Multi-Drug Resistant Staph In 1 Of 4 Supermarket Meat 
Samples
By Maryn McKenna
wired.com
April 18, 2011
http://arstechnica.com/science/news/2011/04/multi-drug-resistant-staph-in-1-of-4-supermarket-meat-samples.ars

Much of the contentious debate over the abuse of
antibiotics in farming boils down to a couple of simple
questions: whether resistant organisms that arise on
farms because of antibiotic use leave the farm, and
whether, once they do, they reach human beings.

A piece of research (PDF) released Friday morning in the
journal Clinical Infectious Diseases helps answer both
those questions.

[moderator: the referenced report is found here
http://cid.oxfordjournals.org/content/early/2011/04/14/cid.cir181.full.pdf]

A team of researchers from Arizona bought meat and
poultry in five cities across the United States, tested
them for bacteria, and found this: 47 percent of the
samples contained the very common pathogen
Staphylococcus aureus, and 96 percent of those isolates
were resistant to at least one antibiotic. Of more
concern: 52 percent of those staph isolates were
resistant to at least three antibiotics that are
commonly used in both veterinary and human medicine.

That is, roughly one in four packages of meat and
poultry from across the United States contained
multidrug resistant staph.

Here are the details: A team from the Translational
Genomics Research Institute in Flagstaff, Arizona, led
by Lance B. Price, Ph.D., bought 136 packages of ground
beef, chicken breasts and thighs, pork chops and ground
pork, and ground turkey, under 80 brand names, in 26
supermarkets in Flagstaff, Chicago, Fort Lauderdale, Los
Angeles and Washington, DC. They analyzed the meat for
the presence of staph, because staph has been found in
the past in several food-animal species. They did a
second round of testing to define which strain of staph
was on the meat, and then they did a third round,
testing the isolates against five important classes of
antibiotics, to see whether the staph they had found was
resistant.

Which it was. Very. The antibiotics to which the staph
was resistant included: penicillin and ampicillin;
erythromycin; tetracycline; oxacillin, the more modern
form of the drug methicillin; the drug combination
quinupristin/dalfopristin, known as Synercid; the
fluoroquinolones levofloxacin (Levaquin) and
ciprofloxacin (Cipro); and the last-resort drugs for
very serious staph infections vancomycin and daptomycin.
One staph isolate was resistant to nine different
antibiotics.

Among the types of meat tested, turkey carried the most
resistance, with 77 percent of the meat samples showing
at least some; that was followed by pork (42 percent),
chicken (41 percent) and beef (37 percent).
Interestingly, it wasn't all the same staph. Though
there was a great diversity of staph types, each animal
species seemed to carry mostly one sequence type or
strain of staph: ST1 in pigs, ST5 in chickens and ST398
in turkey. (More on that below.)

I spoke to Lance Price about his team's work. "This is
the first study to show that antibiotic-resistant staph
is highly prevalent in the American food supply," he
told me.

He added: "There's an important second point: We found
that each of the meat and poultry types had their own
distinctive staph on them. That provides strong evidence
that food animals were the primary source of the
resistant staph. The source wasn't human contamination
of the meat at slaughter, or when it was packaged for
retail sale."

How much resistant staph is present in food animals and
on the meat they become has been an urgent question for
about seven years, since a team of Dutch researchers
identified an unusual staph strain-MRSA ST398-in a
family who operated a pig farm. That strain has since
spread to pigs across the European Union and into
Canada, and has been found to cause human illness in all
those areas. It's also been found in pigs in the United
States, though it has not yet been proven to cause human
illness here. (I've been covering ST398 for several
years, and a long archive of posts is available.)

"I think this extends [our work] and opens up some new
lines of investigation," Tara Smith, Ph.D., an assistant
professor of epidemiology at the University of Iowa and
head of the team that identified MRSA ST398 in US pigs,
told me. "The research showed the `pig' strain, ST398,
in poultry products, and an especially high percentage
of the turkey products. Why is that? Where's it coming
from, and if it's on the farms, how is it moving between
species? Or has it been in these animals for a while?"

And here is where this new finding gets especially
complex. The United States has a surveillance system
that tests for the spread of antibiotic-resistant
bacteria, including in meat animals and food. That
surveillance system doesn't look for MRSA, and in the
past few years, because of those pig findings, there has
been a lot of pressure to add MRSA to the list.

But even if we did perform nationwide testing for MRSA
in meat, that would not have found the multi-drug
resistant strains revealed in Price's work today -
because most of them were not MRSA.

What defines MRSA, technically, is resistance to a class
of antibiotics known as the beta-lactams, of which the
earliest version is methicillin (the "M" in MRSA) and
the most common current one is oxacillin. These meat
samples, hazardous as they are, were mostly not
resistant to oxacillin-and thus a test for MRSA, if we
had created one, would have let them slip through.

"We've carried out similar testing (unpublished as
yet)," Smith told me. "And our results are similar, so
it's not surprising. I do think we need to cast a wider
net and not focus only on MRSA."

People who treat humans tend to dismiss non-MRSA (the
technical term is MSSA, for methicillin-sensitive staph)
as being no big deal. Price thinks that would be a
mistake. He pointed out that several of the strains they
found, especially ST1 and ST5, are already known to
cause disease in humans.

"You look at these strains and you see they are multi-
drug resistant," he said. "Say someone got infected with
one of these, and was treated with oxacillin because it
is MSSA, and it developed resistance and became MRSA.
That would create a really scary strain."

So, what's the risk here? These strains of staph are not
the kind that causes food poisoning; their favored spot
is not our guts, but our skin and other warm, damp salty
places on our bodies, where they can live without
causing illness for an unpredictable period of time. So,
the issue is not that we will bring the meat home, eat
it and develop drug-resistant illness. Instead, it is
that we will bring it home, handle it badly and transfer
that staph-and its mobile resistance factors capable of
moving into other bacteria-onto our bodies and into our
homes.

"We know already that most food-borne illness occurs not
because of undercooked food" in which pathogens
survived, Price said. "It's from mishandling in the
kitchen and cross-contamination. I think there is a risk
of these strains contaminating a local environment. We
don't know what that risk is, because it has never been
evaluated - but anyone who dismisses that risk is doing
so without any data."

Price's research was partially funded by the Pew
Charitable Trusts, which several years ago supported a
massive investigation and report on the impact of
antibiotic overuse in agriculture, the Pew Commission on
Industrial Farm Animal Production. Shelley Hearne,
managing director of the Pew Health Group, told me, "The
bottom line is, the more we use antibiotics in
injudicious ways, the more we are compromising our
ability to save human lives in the future."

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