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<OL> <LI>Experimentally determine the frequency with which blacmy-2 and adjacent plasmid genes are transferred to previously ceftiofur susceptible E. coli strains, and the degree to which ceftiofur therapy affects the frequency of transfer. <LI> Perform observational field studies in calves and adult cows to determine the relative effects of ceftiofur therapy (as compared with the effects of alternative therapeutic drugs) on the frequency, diversity, and persistence of cmy-2 resistance in the commensal E. coli population on dairy farms.
Non-Technical Summary: The use of antimicrobials in food animal production creates a pressure to select for more resistant bacteria that may affect the food supply and public health. Examine the mechanism and duration of selection pressure for antimicrobial resistant bacteria in food animal production <P> Approach: Aim 1. We will measure the rate of dissemination of blacmy-2 to E. coli of new genetic backgrounds as it is affected by ceftiofur selection pressure. Calves entering this experiment will first be exhaustively screened to ensure that they lack a pre-existing blacmy-2 E. coli flora. Three groups of calves will receive three different treatments: 1) ceftiofur therapy during exposure to blacmy-2 positive donors, 2) no antibiotic treatment during exposure to blacmy-2 positive donors, and 3) ceftiofur therapy during exposure to blacmy-2 negative (sham) donors. Aim 2. We will enroll calves and mature post-parturient cows identified by farm personnel as ill and designated for antibiotic treatment. The animals will come equally from dairy farms that use ceftiofur as first choice compared to farms that use other antibiotics as first choice. Commensal E. coli will be monitored pretreatment and daily for duration of treatment and 10 days following the last treatment. Untreated controls will also be monitored.