Samuel Schriever and Faculty Mentor: Bradford Berges M&MB
Bacteria can be found everywhere. While some bacteria can help humans to make
medicine and clean up environmental disasters, other bacteria can cause horrible disease. After
the discovery of antibiotics in the early twentieth century humans have been using them to treat
human and animal disease. At first antibiotics very effective at clearing bacterial infections.
Bacteria that were able to survive the antibiotics were able to spread and become more common.
Bacteria have the ability to share genes and those genes are very favorable for the bacteria that
have them. Today, animals are given more antibiotics than humans are. This massive use of
antibiotics has created a situation where antibiotics may no longer effective and we don’t have a
good way to treat disease. One such bacteria is known as MRSA, or Methicillin Resistant
Staphylococcus Aureus (MRSA). MRSA is a specific type of Staphylococcus Aureus, and they
are both detected by the same method. By measuring the level of antibiotic resistance in meat
from grocery stores we hope to estimate the prevalence of Staphylococcus Aureus and
Methicillin Resistant Staphylococcus Aureus generated by the antibiotics use on the animals.
In order to measure how resistant the bacteria are we first had to collect bacteria from
meat. We collected samples from eleven different stores in Utah valley and put a small piece of
the sample on an agar plate. The bacteria that grew were tested with several different methods to
verify that they are S. Aureus. We used a mannitol salt agar, catalase test, coagulase test, gram
stain, and a PCR test to double check the identification. Once we ensured that the bacteria we
isolated was Staphylococcus Aureus we placed the bacteria on an agar plate with antibiotics to
see how close the bacteria would grow. We measured the distance and correlated it to known
values to determine how resistant the Staphylococcus Aureus was to that antibiotic. We tested
the bacterial strains against ten different antibiotics of different classes.
Of the 159 samples that we tested we were able to isolate 36 bacteria strains. This gave us
an overall prevalence of 22.6% for Staphylococcus Aureus. Broken down, 1 of 36 beef samples
was positive for SA (2.78%), 14 of 49 chicken samples were positive (28.57%), 12 of 39 pork
samples were positive (30.77%), and 9 of 35 turkey samples were positive (25.71%). Beef was
significantly less contaminated than the other types of meat. When we tested specifically for
MRSA it was detected only rarely in beef (1 of 36 positive; 2.78%), but was common in the
other three meat types: 10 of 49 positive in chicken (20.41%), 7 of 39 positive in pork (17.95%),
and 7 of 35 positive in turkey (20.00%). The overall frequency of MRSA isolation from the 159
samples was 15.72% with 25 strains found. The other thing that we tested for was how many
antibiotics the strains were resistant to. We found that the frequency of isolates that were multidrug
resistant (defined as complete resistance to three or more antibiotics tested) per meat group
were found to be (in descending order): 100% (beef; but n=1), 66.7% (chicken), 55.5% (turkey),
and 54.5% (pork); the overall frequency of multi-drug resistance was 20/33 or 60.6%.
We were surprised to find that there was so little Staphylococcus Aureus in the beef,
while chicken, turkey and pork had such high prevalence levels. It was interesting to find that
there was also such a high level of multi-drug resistance among all of the isolates found. It is
interesting to note that some of the antibiotics that have a low level of resistance are no longer in
agricultural use, suggesting that discontinuing their use decreases their level of resistance.
Further study into methods of switching antibiotic use routinely could help to find cost effective
methods of reducing antibiotic resistance in bacteria.
We found a disturbing level of microbes and antimicrobial resistance in the meet from
local grocery stores. These bacteria are something that can all be exposed to, and can even spread
their resistance genes to other pathogenic bacteria. We found that most of the bacteria were
resistant to multiple types of antibiotics. Further research into methods to control antibiotic
resistance should be performed. We are currently working on collecting enough bacteria in meat
from antibiotic-free raised animals to determine if that method would be effective at preventing
the spread of antibiotic resistance genes.