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