Thyme-synthesized silver nanoparticles mitigate immunosuppression, oxidative damage, and histopathological alterations induced by multidrug-resistant Enterococcus faecalis in Oreochromis niloticus: in vitro and in vivo assays

The escalating prevalence of antibiotic resistance poses a significant challenge to controlling aquatic diseases, underscoring the need for innovative antimicrobial approaches. Therefore, the current study aimed to investigate the effects of thyme-synthesized silver nanoparticles (thyme-AgNPs) on modulating the immunosuppressive, oxidative stress, and histopathological changes induced by multidrug-resistant Enterococcus faecalis (MDR E. faecalis) infection in Oreochromis niloticus (Nile tilapia). The antibiotic susceptibility profiling of the isolated E. faecalis (PQ084785) revealed resistance to multiple antibiotic classes. The in vitro bactericidal efficacy of thyme-AgNPs was confirmed, with minimum bactericidal concentrations (MBC) and minimum inhibitory concentrations (MIC) of 62.5 µg/mL and 31.5 µg/mL, respectively. In vivo, the 96-h median lethal concentration (LC₅₀) of thyme-AgNPs was determined to be 1 mg/L. Meanwhile, eighty O. niloticus (30.00 ± 5.00 g) were categorized into four groups in duplicate: group I was the negative control group, group II was intraperitoneally injected with MDR E. faecalis (0.2 mL of 0.79 × 10⁸ CFU/mL), group III was exposed to the sublethal dose of thyme-AgNPs in water (0.1 mg/L), and group IV was challenged with the E. faecalis and exposed to thyme-AgNPs simultaneously for 15 days. Results outlined that E. faecalis induced clinical abnormalities, reduced immunoglobulin M levels, elevated oxidant indicators (malondialdehyde), decreased antioxidant capacity (reduced glutathione content, catalase, and superoxide dismutase), and significantly upregulated immune gene expression (tumor necrosis factor-α and interleukin-1β) with marked histopathological and histochemical alterations in gill and liver tissues. Interestingly, the thyme-AgNPs intervention resulted in a noticeable improvement in all measured biomarkers as well as histopathological regeneration. Overall, thyme-AgNPs are a recent antibacterial tool against immune-oxidant/antioxidant and histopathological changes triggered by MDR E. faecalis infection.