The combination of efficacious treatment against bacterial infections and mitigation of antibiotic resistance amplification in gut microbiota is a major challenge for antimicrobial therapy in food-producing animals. In rats, we evaluated the impact of cefquinome, a fourth-generation cephalosporin, on both Klebsiella pneumoniae lung infection and intestinal flora harboring CTX-Mproducing Enterobacteriaceae. Germfree rats received a fecal flora specimen from specific-pathogen-free pigs, to which a CTX-M-producing Escherichia coli strain had been added. K. pneumoniae cells were inoculated in the lungs of these gnotobiotic rats by using either a low (10 5 CFU) or a high (10 9 CFU) inoculum. Without treatment, all animals infected with the low or high K. pneumoniae inoculum developed pneumonia and died before 120 h postchallenge. In the treated groups, the low-inoculum rats received a 4-day treatment of 5 mg/kg of body weight cefquinome beginning at 24 h postchallenge (prepatent phase of the disease), and the high-inoculum rats received a 4-day treatment of 50 mg/kg cefquinome beginning when the animals expressed clinical signs of infection (patent phase of the disease). The dose of 50 mg/kg targeting the high K. pneumoniae inoculum cured all the treated rats and resulted in a massive amplification of CTX-M-producing Enterobacteriaceae. A dose of 5 mg/kg targeting the low K. pneumoniae inoculum cured all the rats and averted an outbreak of clinical disease, all without any amplification of CTX-M-producing Enterobacteriaceae. These findings might have implications for the development of new antimicrobial treatment strategies that ensure a cure for bacterial infections while avoiding the amplification of resistance genes of human concern in the gut microbiota of food-producing animals.A ntimicrobial resistance is a major threat to human health, and the overuse of antibiotics in both human patients and animals is considered to be the main factor leading to the selection of resistant bacteria. It is also increasingly recognized that the gut microbiota constitutes one of the main reservoirs of resistance genes among commensal bacterial ecosystems (1-4), and that the antibiotic doses currently used in human and animal patients have not been optimized to prevent the collateral selection of antimicrobial resistance in the gut microbiota or its colonization by exogenous resistant strains (3, 5).An examination of the interactions between antibiotics, pathogens, and the commensal flora, as well as an increased understanding of the key factors governing antimicrobial activity and resistance selection, might lead to the development of strategies combining maximal efficacy with minimal impact on the commensal bacterial ecosystems (2, 6, 7). For example, recent studies demonstrated that the degree of amplification of antimicrobial resistance in the gut microbiota was directly correlated with the magnitude of the antibiotic dose, regardless of the route of administration (8, 9).Interestingly, some other studies have shown that the i...