The swine gastrointestinal microbiota is comprised of a diverse and complex microbial population that coexists in a coordinated, complex mucosal ecosystem that contributes to host gastrointestinal and immunological development. While antimicrobial are cost-effective tools for prevention and treatment of infectious diseases, the impact of their use on potentially beneficent mucosal microbial communities has not been widely explored. Optimization of antimicrobial use in swine management systems requires full understanding of antimicrobial-induced changes on developmental dynamics of gut microbiota and prevalence of antimicrobial resistance genes (ARGs). While the antibiotic resistance profiles of pathogens have been characterized, the antimicrobial resistance bacteria and ARGs from the whole gut microbiota have received far less attention. The objective of this study was to characterize the impact of parenteral antibiotics administration on composition and diversity of the resident fecal microbiota in pigs. In commercial swine farm, five antimicrobial treatment groups, each consisting of four, eight-week old piglets, were administered one of the antimicrobial; Tulathromycin (TUL), Ceftiofur Crystalline free acid (CCFA), Ceftiofur hydrochloride (CHC), Oxytetracycline (OTC), and Procaine Penicillin G (PPG) at label dose and route. Individual fecal swabs were collected immediately before antimicrobial administration (control = day 0), and again on days 1, 3, 7, and 14 after dosing. Additionally, a randomized complete block design was used to study the impacts of early-life antimicrobial intervention on fecal microbiota structure, and their …