The influence of trace metal supplementation on the presence of ceftriaxone resistance in Enterobacteriaceae in the gastrointestinal tract of dairy cattle.

The addition of trace minerals into the diet of lactating cows frequently exceeds national recommendations for industry practices. However, the presence of certain heavy metals, such as zinc and copper, has been shown to exert selection pressure on the gut microbiota, favoring metal resistance and potential co-selection for antimicrobial resistance. To determine whether oversupplementation of dietary zinc alters the gut microbiota of dairy cattle, a cross-over design was used to feed either recommended or surplus levels of dietary zinc (0.89×; high mineral diet) compared to the recommended levels (control diet). Rumen, duodenum, and fecal samples were collected to analyze the 16S rRNA microbial community for diversity and relative abundance, with a greater focus on the Enterobacteriaceae family, while mixed enriched gut content samples were cultured to determine the presence of zinc, copper, and ceftriaxone resistances in gram-negative bacteria. Alpha-diversity analysis showed a decrease in richness and evenness (Simpson index) when cows were in the HIGH treatment (P = 0.0464) and a tendency to decrease (P = 0.0592) diversity according to the Shannon index. Despite alpha-diversity differences, Enterobacteriaceae abundance showed no difference between treatments. For culturing, a tendency (P = 0.0956) for decreased fecal resistance to zinc on MacConkey mixed enriched isolates was observed for the HIGH group. This study showed that there were differences between niches but no significant increase in resistance in response to zinc, copper, and ceftriaxone in the enriched Enterobacteriaceae populations from the rumen, duodenum, and fecal niches and that zinc oversupplementation had minimal impact on gut microbiota communities.

Importance: The addition of trace minerals into the diet of lactating cows, at levels exceeding national recommendations, is a common industry practice. However, there are new concerns as the presence of certain heavy metals, such as zinc, has been shown to exert selection pressure on the gut microbiota, favoring metal resistance and potential co-selection for antimicrobial resistance. We evaluated how the addition of zinc in the diet of lactating cows affects the bacterial community's relative abundance and diversity, with a focus on the Enterobacteriaceae family throughout the gastrointestinal tract, due to their importance for human health. Using samples from the rumen, duodenum, and feces, we cultivated gram-negative bacteria from enriched samples in the presence of zinc, copper, and ceftriaxone resistances to confirm phenotype resistances. This study contributes to our understanding of how dairy diets oversupplemented with minerals may alter the microbial community of the animal and could contribute to the dissemination of antibiotic resistance.