Removal and restriction of subtherapeutic antibiotics from poultry diets in many parts of the world has amplified interest in improving intestinal health and nutrient utilization. Some probiotic (direct-fed microbials) and plant-derived (phytogenic) feed additives are gaining market presence. Defined probiotic cultures have the potential to succeed, in large part because of in vitro screening and selection. However, regulatory approval delays, particularly in Europe, have stymied the commercial application of some microorganisms in poultry diets. Phytogenic feed additives have demonstrated ranges of antimicrobial activities in vitro and are building a track record of improvements in bird performance. Hesitation by nutritionists to incorporate these feed additives are due in part to 1) unfamiliarity, 2) the overselling of plausible effects by industry, 3) product inconsistency, 4) a lack of documented physiological and microbiological effects in vivo, and, in the case of probiotics, 5) a lack of documentation of persistence.

In this study, shotgun metagenomics was employed to monitor the effect of oxytetracycline, administered at a therapeutic dose, on the dynamics of the microbiota and resistome in the feces of weaned pigs. Sixteen weaning pigs were assigned to one of two treatments including standard starter diet for 21 days or antibiotic-supplemented diet (10 g oxytetracycline/100 kg body weight/day) for 7 days, followed by 14 days of standard starter diet. Feces were collected from the pigs on days 0, 8, and 21 for microbiota and resistome profiling. Pigs receiving oxytetracycline exhibited a significantly greater richness (ANOVA, P = 0.034) and diversity (ANOVA, P = 0.048) of antibiotic resistance genes (ARGs) than the control pigs. Antibiotic administration significantly enriched the abundances of 41 ARGs, mainly from the tetracycline, betalactam and multidrug resistance classes. Compositional shifts in the bacterial communities were observed following 7 days of antibiotic adminstration, with the medicated pigs showing an increase in Escherichia (Proteobacteria) and Prevotella (Bacteroidetes) populations compared with the nonmedicated pigs. This might be explained by the potential of these taxa to carry ARGs that may be transferred to other susceptible bacteria in the densely populated gut environment. These findings will help in the optimization of therapeutic schemes involving antibiotic usage in swine production.

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Increasing evidence shows that the chicken gastrointestinal microbiota has a major effect on the modulation of metabolic functions and is correlated with economic parameters, such as feed efficiency and health. Some of these effects derive from the capacity of the chicken to digest carbohydrates and produce energy-rich metabolites such as short-chain fatty acids (SCFA) and from host-microbe interactions. In this study, we utilized information from metagenomic assembled genomes (MAGs) from chicken gastrointestinal tract (GIT) samples, with detailed annotation of carbohydrate-active enzymes (CAZymes) and genes involved in SCFA production, to better understand metabolic potential at different ages. Metagenomic sequencing of 751 chicken GIT samples was performed to reconstruct 155 MAGs, representing species which belong to six phyla, primarily Firmicutes followed by Proteobacteria. MAG diversity significantly (p < 0.001) increased with age, with early domination of Lachnospiraceae, followed by other families including Oscillospiraceae. Age-dependent shifts were observed in the abundance of genes involved in CAZyme and SCFA production, exemplified by a significant increase in glycosyltransferases (GTs) and propionic acid production pathways (p < 0.05), and a lower abundance of glycoside hydrolases (GHs) (p < 0.01). Co-occurrence analysis revealed a large cluster highly interconnected by enzymes from GT2_2 and GH3 families, underscoring their importance in the community. Furthermore, several species were identified as interaction hubs, elucidating associations of key microbes and enzymes that more likely drive temporal changes in the chicken gut microbiota, and providing further insights into the structure of the complex microbial community. This study extends prior efforts on the characterization of the chicken GIT microbiome at the taxonomic and functional levels and lays an important foundation toward better understanding the broiler chicken gut microbiome helping in the identification of modulation opportunities to increase animal health and performance.

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