## 3 Results & Discussion  
### Evaluation of the Anti-Listerial Potency of LRE  
The anti-Listerial potency of LRE was investigated by the employment of the broth microdilution method. Initially, the MIC of the licorice root extract was determined for each _Listeria_ species and strain, as this value reflects the concentration required to inhibit visible bacterial growth and serves as an indicator of the extract's bacteriostatic effect against each test organism. LRE was obtained by supercritical carbon dioxide extraction; thus, water and ethanol were used for the solubilization of the extract for the assessment of its inhibitory effect against _Listeria_ bacteria. Table 1 summarizes the MIC values of LRE dissolved in ethanol and water against _Listeria_ strains. In particular, the MICs of LRE ranged from 31.3-65.5 \(\upmu\)g mL\({}^{-1}\) when dissolved in ethanol, whereas MICs were significantly higher when water was used for solubilization (125-250 \(\upmu\)g mL\({}^{-1}\)). The significant differences in MIC values may be attributed to the better solubilization of LRE phytochemicals in ethanol than water. The superiority of alcohol for the solubilization of phytochemicals against water has been described thoroughly [28]. Findings also support the great potential of LRE as a growth inhibitor of _Listeria_ bacteria, since its MIC values were similar to well-known classes of antibiotics such as cephalosporions and quinolones and comparable with streptograms and imcosamides [29]. The MIC values of LRE are also substantially lower than those of over a hundred plant materials (entire plants, bulbs, fruits, flowers, roots, seeds, and by-products) published from 2017 to 2023 [6]. The potential of LRE to inhibit the growth of _Listeria_ bacteria is equivalent to extracts of _Hypericum perforatum_ L.  
leaves [30], _Berberis libanotica_ Ehrenb. leaves [31], _Pistacia lentiscus_ leaves [32], _Origanum ehernbergii_ Boiss aerial parts [33], and _Alpinia galanga_ ([PERSON]) flowers [34]. In addition, LRE is more effective in controlling the growth of _Listeria_ bacteria than extracts originating from the aerial parts and roots of _Glycyrrhiza glabra_. Its aerial parts and root extracts present MIC values of 620 \(\upmu\)g mL\({}^{-1}\) and 290 \(\upmu\)g mL\({}^{-1}\), respectively [18; 19]. Since bacteriostatic effects alone are insufficient to confer antiseptic and disinfectant properties on an extract or antibacterial agent, the bactericidal activity of LRE was also determined. According to Table 1, the MBC values of the extract dissolved in ethanol ranged from 62.5 \(\upmu\)g mL\({}^{-1}\) to 250 \(\upmu\)g mL\({}^{-1}\). Interestingly, the variation of MBC values is greater than that of MICs. _L. monocytogenes_ ATCC 23,074 and _L. monocytogenes_ NCTC 7973 were the most susceptible strains among those studied. On the other hand, significantly higher amounts of LRE were required to kill the bacteria of _L. monocytogenes_ EGD, _L. grayi_ NCTC 10815, and _L. seeligeri_ NCTC 11856. Results also highlighted that non-pathogenic _Listeria_ species are more resistant to the effect of the extract in comparison with pathogenic _Listeria_. Results disclose remarkable differences in inherent susceptibility among the studied species and strains. The susceptibility diversity of _Listeria_ species to antibiotics and natural products has also been described [29; 35; 36]. In addition, the MIC values reveal a great potential of LRE to act as a potent inhibitor of _Listeria_ bacteria growth, since they meet stringent end point criteria for \"activity\" as a previous study described [37]. In particular, plant extracts should be considered efficacious if they exhibit IC\({}_{50}\) values \(\leq\) 100 \(\upmu\)g mL\({}^{-1}\); the present extract is notably promising, as its MBC values are lower than the described critical concentration. Therefore, the elucidation of phytochemical composition is of great importance in order to decipher the antimicrobial potential of LRE.