SCROLL DOWN FOR ANSWERS TO DISCUSSION QUESTIONS OF THIS ARTICLE
Antibiotic resistant bacteria at the meat counter
May 2013
The pork chops you buy in the supermarket neatly packaged in plastic and styrofoam may look completely sterile, but
are, in fact, likely to be contaminated with disease-causing bacteria — and not with just any old bugs, but with hard-totreat, antibiotic resistant strains. In a recently published study, researchers with the National Antimicrobial Resistance
Monitoring System bought meat from a wide sampling of chain grocery stores across the country and analyzed the
bacteria on the meat. Resistant microbes were found in 81% of ground turkey samples, 69% of pork chops, 55% of
ground beef samples, and 39% of chicken parts. Of course, thoroughly cooking the meat will kill the germs, but if the
meat is undercooked or contaminates other food with its bacteria — perhaps via a shared cutting board — the result
could be an infection that can't be cured with common medications. Such infections are a serious health concern — a
strain of antibiotic resistant staph was recently estimated to cause nearly 20,000 deaths per year in the U.S. — and the
problem seems to be getting worse. An evolutionary perspective helps us understand how antibiotic resistance arises
in the first place and why the prevalence of resistant bugs in livestock has health professionals and scientists worried.
It should be no surprise that antibiotic resistant bacteria are the products of evolution via natural selection: as bacteria reproduce, small, random errors
(i.e., mutations) occur as their DNA is copied. Just by chance, some of those mutations may help their bearers survive and reproduce better and so will increase in
frequency in the bacterial population. Other mutations may be detrimental and will be weeded out of the population. Still others may have no impact at all to the
bacterium (i.e., neutral mutations). When antibiotics flood the environment of the bacteria, individuals that happen to carry random mutations that allow them to
survive and reproduce despite the drug will be favored. Eventually, the entire lineage of bacteria may carry genes that confer antibiotic resistance.
This process seems to be inevitable. If a bacterial lineage is consistently exposed to a particular antibiotic, it will eventually evolve resistance to that drug, and this
will occur in the soil, in livestock, in the human body — wherever bacteria are exposed to antibiotics. This same basic process is responsible for the evolution of
advantageous traits in familiar organisms, like a hawk's keen eyesight or a polar bear's insulating fur. However, bacteria have a leg up on birds and bears when it
comes to evolution. Most species rely on mutations somewhere in their historical lineage for their genetic variation — that is, an improved ability to spot prey will
evolve in a lineage of hawks only if mutations conferring keener sight occurred somewhere in the hawk lineage and were then passed down to the generation of
hawks experiencing natural selection. Bacteria, on the other hand, get their genetic variation both from their ancestral lineage and through a process known as
horizontal transfer.
In horizontal transfer, organisms share genetic material with one another directly, as opposed to passing genetic material only to their offspring. In this way, genes
from distantly related lineages of bacteria can wind up in the same individual. A gene version that first arose in Escherichia coli could easily be passed on
to Salmonella.
Horizontal transfer represents a special danger when it comes to the evolution of resistance
because, through gene sharing, antibiotic resistance genes that evolve and become common in
one lineage of bacteria that is exposed to a particular antibiotic can be passed to distantly related
bacterial lineages. In other words, a bacterial lineage can evolve resistance to a particular
antibiotic even if its ancestors never carried a mutation that conferred resistance to that drug.
With all this genetic variation being shared, antibiotic resistant bacterial strains can evolve quickly.
Furthermore, different antibiotics often have similar modes of action (e.g., amoxicillin and
methicillin both work by preventing bacteria from forming cell walls), so resistance to one drug
often means partial resistance to a host of other medications. To make matters even worse,
bacteria often transfer multiple genes for resistance to different antibiotics on the same piece of
DNA. Since the genes are physically attached to one another, selecting for one of those
resistance genes lets the others hitchhike to high frequency. So exposing a bacterial population
to say, streptomycin, may also unintentionally favor the evolution of a strain that resists many
other antibiotics as well — making for a particularly hard-to-cure infection.
Bacteria have many characteristics that allow them to evolve resistance to whatever antibiotics
we throw their way — short generation times, high mutation rates, and horizontal transfer — and
current agricultural practices (in particular, the heavy use of antibiotics in livestock) seem
destined to speed this process even further. In the U.S., around 80% of antibiotics are destined for farm animals, not for treating human disease. The majority of
those animal antibiotics are used preventatively and to promote faster growth and speed meat production, not to treat sick individuals. Unfortunately, this approach
also encourages the evolution and proliferation of antibiotic resistant strains on factory farms. So, it should come as no surprise that a large percentage of
supermarket meat carries antibiotic resistant bugs!
Clearly, the ubiquity of antibiotic resistant bacteria in livestock has implications far beyond highlighting the need to cook meat thoroughly. It suggests that, lurking in
farm animals, is a vast pool of dangerous resistance genes that could easily make their way out of the bacteria in which they currently reside and into strains that
would represent an even more significant human health threat. We have many lines of evidence suggesting that horizontal transfer of genes, including resistance
genes, is commonplace among bacteria. What we have not had is a major outbreak of an antibiotic resistant infection that has been definitively linked to resistance
from bacteria inhabiting livestock — yet. If the American Medical Association, the World Health Organization, and the National Academy of Sciences have their
way, we may be able to avoid that fate, at least for certain antibiotics. These groups have all signed on to support new legislation that would prevent widespread
use of certain antibiotics on livestock, helping to protect the effectiveness of these drugs in humans.
Disinfectants could also give rise to antibiotic-resistant superbugs
A new study by the National University of Ireland has provided more evidence that using common disinfectants could promote the growth of antibiotic-resistant
superbugs.
Overuse of antibiotics is known to give rise to antibiotic resistance, but disinfectants could produce the same effect, according to research published in the January
2010 issue of the journal "Microbiology."
Researchers grew a bacteria associated with hospital-acquired infections in the presence of a common disinfectant and found that the bacteria became 12 times
less susceptible to the disinfectant. But they also found that the same bacteria became 256 times more resistant to a widely used antibiotic -- ciprofloxacin -- even
though the bacteria had never been exposed to it.
References:
 National Antimicrobial Resistance Monitoring System. (2013). Retail meat report 2011.
 Smillie, C. S., Smith, M. B., Friedman, J., Cordera, J. X., David, L. A., and Alm, E. J. (2011). Ecology drives a global network of gene exchange connecting the
human microbiome. Nature. 480: 241-244.
 http://www.cnn.com/2009/HEALTH/12/30/disinfectant.superbugs/index.html
Discussion questions
1. Explain how mutation and horizontal gene transfer affect the genetic variation in a bacterial population.
2. What will inevitably happen when a population of bacterium is consistently exposed a particular drug?
3. Why does bacterial horizontal gene transfer represent such a special danger to us?
Answers to Discussion questions
1. Explain how mutation and horizontal gene transfer affect the genetic
variation in a bacterial population.
Random errors or mutations can change existing base
pairings of bacterial genes into new ones which enable
certain individuals of a bacterial population to be genetically
different from the rest of the population. This genetic novelty
is further magnified in the population as different individuals
of the bacterial population exchange their genetic material
through horizontal gene transfer.
2. What will inevitably happen when a population of bacterium is
consistently exposed a particular drug?
Random mutations can confer advantageous traits that help
the bacteria survive a treatment of antibiotic. The gene(s)
codes for such traits is then pass on to the bacteria’s
progeny. Since bacteria reproduce fast through binary fission,
soon enough the entire population will become immune to
the current antibiotic and more powerful drug will be required
for further treatment of such bacteria.
3. Why does bacterial horizontal gene transfer (HGT) represent such a
special danger to us?
HGT allows individuals of a bacterial population to exchange
its resistant genes within their lifetime. Thus, antibiotics
resistance can occur very fast before we can come up with
more powerful drugs. This can cost lives. HGT also allows
distantly related individuals to acquire genes that confer
resistance to multiple drugs.
* Vertical gene transfer: genes passed from parents to offsprings