Temporal Changes in Antibiotic Resistance Among Bacteria Isolated from Common Bottlenose Dolphins (Tursiops truncatus) in the Indian River Lagoon, Florida, 2003-2015

Increases in resistance to commonly used antibiotics have been reported globally in isolates from humans, wildlife, and the environment. To date, few studies have examined long-term trends in antibiotic resistance in organisms isolated from marine mammal populations. The objective of this study was to examine temporal trends in resistance to antibiotics among pathogens isolated from common bottlenose dolphins (Tursiops truncates) between 2003 and 2015. Dolphins were captured and released in the Indian River Lagoon, Florida, an ecosystem with a large coastal human population and significant environmental impacts. Swab samples for microbiology were taken from the blowhole, gastric fluid, and feces and cultured on standard media under aerobic conditions. Isolates were identified using gram stain morphology and growth on selective media. Antibiotic resistance was measured using disc diffusion on Mueller Hinton agar and the Multiple Antibiotic Resistance (MAR) index calculated for each pathogen. A total of 733 isolates was obtained from 171 individual dolphins. The most commonly cultured pathogens included Aeromonas hydrophila, Escherichia coli, Edwardsiella tarda, and Vibrio alginolyticus. The overall prevalence of resistance to at least one antibiotic for the 733 isolates was 88.2%. The MAR index increased significantly between 2003 and 2007 and 2010 and 2015 for Pseudomonas aeruginosa and V alginolyticus. For all bacterial isolates, resistance to cefotaxime, ceftazidime, and gentamicin increased significantly between sampling periods. This is one of few studies to use, the MAR index for bacterial isolates from a marine mammal. The significant increases in resistance for some bacterial species likely reflect shared environmental exposures to antibiotics and transfer of resistance to dolphins from terrestrial sources or from animal or human populations.