SummaryThe objective was to summarize and describe the possible mode of action of dietary organic acids and their effects on growth performance of broiler chickens. Previous experiments have suggested that dietary organic acids decrease pH in diets and subsequently reduce pH in the proximal and distal intestine, increase nutrient utilization, and inhibit pathogenic bacterial growth in the gastrointestinal tract (GIT). The degree of pH reduction is usually greater in the upper part of the GIT (crop, proventriculus, and gizzard) than in the lower part of the GIT (duodenum, jejunum, ileum, and cecum). Bactericidal effects of dietary organic acids have been observed for pathogenic bacteria and even for beneficial bacteria to some extent. However, few significant results regarding bacterial modulation in the GIT have been reported. Dietary organic acids can improve dry matter and protein utilization in some experiments, but the extent of improvement in nutrient utilization is smaller than has been anticipated. Growth performance is likely improved, but results have been inconsistent due to variations in sources and inclusion levels of dietary organic acids. Differences in other dietary components and experimental environments among previous experiments likely contribute to the variable results. This review suggests that the effects of dietary organic acids on broiler chickens are not fully understood. Further experiments are required to reliably demonstrate the mode of action of dietary organic acids and their growth-promoting effects on broiler chickens. Keywords: acidifiers, gastrointestinal pH, growth performance, microbial population, nutrient utilization. ResumenEl objetivo fue resumir y describir el posible modo de acción de los ácidos orgánicos en la dieta y sus efectos sobre el crecimiento de los pollos de engorde. Experimentos previos sugieren que los ácidos orgánicos dietarios disminuyen el pH de la dieta y posteriormente reducen el pH en el intestino proximal y distal, aumentan la utilización de los nutrientes, e inhiben el crecimiento de bacterias patógenas en el tracto gastrointestinal (GIT). El grado de reducción del pH es generalmente mayor en la parte superior (buche, proventrículo y molleja) que en la parte inferior del GIT (duodeno, yeyuno, íleon y ciego). Se han observado efectos bactericidas de los ácidos orgánicos sobre bacterias patógenas e incluso sobre bacterias beneficiosas en cierta medida. Sin embargo, se han reportado algunos resultados significativos con respecto a la modulación bacteriana en el GIT. Los ácidos orgánicos en la dieta pueden mejorar la utilización de la materia seca y la proteína en algunos experimentos, pero el grado de mejora en la utilización de los nutrientes es más bajo que lo esperado. El crecimiento probablemente mejora, pero los resultados han sido inconsistentes debido a las variaciones en las fuentes y a los niveles de inclusión de los ácidos orgánicos en la dieta. Las diferencias en otros componentes de la dieta y entornos experimentales entre los ensayos anterio...

The mechanisms by which antibiotic growth promoters (AGP) enhance growth rates, feed efficiencies, and disease resistance in poultry need to be understood for designing safer and alternative strategies to replace AGP. Avilamycin has been widely used as an AGP in poultry, but its impact on the structure and function of the gut microbiome of broiler chickens has not been fully elucidated. In this study, we investigated the bacterial communities of the ileum and cecum in broiler chickens fed with an avilamycin-supplemented diet, by high-throughput sequencing of bacterial 16S rRNA genes. Alpha diversity metrics indicated that the ileal bacterial diversity was higher in avilamycin-fed chickens than in the control group, whereas the opposite was true for the cecum. Multivariate analyses revealed that the ileal microbiota of the avilamycin-fed group were clearly distinguished from those of the control group, whereas the cecal bacterial communities were apparently not influenced by feeding diets containing avilamycin. In the ilea, 2 operational taxonomic units (OTU) that matched Lactobacillus reuteri and Clostridium were enriched (P = 0.016 and P = 0.007, respectively) in the avilamycin-fed group, and an OTU belonging to Lactobacillus crispatus was decreased (P = 0.016). In the cecal microbiota showing much higher diversity with 1,286 non-singleton OTU, 12 OTU were decreased, and 3 were increased in response to avilamycin treatment (P = 0.005-0.047). Functional profiling of bacterial communities based on PICRUSt analysis revealed that 10 functional categories were enriched by avilamycin treatments, and 4 functional categories were decreased. In conclusion, our results demonstrated that the influence of avilamycin supplementation on the diversity, taxonomic composition, and functional profiles of the microbiota was evidently different in the ileum and cecum. These results further our understanding of the impact of AGP on the composition and activity of commensal bacteria in the chicken gastrointestinal tract to develop novel feeding strategies for improving animal health and performance.