Contaminated Chicken Report
Unfortunately, raw chicken purchased at grocery stores are often contaminated with
bacteria, such as Escherichia coli, Salmonella spp., Shigella, or Campylobacter spp. According
to an analysis of outbreaks conducted by the CDC, most deaths occur from consuming poultry
than from any other commodity. It is normal to expect that raw chicken will contain some
species of bacteria, but some bacteria are more harmful than others, such as the antibioticresistance ones. Antibiotic-resistance infections cause at least 2 million illnesses and 23,000
deaths in the U.S. annually, and these resistant bacteria are commonly found in chicken that we
buy in stores (“Dangerous Contaminated Chicken,” 2014). Therefore, it is important to test for
microorganisms in raw chicken to detect the presence of harmful bacteria strains, mainly those
that are antibiotic-resistant, to prevent them from spreading and to reduce the number of people
from getting food-borne diseases.
In this experiment, we tested for Salmonella, Shigella, and E. coli species living in a
sample of raw chicken using Xylose lysine deoxycholate (XLD) agar, a selective and differential
growth medium. XLD agar contains different sources of fermentable sugar including xylose,
lactose, and sucrose. The yeast extract in it provides nitrogen, carbon, and vitamin sources that
are essential for bacterial growth. This medium also contains sodium deoxycholate, which
inhibits Gram-positive bacteria to grow. All of these factors allow for the growth of Gramnegative bacteria, Salmonella, Shigella, and E. coli species, but prevent the growth of Grampositive bacteria, which makes XLD agar a selective medium. In addition, this medium has a pH
indicator, phenol red, that turns yellow due to acid production after fermentation, which helps
differentiate the types of bacteria that grew on the plate. A red colony indicates a Shigella
because this type of bacteria cannot ferment xylose. On the other hand, Salmonella can ferment
xylose, which turns the medium yellow, but then it later decarboxylates lysine, making the
medium basic and red again. However, Salmonella can metabolize thiosulfate, producing
hydrogen sulfide, resulting in black-centered colonies. Furthermore, E. coli ferments lactose and
sucrose, and the acid byproducts will cause the medium to turn yellow.
The second medium that we used to isolate Campylobacter jejuni from the contaminated
chicken source was Campy CVA agar. This medium contains peptamin, yeast extract, dextrose,
and sheep blood, which all supports the growth of Campylobacter jejuni. The peptamin is a
source of carbon, sulfur, and nitrogenous compounds for the bacteria. Sheep blood supplies the
X factor for the medium and other growth requirements. The yeast extract supplies B vitamins,
and dextrose is utilized as an energy source. Campy CVA agar also includes antimicrobials that
lessen the growth of normal fecal flora and ingredients that inhibit the growth of Gram-positive
bacteria, vancomycin, the growth of aerobic and anaerobic Gram-positive and Gram-negative
microbes, cefoperazone, and the growth of yeast, amphotericin B. Because Campy CVA agar
incorporates ingredients suitable for Campylobacter jejuni growth and ingredients harmful for
the growth of other organisms, it is a selective medium.
After incubation, numerous amounts of red, black, and yellow colonies grew on all XLD
diluted plates. This finding indicates that Escherichia coli, Salmonella spp., and Shigella were all
present in the chicken sample, with Salmonella being the most prevalent. Although not as many
of Campylobacter jejuni grew on the Campy CVA agar plates as did Escherichia coli,
Salmonella spp., and Shigella grew on the XLD agar plates, it was still found to inhabit the
chicken. The Campy CVA agar plates with 10-1 and 10-2 dilutions had bacterial growth on it
while the remaining two plates did not.
All in all, the results from the diluted XLD and Campy CVA agar plates support the
rationale that chicken found for sale in a market has a high possibility of being contaminated
with Escherichia coli, Salmonella spp., Shigella, and Campylobacter spp. Obviously, other
methods should be taken into account in lab to have more efficient results, such as making more
serial dilutions for the chicken sample. As seen in most of the XLD agar plates, there were too
many colonies to count. Therefore, a more accurate count of the bacteria will be obtained with
more dilutions. Another suggestion for this lab experiment is to test more samples of chicken
with the purpose of avoiding results due to chance. Positive results for Escherichia coli,
Salmonella spp., Shigella, and Campylobacter spp. from a larger sample size will end in more
accurate results. Furthermore, testing other meat samples, such as ducks, geese, and turkeys is
essential because poultry, not just chicken, causes more deaths in the U.S. than any other food
products. Like chicken, most of these meats contain the same bacteria when contaminated.
Therefore, using the XLD and Campy CVA agar plates will isolate these specific organisms.
Reference
Dangerous Contaminated Chicken., 2014, Consumer Reports,
http://www.consumerreports.org/cro/magazine/2014/02/the-high-cost-of-cheapchicken/index.htm