## 3 Results and Discussion
### Growth Stimulatory Activity
Polysaccharides and phenolic compounds can increase the proportion of probiotic bacteria in the gut, regulate the intestinal microenvironment, and stimulate the innate immune system to fight infections by promoting macrophages and lymphocytes within the intestinal mucosa [2,45,46]. Figure 5: Prebiotic index values of (**a**) aqueous and (**b**) ethanolic _A. citrodora_ extracts based on _Lacticaseibacillus rhamn_SG and _Lacticaseibacillus rhamn_OXLAL-1. Significant difference (\(p<0.05\)) compared to the control (MRS broth) is indicated with *. Recent studies have demonstrated the beneficial effects of polyphenols in human and animal microbiota [47], as well as in the survival of various lactobacilli strains, such as _L. johnsonii_ and _L. rhamnosus_ GG [48]. Dietary fibers, plants, fruits, and secondary plant metabolites are the most common and richest sources of polyphenols (PP) and polysaccharides (PS) that serve as substrates for the probiotic strains. Hence, they effectively support bacterial growth. However, the level of this probiotic cell growth is dependent on the various components each plant extract possesses. Simply structured components, such as galactose, are more likely to be consumed as substrates by probiotic strains, while more complex compounds, such as pectin, are selectively processed by a small number of bacterial strains [8]. This finding is indicative of the various responses that bacteria have to different prebiotic substrates as a result of differential molecule and compound  
Figure 6: Effect of aqueous _A. citradora_ extract (mg (dry matter)/mL) as growth stimulator of (**a**) _Lacticasibacillus rhamnus_ GG and (**b**) _Lacticasibacillus rhamnus_ OXLAL-1 in MRS supplemented with different NaCl concentrations. Significant difference (_p_\(<\) 0.05) compared to the control (MRS broth, 0% NaCl) is indicated with *. utilization. A recent study investigated the capacity of different ginseng berry extract components to support probiotic bacterial growth. The results revealed that galactose was the most-consumed substrate by _L. plantarum_, _L. reuteri_, _B. longum_ subsp. _Infantiis_, and _B. longum_, while pectin was utilized mostly by lactobacilli strains [8]. Figure 7: Effect of aqueous _A. citrodara_ extract (mg (dry matter)/mL) as growth stimulator of (**a**) _Lacticaseibacillus rhamnus_ GG and (**b**) _Lacticaseibacillus rhamnus_ OLXAL-1 in MRS enriched with bile salts (0.3% \(w/v\)). Significant difference (\(p<0.05\)) compared to the control (MRS broth) is indicated with *. _A. citrodora_ extracts had an overall beneficial and growth stimulatory effect on lactobacilli strains (_L. rhamn_s GG, _L. rhamn_s OIXAL-1). In addition, the aqueous extract of _A. citrodora_ seems to be a valuable substrate, increasing the survival rate of the lactobacilli strain in various assays (osmotic, acid, thermal, and alcohol stress), indicating that it could be used as a prebiotic agent or even as a food preservative. Yet, not many studies exist evaluating plant extracts as prebiotic agents and their protective, supportive effects on probiotic strains. However, further research is required to validate their activity in food systems and provide more insights on their mode of action. Figure 8: Effect of aqueous _A. citrodora_ extract (mg (dry matter)/mL) as growth stimulator of (**a**) _Lacticasebiacillus rhamn_s GG, and (**b**) _Lacticasebiacillus rhamn_s OIXAL-1 under thermal stress (52 \({}^{\circ}\)C). Significant difference (\(p<0.05\)) compared to the control (52 \({}^{\circ}\)C, without plant extract) is indicated with *.