. Direct ROS assays confirmed suppression of DCFH-DA-associated fluorescence and mitochondrial superoxide (MitoSOX) signal. Dose-response analysis supported 20 µM as an effective non-toxic concentration. MTT assay confirmed improved metabolic viability, while CFU assays indicated that quercetin did not substantially reduce bacterial burden under the tested conditions. Under the preventive conditions tested, involving 2 h of quercetin pretreatment before bacterial exposure, quercetin showed an epithelial-cell protective profile in BEAS-2B cells exposed to live acapsular S. pneumoniae R6x. These findings support a preventive cytoprotective effect of quercetin pretreatment in this in vitro model and require validation in primary airway epithelial cultures, encapsulated pneumococcal strains, post-infection treatment designs, antibiotic-combination settings, and in vivo pneumonia models.