Chicken Age Outweighs Feed Additives in Resistance Gene Study

A Penn State experiment suggests that the natural development of a chicken's gut microbiome can shape antimicrobial resistance gene profiles more strongly than several commonly discussed feed additives.

Researchers studied 320 broiler chickens assigned to four diets, a control diet, an antibiotic diet, an essential oil diet and a probiotic diet. Fecal samples were collected when the birds were 1, 10 and 21 days old and analyzed using shotgun metagenomic sequencing.

Across the experiment, the team detected 823 unique resistance genes. The overall diversity of those genes was not significantly changed in a sustained way by the antibiotic, probiotic or essential oil treatments. Age was the much stronger factor separating resistance gene profiles.

The researchers explain that the gut bacterial community changes naturally as chickens grow. Because different bacteria carry different resistance genes, that microbial succession can also reshape the resistome, the collection of resistance genes detected in the microbial community.

The result should not be read as evidence that feed additives never influence antimicrobial resistance. The sequencing approach identifies resistance gene sequences and their relative abundance, but does not establish whether bacteria carrying those genes are resistant in practice or whether the genes are actively expressed.

For poultry producers and researchers looking for alternatives to routine antibiotic use, that distinction is important. Probiotics and essential oils remain areas of investigation, but evaluating them requires separating treatment effects from the substantial biological changes that occur as birds mature.

The study was supported by the USDA National Institute of Food and Agriculture, Hatch appropriations and Penn State research funding.