On a crisp morning at the Penn State Poultry Education and Research Center, 320 fluffy broiler chicks peeped in unison, unaware they were part of a study that could reshape how we raise poultry. Scientists wanted to know: do feed additives like probiotics or essential oils reduce antibiotic resistance genes in chickens? The answer surprised them. It wasn’t what the birds ate—it was how old they were.
Antibiotic resistance is a growing global threat. When bacteria evolve to survive drugs meant to kill them, infections become harder to treat. For decades, farmers added antibiotics to chicken feed to keep flocks healthy and help them grow faster. Now, many are switching to alternatives like probiotics—live helpful bacteria—and plant-based essential oils. But do these swaps actually reduce the number of resistance genes in chickens’ guts?
To find out, researchers fed the 320 chicks one of four diets: plain feed, feed with antibiotics, feed with probiotics, or feed with essential oils. They collected fecal samples when the birds were 1, 10, and 21 days old. Using a powerful DNA sequencing method called shotgun metagenomics, they analyzed the gut bacteria and identified 823 unique antimicrobial resistance genes. The results, published in Poultry Science, showed no significant change in the overall resistome—the full collection of resistance genes—across the different diets.
Instead, age was the biggest factor. The gut microbiome of a 1-day-old chick looked nothing like that of a 21-day-old. As the chickens grew, their gut bacteria naturally shifted in a process called microbial succession. New microbes moved in, others faded away, and with them, the mix of resistance genes changed. Even the antibiotic-fed birds showed no sustained increase in resistance genes, which surprised the team.
"The resistome of a 1-day-old chick looked quite different from that of a 10-day-old chick, and the resistome changed again by day 21," said Ana Fonseca, the study’s lead author and now a poultry management educator with Penn State Extension. "As chickens grow, the community of bacteria living in their gut changes naturally. Because different bacteria carry different resistance genes, natural changes in the microbial community also reshape the resistome."
This doesn’t mean feed additives have no effect, the researchers stress. But under these conditions, age mattered far more than diet. The findings support the use of probiotics and essential oils as safe antibiotic alternatives, since they didn’t worsen resistance. Understanding what shapes the resistome helps scientists design smarter, more sustainable farming practices.
With computing power from Penn State’s Roar Collab cluster, the team handled vast amounts of genetic data, paving the way for future studies. As the world seeks solutions to antimicrobial resistance, this research reminds us that sometimes, nature’s timeline plays a bigger role than we think.