I am pleased to announce that my master thesis is available in the University of Lisbon Repository: https://www.repository.utl.pt/handle/10400.5/6754 Title: Antimicrobial resistance of the upper respiratory tract commensal microbiota, in bottlenose dolphins (*Tursiops truncatus*), under human care ABSTRACT: Respiratory affections, especially bacterial pneumonia, are a major cause of death in dolphins, both free-range individuals and those under human care. Animals affected by stress, immunocompromised or with underlying affections are more likely to be infected by opportunistic agents, usually present in the host in the commensal microbiota. Several colonizing microorganisms were recovered from the upper respiratory tract of nine healthy bottlenose dolphins, living under human care at the entertainment and educational oceanographic park Zoomarine - Mundo Aquático, S.A., Portugal. The isolated bacteria belonged to the species Escherichia coli, Enterococcus faecalis, Morganella morganii, Klebsiella oxytoca, Staphylococcus hominis, Staphylococcus haemolyticus, Staphylococcus aureus, Staphylococcus simulans e Staphylococcus delphini group A. Disk diffusion method and genotypic characterization through PCR were the techniques performed in order to evaluate the antimicrobial resistance of these strains, regarding different families of antibiotics. The presence of resistance genes to β-lactams was investigated in the E. coli isolates through PCR, in order to identify β-lactamases’ producing strains (TEM, SHV, OXA- 1, CTX-M, AmpC), as well as the resistance genes to aminoglycosides (aaC(3’)-IV and aaC(6’)-Ib). The mecA and mecC genes were investigated in the Staphylococcus spp. isolates. The results demonstrated that the majority of the isolates were multidrug-resistant, 76% of the isolates were considered clinically resistant to more than three antibiotic families (R>3), followed by 17% of resistant strains (1≤R≤3) and a small representation of 7% of fully susceptible bacteria (R=0). Resistance genes were detected in all the E. coli isolates, most frequently the blaTEM, followed by blaOXA-1, blaCTX-M-15 and aaC(6’)-Ib and less frequently the blaDHA-1.. The mecA gene was identified in one S. aureus and in the S. hominis isolates. The isolation of multidrug-resistant bacteria from the commensal microbiota is relevant in that these microorganisms are capable of inactivating a wide spectrum of antibiotics, limiting the therapeutic options. Associated with the colonization of the respiratory tract of dolphins by these organisms, the question arises of the potential risk of colonization and transmission between these animals and humans. Keywords: bottlenose dolphin, pneumonia, commensal microbiota, antimicrobial resistance, resistance genes, Methicillin-resistant S. aureus Machado, C., DVM, MSc