**5. Effects on gut microbiota**

The gut microbiota is considered one of the key elements contributing to the efficiency, welfare, and health of birds. The evaluation of gut microbiota changes in response to the use of feed additives is a key indicator to evaluate the efficiency of natural alternatives to the use of feed antibiotics in poultry production. The latest generation molecular techniques have contributed to characterizing the microbial

communities in a complex environment such as the digestive tract [33, 37, 38] and have also allowed them to transcend to the animal's metabolome. In our studies, a positive correlation was found between body weight and OEO from *L. origanoides* supplementation with the Firmicutes:Bacteriodetes ratio and Firmicutes levels in cecal content. Quite fewer Bacteroidetes bacteria than those of Firmicutes have been found in fat broilers [39], and a similar relationship was found in obese humans experiencing insulin resistance [40]. The addition of 200 ppm of dietary OEO increased the abundances of Burkholderiales, *Actinobacteria*, Bifidobacteriales, Enterococcaceae, and Bacillaceae, whereas it decreased *Shigella* abundance in the ileum [8].

We also explored the effects of OEO from three chemotypes through the use of denaturing gradient gel electrophoresis (DGGE) technology on broiler chickens at high altitude, *Origanum vulgare* L. (OL), *O. vulgare* L. ssp. Hirtum (OH), and *O. majorana* (OM) from a greenhouse of Sabana de Bogotá and *O*. *vulgare* L. ssp. Hirtum (OG) from Greece. Bacterial community DNA revealed two main clusters: OEOtreated chicks and nontreated control chicks. These results indicate that all treatments with OEO had some effect on the gut microbial communities' changes. A reduction of about 50% in mortality caused by Ascites with *O. Majorana* essential oil and an average of 68% of all additive-supplemented groups compared to controls indicate a possible association of pulmonary hypertension syndrome with the use of OEO and gut microbiota [41].

It is also reported that OEO supplementation increases the abundances of *Ruminococcus*, *Bifidobacterium*, and *Enterococcus* as well as its Spearman's correlation positively correlated with the messenger RNA (mRNA) expression of mucins. Moreover, the relative abundance of *Enterococcus* was positively correlated with amylase activity [42]. The OEO supplementation enriched the genera *Propionibacterium*, *Brevinema*, and *Corynebacterium*, whereas the genus *Vibrio* was enriched in the control with no OEO supplementation in *Cyprinus carpio* [3].
