Abstract
L. acidophilus bacteria are found in foods and are used in pharmaceuticals to supplement the natural gut flora associated with human digestive health. They also possess excellent therapeutic properties, attributed to their biological activity. These bacteria produce serotypes that inhibit the growth of pathogenic bacteria and adhere to epithelial cells, preventing intestinal pathogens from attaching to them through a process called competitive exclusion. In this study, L. acidophilus strains were isolated and identified from local yogurt, and the efficacy of the isolated strains' metabolites against the diarrheal serotype of Escherichia coli was investigated. Additionally, some L. acidophilus strains were examined for their probiotic properties. The research was conducted in the laboratories of the Department of Biological Sciences and the Central Laboratories at Samarra University from September 1, 2025, to November 1, 2025. In this study, one of the L. acidophilus strains was obtained from lactic acid bacteria. The study results showed that the metabolites of the studied strains exhibited good inhibitory activity against the diarrheal serotype of E. coli. Regarding the probiotic assay, the results confirmed the ability of all strains to adhere to epithelial cells. The group treated with a combination of pathogenic bacteria and copper oxide nanoparticles (G2) showed a significantly higher level of IL-8 (9.78±3.45 pg/mL), which is considerably higher than G1, suggesting that the nanoparticles may have stimulated a level of inflammatory stress or an immune response. The control group (G3: pathogenic bacteria only) recorded the highest mean IL-8 (12.21 ± 3.09 pg/mL). In comparison, the presence of gut microbiota (G1) appeared to have a modulating effect on the IL-8 response to pathogenic infections, resulting in a marked decrease in its mean concentration (4.54±0.99 pg/mL) compared to the control group (G3) (p<0.05). Interestingly, the group treated with pathogenic bacteria and copper oxide nanoparticles (G2) exhibited an exceptionally high mean IL-6 (63.5 ± 22.28 pg/mL), significantly higher than the mean of both the control group (G3: 18.13 ± 9.49 pg/mL) and the gut microbiota group (G1: 9.52 ± 4.11 pg/mL). Copper oxide nanoparticles not only directly and effectively affect bacteria but also influence the host's immune system. The large increase in IL-6 may be due to the recognition of nanoparticles as foreign bodies, causing phagocytic cells to activate and release pro-inflammatory cytokines in an attempt to remove the particles.