Oregano: A potential alternative to antibiotics as growth promoter within the broiler industry

This thesis presents the investigation of the effects of oregano supplementation on broiler microbiota and intestinal health presented as literature review (Chapter 1) followed by four published manuscripts (Chapters 2 to 5) and final conclusions (Chapter 6). The initial in vitro assay uses specially designed microbiological media that could sustain a large proportion of chicken intestinal ileal and cecal bacteria. 16S rRNA gene sequencing was used to investigate the overall changes to the microbial community as well as individual taxa. This was followed by in vivo effects of different concentrations of oregano on microbiota structure of broilers, including alpha and beta diversity, pathogen presence and abundance, effects on beneficial microbiota and dosage effects. Different sections of the gut were investigated for a complete understanding of where the oregano has significant effects due to the fast absorption of phytogens. Short-chain fatty acids and histological analysis of the different gut sections were also investigated. The investigation of the influence oregano on the development and maturation of intestinal microbiota followed. Surprisingly, even a high concentration of oregano did not result in very different mature microbiota; the differences were small and variable from week to week. Lastly, the change of functional abilities of microbiota from the gut of broilers supplemented with 2% oregano powder was investigated. The 16S amplicon sequencing was used in combination with PiCrust algorithm to predict the genes and functions present in microbiota of control and oregano. The data suggested significant changes in function rather than in microbiota. Oregano reduced the abundance of genes involved in the ability of bacteria to invade epithelia, make toxins and move. It also reduced a range of infection and disease-related genes and reduced abundance of genes involved in carbohydrate and lipid metabolism and steroid synthesis. Finally, in the last chapter, a discussion of the outcome of the thesis and the possibility of oregano and other phytogens causing and selecting for antimicrobial resistance on farms, which was absurdly the very reason the antibiotics were replaced with phytogens. And a proposition of future work and directions recommended from the outcomes of this thesis.