FOOD3230 Lab Report 1 (6)
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University of Manitoba *
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3230
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Biology
Date
Dec 6, 2023
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6
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Lab 1 Report
1) Construct a table that describes the colony appearance, Gram reaction,
oxidase and catalase test. From the results obtained determine the bacteria
genera that were in the original cell suspension. (20 marks)
Table 1
: Characteristics of bacteria grown in nutrient agar plates and incubated at 37°C.
Nutrient Agar
Plate
Colony
Appearance
Gram Stain
Reaction (+/-)
Oxidase Test
Reaction
Catalase Test
Reaction
Escherichia coli
- Large
- Yellow
- Round form
- Entire
margin
- Smooth
edge
- No elevation
Positive
(Purple)
Negative (no
color)
Positive
(Small, sparse
bubbles)
Listeria
- Large
- Raised
elevation
- Round form
- Blue with
hint of yellow
- Round
margin
Positive
(Purple)
Negative (no
color)
Positive
(Small, sparse
bubbles)
Lactobacillus
- Small
- Low-opacity
- Yellow
- Smooth
- Raised
elevation
Round form
Entire margin
Positive
(Purple)
Negative (no
color)
Negative (No
bubbles)
The result of Gram staining of the E.coli culture has been determined to be incorrect
(Smith, 2012). Research has identified E. coli as a Gram-negative bacteria with a thin
peptidoglycan layer in the cell wall (Smith, 2012). Possible errors that resulted in the
incorrect reaction include over/under-decolorization of smear, improper drying, incorrect
timing for staining, inadequate slide rinsing, etc.
2) How did the Germ Theory change approaches to infection control? (10 marks)
Louis Pasteur and Robert Koch’s Germ theory had a tremendous impact on
microbiology (Smith, 2012). The theory states that germs (microorganisms) are the root
cause of infection and disease (Smith, 2012). This theory allowed us to study different
types of microbes by isolating and classifying them (Smith, 2012). The identification of
microorganisms allowed for the development of treatments targeted for specific
diseases (Smith, 2012). Examples of treatments include antibiotics and antifungals
(Smith, 2012). The Germ Theory set the foundation for sanitation and hygiene when it
comes to infection control, the spreading of bacteria was further studied and linked to
handwashing and sterilization (Smith, 2012).
3) What is the principle behind the selectivity of MRS, MacConkey and Oxford
Formula agar? (12 marks)
Different types of agar mediums are used in microbiology to act as conditions suitable
for microorganisms to not only survive, but to grow rapidly in. When it comes to
identifying microorganisms, selective agar is not necessarily specific, however, it does
give us general information regarding the microorganisms’ characteristics and genera.
MRS agar, also known as de Man, Rogosa, and Sharpe agar, is used to cultivate lactic
acid bacteria such as Lactobacillus (panel & Man, 2007). It’s typically composed of beef
extract, glucose, and pH indicator (panel & Man, 2007). MRS agar is considered to be
selective due to its low pH environment, which prevents many types of microorganisms
from surviving, this low pH medium is the perfect environment for lactic acid bacteria to
prosper in (panel & Man, 2007). MacConkey agar was developed by Alfred MacKonkey
in the 1900, where it is used to isolate Gram-negative bacteria, specifically
Enterobacteriaceae (Statpearls, 2018). This agar is typically made of crystal violet as
well as bile salts, which allows the Gram-negative bacteria to grow while inhibiting the
growth of bacteria that are Gram-positive (Statpearls, 2018). This type of agar contains
lactose as a source of sugar, certain bacterias have the ability to ferment it while others
cannot (Statpearls, 2018). This fermentation results in the production of acid, lowering
the overall pH of the environment (Statpearls, 2018). Bacteria that are able to perform
fermentation will result in a color change due to the interaction of acid with the indicator
(Statpearls, 2018). E. coli is an example of lactose- fermenting bacteria, causing
colonies to turn a pink color due to the acid’s reaction with the indicator (Statpearls,
2018). Oxford Formula agar contains an antibiotic called cefoxitin, which promotes the
growth of the bacterium Clostridium difficile (Miller et al., 2018). It also contains egg
yolk, which forms a halo around Clostridium difficile in its presence (Miller et al., 2018).
4) Why are confirmation tests required even though typical colonies appear on
selective agar plates? (5 marks)
Conformational tests such as the oxidation and catalase tests are required to determine
the exact strain of microorganisms present, even though typical colonies appear on
selective agar plates (Miller et al., 2018). In order to identify the microorganisms,
confirmation tests should accurately be performed to tell them apart from the expected
typical colony. Selective agar plates are designed to help a certain microorganism to
grow while inhibiting others from surviving (Miller et al., 2018). It is important to note that
this does not allow us to identify the microorganism because many different strains can
survive in it as well, due to similarities in characteristics. When it comes to safety,
misidentifying microorganisms can pose a health risk in food and beverage production
to ensure the safety and quality of products.
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