Sharks Incubators for Drug-Resistant Bacteria.

Even the monstrous Jaws might
look tame compared to the
swarm of antibiotic-resistant
bacteria found in sharks and
redfish, according to a new
study.
The bacteria don't harm the
toothy fish, but scientists worry
that the sharks and redfish may
become incubators for nasty
multidrug-resistant strains,
which could possibly infect
humans someday. The exact
sources for the antibiotic-
resistant microbes remain
unknown.
"It's not unexpected to find some
drug resistance, but we were
surprised to find the amount of
resistance to that large suite of
drugs," said Jason Blackburn, a
spatial ecologist at the University
of Florida in Gainesville.
Sharks swimming off the Florida
Keys and Belize were hosts to
bacteria resistant to the
antibiotics amikacin, ceftazidime,
chloramphenicol, ciprofloxacin,
doxycycline, penicillin,
piperacillin, sulfamethoxazole
and ticarcillin.
By contrast, fish found off
Martha's Vineyard in
Massachusetts and in offshore
Louisiana appeared to have the
fewest number of antibiotic-
resistant strains.
The study lacked large enough
sample sizes to statistically
compare different fish
populations of wild sharks such
as nurse sharks and bull sharks,
as well as redfish popular among
sport fishermen, but it still found
bacteria resistant to 13
antibiotics in all among the
sampled fish. That likely
represents an underestimate of
true drug resistance, the
researchers said.
Just how the sharks and redfish
acquired their antibiotic-resistant
bacteria remains unclear.
Humans may have reason for
concern if some of the bacteria
built up over time from the
animals feasting on fresh
seafood.
"These species that we've
targeted, these are apex or top-
end predators in a similar
position in the marine food web
as we are," Blackburn told
LiveScience. [ Gallery of
Endangered Sharks]
Blackburn conducted the work
alongside other researchers as
his master's thesis while
studying at Louisiana State
University in Baton Rouge. The
study is now detailed in the June
issue of the Journal of Zoo and
Wildlife Medicine.
Finding the source
Researchers want to begin
tracking down the source of the
antibiotic resistance in various
locations, even if the top
predators don't suffer any ill
effects so far from playing host
to the bacteria.
"Either their food source had
directly ingested drug-resistant
bacteria, or bacteria in the
[shark] gut were exposed to
antibiotic components,"
Blackburn explained.
Sharks sampled off the Florida
Keys ended up showing the
greatest resistance to a wide
selection of drugs. But different
locations may have different
routes for exposure to
antibiotics or antibiotic-resistant
bacteria.
Some of the antibiotic resistance
may come from exposure to
sewage. For instance, Blackburn
and his colleagues noted an
overflowing septic tank problem
at the Dry Tortugas islands of the
Florida Keys.
Sharks could also get their
exposure to antibiotics directly
from the humans popping the
pills. Tourists in Belize often get
right into the water to swim with
or touch the relatively docile
nurse sharks that were part of
the study.
Age could make yet another
difference, because the elderly
redfish sampled had significantly
greater drug resistance
compared with younger spinner
sharks that fed on the same fish.
Both fish were sampled in
Louisiana waters near offshore
oil rigs.
Swimming with sharks
A next experiment might look
more closely at age and
antibiotic-resistance within the
same individual fish over time,
Blackburn said.
Slow-moving nurse sharks may
prove ideal for the next
experiment. The bottom-feeders
browse for crustaceans and
small fish, and also allow humans
to interact with them for capture
or tagging purposes. One nurse
shark study at the Dry Tortugas
has already reached its second
decade.
Blackburn and his colleagues also
went out several times with a
park ranger or tourist boat at the
Hol Chan Marine Reserve in Belize.
They would swim out and grab
an approaching nurse shark,
turn it upside down and then
allow a fellow scientist to swab
the shark's genital region.
By contrast, capturing a faster
shark requires longlines, rod and
reel or gillnets.
"The bull sharks and spinner
sharks have very low recapture
rates when you tag and release
one, so you'd have to bring
[them] into captivity," Blackburn
pointed out. "Nurse sharks
provide a nice model because
you can go get them again."
The researchers hope to include
more sharks in any future
studies so that they have larger
sample sizes, and they also want
to identify the antibiotic-resistant
bacteria by species.
"Knowing that this pool of drug
resistance is out there, certainly
we've presented enough
evidence to argue for more
routine sampling," Blackburn
said. "The survey showed
resistance everywhere we
looked, even in remote areas."