Drivers and vectors for AMR selection and transmission on dairy farms.

Background and context of the work: Minimum selective concentrations (MSC) have become a topic of interest in the investigation of environmental AMR selection. Studies indicate that even at concentrations well below the minimum inhibitory concentration (MIC) a selective advantage can be gained by bacteria with existing resistances compared to those without. Existing studies have focused on in vitro experiments and study of environmental residues in soils/sediments but none focus on the impact of MSC in populations caused by specific therapeutic regimens. Current dairy farm practices use a number of antimicrobials on cattle which can end up in the milk (Varied dry cow therapies, systemic treatments). Milks from cattle post treatment cannot be used commercially due to AMR residues until past the minimum withdrawal time but do get used for calves for feed and for colostrum transfer depending on specific farm practices. Different intramammary dry cow preparations can differ in their formulations and can be biased in their coverage for control of Gram negative and Gram positive bacteria. For example Leo Red Dry Cow has significant Gram negative coverage while Orbenin Extra DC has no specific Gram negative coverage. There are different potentials for selection within the bacterial communities with which they interact. The use of penicillins still have the potential to drive selection for resistances to ESBLs and that could circulate between Gram negative bacteria. the field. While aminoglycosides are often used for treatment of MDR Gram negative infection so selections which can increase resistance to aminoglycosides in potentially zoonotic reservoirs pose a potential problem for the wider health community. Prelim airy Data form the EVAL farms project would indicate that antibiotic use can lead to blooms of specific resistances. Aims: (i) We will investigate dairy herds which have different management practices to determine if there is potential risk of selection using the products in developing resistance in the guts of claves that can be maintained as they grow. (ii) This project will assess the impact of varied antimicrobial therapies on cattle that can lead subsequent low level concentrations of antimicrobial in the dairy environment. (iii) In addition we investigate the mechanism of carriage of these resistances and their potential for co-selection of other antimicrobials and transfer to related bacterial genera. Outcomes: We aim to identify on-farm practices, which have selective pressure. Such that recommendations on use of milk during dry cow therapy with specific products could be considered as part of an AMR stewardship program.