## 6 Essential Oil, Gut Microbiota, and Host Health  
The gut microbiota plays a crucial role in regulating the health and physiology of the host [101]. It is also closely associated with several diseases and disorders [102]. Supplementing phytocompounds and natural products could improve the composition and diversity of the microbiome [13; 103]. Dietary EO supplementation could increase the abundance of beneficial bacteria, promoting the growth of beneficial microbes in the colon [104]. Importantly, the gut microbiota significantly influences the microbiota-gut-brain axis, which is the bidirectional communication between the gastrointestinal and brain systems [102]. A healthy gut microbiota is more important for the intestinal integrity, immunity, feed utilization, and growth of nursery pigs [105; 106; 107]. Dietary supplementation of cinnamaldehyde and carvacrol enhances performance and appetite in nursery pigs by modulating intestinal health and microbiota. In detail, supplementing cinnamaldehyde (100 mg/kg), carvacrol (100 mg/kg), or a combination improved body weight, average daily weight gain, feed intake, and diarrhea rates. Also, the supplementation increased intestinal absorption area, tight junction protein abundance, and intestinal development, decreasing intestinal permeability and local inflammation. Goblet cell numbers were also increased, indicating enhanced mucus barrier function. Furthermore, piglets in the supplemented group showed upregulated transporters and digestive enzyme levels in the intestine, which correlated significantly with daily weight gain and feed utilization. The abundance of _Lactobacillus_ spp., _Prorotella_ spp., _Megamonas_ spp., _Megasphaera_ spp., and _Blautia_ spp. increased after the supplementation. Dietary cinnamaldehyde and carvacrol improve intestinal barrier function and development, increase digestive enzyme activity and absorption capacity and microbial communities, and enhance appetite, leading to improved performance and reduced diarrhea in nursery pigs [101] (Figure 6). Healthy white \(\times\) landrace sows were supplemented with or without 500 g/t Merident-Stim(r) (which contains 5% OEO (extracted from _Origanum vulgare_ subsp. hirthum) and 95% natural feed grade inert carrier) from day 100 of gestation until day 21 of lactation. OEO caused a notable decline in the phylum Proteobacteria and Actinobacteria (\(p<0.05\)). Additionally, OEO significantly increased the relative abundance of _Lactobacillus_ spp. and _Prerottellaceae_ _UCG 003_ and _UCG 005_ and reduced _Escherichia_ spp. and _Shigella_ spp. abundances. Overall, supplementing maternal diets with OEO during late gestation and lactation improved serum metabolites and antioxidant capacity. It balanced the intestinal flora of sows, potentially resulting in increased piglet weight at weaning [105]. Notably, an increased prevalence of Proteobacteria signifies microbial community instability and could serve as a disease diagnostic marker [106]. At the phylum level, a notable decline in relative abundance was observed in the OEO group, suggesting potential energy conservation in sows for enhanced milk quantity and quality [108]. Likewise, _Escherichia_ and _Shigella_ species are closely linked with gut dysbiosis [109] and demonstrated reduced abundance in the OEO-treated group. _E. coli_ O157:H7, a zoonotic pathogen, represents a significant serotype of Shiga toxin-producing _E. coli_[108]. A study indicates that _E. coli_ O157:H7 triggered notable increases in serum creatinine and urea levels and reported _Thymus vulgaris_ EO mitigating hepatoreal dysfunction induced by _E. coli_ O157:H7 [110]. Another study revealed how carvacrol inhibited flagellin production and subsequent flagellar development, rendering _E. coli_ O157:H7 immobile through protein heat shock [111]. Figure 6: EO-mediated improvement of microbiota and intestinal health in pigs.