**6. The OEO and tight junctions as a barrier between intestinal cells**

The intestinal barrier is formed by a layer of epithelial cells with intercellular junction complexes that form a regulated, selectively permeable barrier between luminal contents and the underlying tissue compartments. These include tight junction (TJ) proteins, such as claudins (CLDN), occludins (OCL), and *zonula occludens* (ZO), which form the continuous intercellular barrier between epithelial cells required to regulate the selective permeability across the intestinal epithelium to a barrier function [43, 44]. Although there is limited research about oregano's effects on tight junctions in birds, some studies have investigated the potential benefits of its active compounds on intestinal barrier function. Dietary OEO (150–300 mg/kg) increased the content of secretory immunoglobulin A and the relative expression of Claudin 1, Mucin 2, and Avian β-defensin 1 in yellow-feathered chicken's ileum [31]. The OEO has been shown to improve intestinal morphology and expression of tight junction proteins, which are associated with the regulation of selected intestinal bacteria and immune status in pigs supplemented with 25 mg/kg of OEO for 4 weeks. These results show that OEO promotes intestinal barrier integrity, probably through modulation of intestinal bacteria and immune status in pigs [9]. The OEO supplementation in pig decreased serum endotoxin levels, increased the villus height and expression of the TJ zonula occludens-1 (ZO-1), signaling pathways and expression of inflammatory

### *Oregano Essential Oils as a Nutraceutical Additive in Poultry Diets DOI: http://dx.doi.org/10.5772/intechopen.113313*

cytokines such as mitogen-activated protein kinase (MAPK), protein kinase B (Akt), and nuclear factor kappa B (NF-kB) in the jejunum. A lower population of *Escherichia coli* in the jejunum, ileum, and colon, and inactivation of inflammation were observed too. The supplementation of OEO in production hens' late phase resulted in a quadratic reduction in the mRNA expression of interleukin 1 (IL-1), tumor necrosis factor alpha (TNF-α), interferon gamma (IFN-γ), and toll-like receptor-4 (TLR-4), but a linear and quadratic increase of ZO-1 expression in the ileum. It was observed that the addition of 200 mg/kg of OEO maximized the results [8]. The effect of OEO on sheep was studied too and it showed that this effect reduces the serum interleukin 2 (IL-2) and TNF-β levels as well as mRNA levels of NF-kB, p65, toll-like receptor-4 (TLR-4), and interleukin 6 (IL-6) in the jejune mucosa [42]. All these studies show that the integrity of the intestinal barrier was improved by OEO treatment.

As mentioned earlier, carvacrol and thymol, two major components of oregano essential oil, have been studied for their antimicrobial and anti-inflammatory properties. These compounds have been shown to inhibit the growth of certain pathogenic bacteria and reduce inflammation in the gut. By modulating the gut microbiota and reducing inflammation, oregano compounds might indirectly support the integrity of tight junctions. Additionally, oxidative stress has been implicated in the disruption of tight junction proteins, potentially compromising the intestinal barrier. The OEO antioxidant effects could counteract the oxidative stress, thus reducing the oxidative stress, and may help protect tight junction integrity [45]. However, it's important to note that the research on oregano's effects on tight junctions and the intestinal barrier is limited, and more studies are needed to fully understand its mechanisms and potential benefits.
