Antimicrobial resistance monitoring in animal production and use of plant additives as alternatives to antimicrobials and zinc oxide

Abstract

This PhD thesis has been written as a compendium of publications and is composed of three papers published in international journals (Q1). All the publications form a scientific unit within the framework of research on antimicrobial resistance in animal production and the development of alternative products to antimicrobials and zinc oxide (ZnO). Two of the publications studied the presence and characterization of antimicrobial resistance in commensal bacteria in laying hens farms in Spain. This studies were carried out due to the scarcity of current data on laying hens, in contrast to other sectors more studied and monitored at the Spanish and European level, such as broiler production. In these studies, resistance to antibiotics was studied according to the categories established by the European Medicines Agency (EMA), paying special attention to antibiotics of special importance for human medicine, with a "One Health” approach. The first study dealt with resistance to category C and D antibiotics in commensal Campylobacter spp., Escherichia coli and Enterococcus spp. in Spanish laying hen farms, which was determined by the Minimum Inhibitory Concentration (MIC). High values of resistance to various antibiotics were found, including some important for the treatment of zoonoses. Particularly high resistance values were found for tetracycline, tetracyclines family, from category D, and for erythromycin, macrolides family, from category C. The second study focused on the resistance of Campylobacter jejuni, Escherichia coli and Enterococcus faecalis to antimicrobial categories A and B by determining the MIC. High levels of resistance were found to important drugs for human medicine, and the results were different from the data available for poultry in Spain and in the EU. There were values particularly concerning for ciprofloxacin and nalidixic acid, quinolone family, from category B, and for tigecycline, glycylcyclines family, from category A. From the results of these two studies, it can be concluded that laying hen farms in Spain have high levels of resistance to all categories of antimicrobials (A, B, C and D), including those used in human medicine. This poses a risk to public health from a “One Health” perspective. Furthermore, the differences found between the information previously available and these results, evidence that more detailed sampling and monitoring methods are necessary for the laying hen sector. These investigations represent a step towards the correct characterization of antimicrobial resistance in animal production, which is essential to make possible the design and implementation of appropriate measures, with the aim of preventing the increase in resistance and preserving the efficacy of antimicrobials. The development of methods and products alternative to antibiotics is an essential step to prevent and reduce the presence of antimicrobial resistance in animal production. Intensive pig production is one of the main consumer sectors for antibiotics and products with similar characteristics such as ZnO. Weaning is one of the most critical points in the pig production cycle, and ZnO is often used to prevent digestive problems, which causes serious environmental pollution and contributes to the increase of resistance to antimicrobials. Due to these negative consequences, ZnO will be banned in the EU from June 2022. In the third publication of this thesis, the effects of supplementing purple garlic (A. sativum) powder and micro-encapsulated oregano (O. vulgare L.) essential oil were studied on biomarkers of oxidative status, stress and inflammation, as well as on the average daily gain and the feed conversion index, compared to ZnO and the control diet, in weaned piglets. The doses of micro-encapsulated oregano essential oil and 0.4% garlic powder did not cause inflammation, stress or negative changes in oxidative biomarkers in piglets, and gave a better productive performance than the control and ZnO diets. These results show that the products derived from garlic and oregano as feed additives, thanks to their beneficial properties, can allow to reduce the need of using antibiotics and ZnO in animal production, since they contribute to keep the animals in good sanitary and productive conditions. This helps to reduce the use of antimicrobials and ZnO, contributing to the sustainability of animal production and the protection of animal, human and environmental health within a “One Health” context.