## 6 Essential Oil, Gut Microbiota, and Host Health
Dietary EO supplementation could improve nursery pig performance and feed digestion and absorption efficiency by ensuring the integrity, turnover, and development of the intestinal barrier and increasing digestive enzyme activity and nutrient transporter abundance. Besides, the increased diversity and structure of the intestine microbiota may affect appetite-related hormones, thus promoting appetite (adapted from [PERSON] et al. [101] under the copyright licensing of CC BY-NC-ND 4.0). Prevotella_ spp. possess a substantial repertoire of carbohydrate-degrading enzymes. They are recognized for producing short-chain fatty acids that regulate long-term energy metabolism. It is important to underscore the need for increased research on the relationship between _Prevotella_ spp. and nutrition, as multiple studies have demonstrated conflicting results regarding their association with dietary patterns [112]. Furthermore, high _Lacticaseibacillus_ spp. abundance is beneficial for preserving intestinal flora equilibrium and inhibiting the proliferation of harmful bacteria. Previous research has indicated that _Lacticaseibacillus_ spp. competes with intestinal pathogens for binding sites in the mucosal layer, thus suppressing bacterial infections [113]. In EO-treated calves, Firmicutes decreased initially but rebounded after two weeks, while Proteobacteria and Bacteroida declined and Actinobacteria increased. Moraxelaceae (36.9%), Burkholderiaceae (14.7%), Microbacteriaceae (10.6%), Mycoplasmaaceae (8.8%), and Pasteurellaceae (5.9%) are the dominant families. _Moraxella_, _Ralstonia_, _Mycoplasma_, _Mannheimia_, _Filobacterium_, _Pseudomonas_, and _Pasteurella_ are the major genera found in the EO-treated group. EO-treated calves exhibited reduced _Mannheimia_ spp. and _Filobacterium_ spp. abundances on day 2, while _Corynebacterium_ decreased on day 1 post-treatment. EO administration altered the nasopharyngeal microbiota composition and diversity, particularly within the first 24 to 48 h, with notable decreases in _Mannheimia_ spp. abundance [114]. The impact of dietary supplementation with _Lippiia sidoides_ EO (LSEO) on the microbiota and intestinal structure of _Danio rerio_ has been reported. _D. rerio_ was supplemented with varying concentrations of LSEO (0.25%, 0.50%, 0.75%, 1.00%, and 1.25%). Microbiological counts did not significantly differ among groups, but microbiome diversity was higher in the 0.25% LSEO-supplemented group compared to the control. LSEO demonstrated inhibitory effects on potential pathogenic bacterial growth. Fish receiving 0.25% LSEO exhibited improved intestinal histomorphometric indices, indicating enhanced intestinal integrity. Thus, incorporating 0.25% LSEO in the diet of _D. rerio_ could improve the microbiota and intestinal health [115]. [PERSON] et al. [116] noted increased species richness and diversity in _Litopenaeus vannamei_'s intestinal microbiota when fed a diet supplemented with organic acid and EO. [PERSON] et al. [117] found no significant differences in rainbow trout (_Oncorhynchus mykiss_) microbiota operational taxonomic units between groups receiving EO-supplemented diets and the control. It has been proposed that organic acid and EO contribute to midgut health by modulating microbial communities [117]. The impact of early OEO supplementation in milk replacers on growth, immune responses, microbiota, and metabolomics in dairy calves during preweaning and adulthood was reported. The EO group exhibited elevated average CD14+ population values and decreased abundance of _Ruminococcaceae UCG-014, Faecalibacterium, Blautia_ species, and _Alloprevotella_, while _Allistipes_ and _Akkermans_ species abundances increased. Changes in plasma metabolites like butyric acid, 3-indole-propionic acid, and succinic acid, especially at 45 days, were correlated with alterations in the intestinal microbiota. Early EO supplementation improved feed efficiency only during the suckling period, accompanied by significant shifts in the microbiota and plasma metabolome.