Humans could face new threat from sharks

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Humans now have one more thing to fear from sharks other than sharp teeth - antibiotic-resistant bacteria.

In a study published in a recent issue of the Journal of Zoo and Wildlife Medicine, Jason Blackburn and coauthors have identified antibiotic-resistant bacteria in seven species of shark and redfish (Sciaenops ocellatus) collected from six sample sites in the western Atlantic.

The authors obtained 134 viable bacterial samples from cloacal swabs of the fishes collected and tested these for resistance to 13 different antibiotics.  

In all cases, they found resistance to one or more antibiotics, with sharks off Martha's Vineyard in Massachusetts and in offshore Louisiana having bacteria that were resistant to the fewest antibiotics, while those from the Florida Keys and Belize harboured bacteria that were resistant to amikacin, ceftazidime, chloramphenicol, ciprofloxacin, doxycycline, penicillin, piperacillin, sulfamethoxazole and ticarcillin.

Human link
Although random mutations cannot be ruled out as the source of this multi-drug resistance, evidence points to a human origin.  

The sharks from Belize, for instance, are found in an area of high tourist traffic and in the vicinity of a sewage treatment plant.  

This makes it likely that the bacteria are developing resistance from exposure to higher ambient levels of antibiotics from anthropogenic sources, or that resistant bacteria already present in sewage is colonizing the sharks.  

Although it appears that the resistant bacteria in sharks and predatory fish do not appear to have any adverse effects on humans or other fish, the authors assert that the threat cannot be taken lightly.

Reservoirs for drug resistance
According to the authors, "The marine environment may be considered a reservoir for resistance to such drugs, and future surveillance of predatory fishes should continue.

"The marine predatory fishes sampled in this study may serve as valuable sentinels, because they are long lived and slow growing and, therefore, have a potentially long exposure to resistant bacteria in the ocean.

"In addition, these data support the hypothesis of previous work that resistance is present in marine species from multiple food webs and habitats. Because fisheries remain an important component of the human diet, this information may be used to determine zoonotic health risks."

For more information, see the paper: Blackburn, JK, MA Mitchell, M-C Holley Blackburn, A Curtis and BA Thompson (2010) Evidence of antibiotic resistance in free-swimming, top-level marine predatory fishes. Journal of Zoo and Wildlife Medicine 41, pp. 7–16.