Impact of ceftiofur administration and <i>Escherichia coli</i> inoculation on the calf fecal microbiome

ABSTRACT The cattle gastrointestinal tract harbors a diverse community of microorganisms, including pathogenic and commensal strains of Escherichia coli . Antimicrobial use in cattle can disrupt the gut microbiome, leading to shifts in bacterial diversity and abundance. Here, we combined shotgun metagenomics and single-cell sequencing to assess how ceftiofur antibiotic treatment impacted microbial diversity and structure. At the start of the experiment, ceftiofur was administered intramuscularly in parallel with the inoculation of a cocktail of extended-beta-lactamase-producing E. coli strains to simulate environmental exposure and acquisition of resistant strains while animals are under antibiotic treatment. Fecal samples were collected from both the antibiotic-treated (ceftiofur and inoculation) and control (inoculation only) calves over the course of 35 days. Read mapping to genome and gene databases showed substantial differences in microbial richness and beta diversity between treatment groups. Treatment group-enriched taxa included Bacteroidaceae and Fibrobacter , which were more abundant in samples that did not receive ceftiofur, and Akkermansia in ceftiofur-treated calves. In ceftiofur-exposed animals, we observed a gradual loss of virulence factors alongside increased abundances of beta-lactam resistance genes, including cfxA5 and cfxA6 , likely encoded by CAG-485 ( Muribaculaceae ). We further profiled individual cells using single-cell sequencing, which revealed a high number of Clostridium carrying macrolide resistance genes lnu (P) and mph (N) in both ceftiofur-treated and control samples. Overall, our complementary approaches reveal distinct remodeling of the calf microbiome following antibiotic and E. coli administration, tied to key functional genes that can be assigned to specific genera or recurrently detected across diverse taxa. IMPORTANCE Cattle serve as natural reservoirs of zoonotic strains of Escherichia coli , which can cause severe gastrointestinal infections in humans. Antibiotic usage on cattle farms can drive the emergence of antimicrobial-resistant bacterial strains and alter the underlying cattle gastrointestinal microbiome. Consequently, there is a need to understand how antibiotic administration impacts population dynamics of cattle rumen and intestinal microbes. In this study, we combined both shotgun metagenomics and single-cell genomics on feces from ruminating calves to determine microbiome changes following administration of both ceftiofur and E. coli cocktails. We observed considerable variation in the prevalence and abundance of virulence factors, antimicrobial resistance-related genes, and taxa with key roles in animal nutrition and health between the microbiomes of antibiotic-treated and antibiotic-free calves, with potential implications for their subsequent development and overall well-being.