# Plant-Derived Phytobiotics as Emerging Alternatives to Antibiotics Against Foodborne Pathogens
## 4 Impact of Phytobiotics on Microbiota of Food and the Intestine
### Impact on Gut Microbiota
Animal model studies have also demonstrated that dietary supplementation with astaxanthin can alter the composition of the gut microbiota. Astaxanthin is a carotenoid responsible for the red pigmentation in certain algae, fish, and crustaceans [188]. Administration of astaxanthin to mice at a dose of 50 mg/kg significantly modified the relative abundance of major bacterial phyla--increasing _Bacteroidetes_ and decreasing _Proteobacteria_--as well as affecting composition at lower taxonomic levels [189]. Furthermore, in _Helicobacter pylori_-infected mice, a higher daily dose of astaxanthin (200 mg/kg body weight) reduced gastric colonization by the pathogen, alleviated mucosal inflammation, and decreased the release of pro-inflammatory cytokines associated with _H. pylori_ infection [190]. Thesefindings support the selective action of phytobiotics in modulating both beneficial and pathogenic gut microorganisms. Tannins may act synergistically with probiotic bacteria. Certain _Lactobacillus_ strains have been shown to tolerate higher concentrations of tannins due to the presence of tannase enzymes, which allow them to utilize tannins as a source of carbon and energy [191]. This enables dietary tannins to favor tannase-positive probiotics, thereby limiting the growth of competing pathogenic species in the gut. However, the dosage is critical--excessive tannin intake can have antinutritional effects and overly suppress the microbiota. Thus, beneficial outcomes are typically observed at moderate intake levels [177,192]. Although essential oils are best known for their potent antibacterial and antifungal activities, they also exert selective effects on the gut microbiome. For instance, orange essential oil--rich in d-limonene and terpen--more strongly inhibited the growth of enterotoxicigenic _Escherichia coli_ than that of beneficial _Lactobacillus_ strains [193]. While pathogenic strains showed clear inhibition, _Lactobacillus rhamnosus_ remained relatively unaffected--MIC for _E. coli_ was several times lower than that for lactic acid bacteria (LAB) [193]. A similar selectivity has been reported for oregano and thyme essential oils, which contain phenolic compounds such as carvacrol and thymol. These oils markedly suppressed _Clostridium perfringens_, _Salmonella_, and _E. coli_ in vitro, with only moderate effects on _Lactobacillus_ spp. [119]. As essential oils act primarily by disrupting microbial cell membranes, depolarizing membrane potential, and inducing leakage of intracellular contents, they tend to be more effective against Gram-positive bacteria. This is due to the more permeable peptidoglycan layer of Gram-positive cells compared to the outer membrane of Gram-negatives. However, phenol-rich oils--such as those containing thymol--can also overcome the Gram-negative barrier by incorporating into the lipid bilayer and increasing membrane fluidity [191,193]. Importantly, when used at moderate doses, essential oils do not sterilize the gut but selectively reduce problematic microbial populations, allowing beneficial fiber-fermenting bacteria to thrive. Moreover, some components of essential oils may actually support probiotic bacteria. For example, low concentrations of limonene--the main terpenoid in citrus oil--do not inhibit and may even stimulate the growth of _Lactobacillus_[194]. There is a hypothesis that minor constituents of complex essential oils may exert a protective effect on beneficial microbes, while major phenolic components selectively target pathogens. The final outcome is an enhancement of the gut's natural microbial barrier [193]. In summary, essential oils represent promising phytobiotics that contribute to maintaining microbial balance in the gastrointestinal tract. Sulfur-containing compounds found in Allium vegetables--such as garlic, onion, and leek--have long been valued for their antiseptic properties. When garlic is crushed, the enzyme alliinase converts alliin into allicin, a potent sulfur-containing compound with bactericidal activity.