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STRAIGHTERLINE BIO250L
Lab 4 Selective Media & Agar
questions with 100% accurate
solutions + rationales 2026
Student Name: Courtney Chang
Access Code (located on the lid of your lab kit): AC-82HRFL
Lab Report Format Expecta0ons
U"lize college level grammar and professional forma4ng when comple"ng this
worksheet.
Submissions without proper forma4ng, all required photos or sufficient responses
will be rejected.
Pre-lab Ques>ons
1. In this lab, you will conduct three different experiments that each
cover the topic of selecFve media and agar. How are these
experiments different from one another? Explain how the different
media in each experiment are uFlized in the context of selecFve vs
differenFal media. In Experiment 1, we uFlized starch agar instead of
nutrient agar as well as iodine. The purpose of this experiment was to
test bacteria capable of digesFng starch and observe the bacteria
growth. The iodine helps detect the presence of starch since it turns
blue/black under these condiFons. In Experiment 2, we uFlized
mannitol salt agar (MSA) which is both selecFve and differenFal for
the gram-posiFve bacteria Staphylococcus. This species of bacteria is
commonly found on our skin and responsible for many skin
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infecFons. However, most types of Staphylococcus are harmless to
humans. The purpose of this experiment was to use MSA to idenFfy
some microbes growing on our skin. In Experiment 3, we used
MacConkey agar which is both a selecFve and differenFal media. The
goal was to collect various liquid samples and test them for the
presence of coliform, a type of bacteria commonly found in
contaminated water. Although these three experiments were very
different from one another, they all had the goal of using different
types of selecFve media in idenFfying certain microbes.
2. What is the difference between chemically defined and chemically
complex media? Give an example (a clinical or environmental
research applicaFon) of when you would use each type of medium to
culture different microorganisms.
Chemically defined media are media in which the composiFons are
known. They are also made with purified ingredients. This type of
media is beneficial for microbiologists since it gives them more
control in research and medical labs. Some examples include water as
a diluent, sodium acetate for carbon source and energy, ammonium
sulfate for nitrogen and sulfur source, and potassium phosphate
mono basic for phosphorus and potassium. Chemically complex
media is composed of yeast, plant, and animal extracts, and refers to
media where the complete composiFon is not fully known or
idenFfied. This type of media can be beneficial since it is likely to
provide more essenFal nutrients and vitamins to a wide range of
microorganisms. However, it gives less control to microbiologists
since they cannot always restrict the nutrients that are present. Some
examples include beef extract for carbon, energy, vitamins, nitrogen,
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and sulfur, lepton for pepFdes and single amino acids, and sodium
chloride to create an isotonic environment that prevents cell lysis.
3. Why do we grow microorganisms on general growth media first,
before inoculaFng differenFal media?
It is important to have a pure, isolated colony that has been able to
grow without a medium affecFng its growth. This allows us to grow
and study microorganisms in controlled condiFons, yielding more
precise results. By growing microorganisms on general growth media
first, we are ensuring that the targeted bacteria is isolated and
capable of growing in a neutral environment.
4. What changes might you see on a differenFal plate that helps idenFfy
the bacteria? DifferenFal media allows microbiologists to idenFfy
microorganisms more easily due to their unique composiFons. This
type of media contains compounds that allow certain microbes to be
disFnguished on a culture plate. For example, we could potenFally
see change in the color of microbial colonies. Some differenFal media
can contain the pH indicator phenol red that changes color from red
to yellow as acidity increases. This would allow us to idenFfy a
microorganism based on their ability to produce acid during
fermentaFon.
EMAIL:
STRAIGHTERLINE BIO250L
Lab 4 Selective Media & Agar
questions with 100% accurate
solutions + rationales 2026
Student Name: Courtney Chang
Access Code (located on the lid of your lab kit): AC-82HRFL
Lab Report Format Expecta0ons
U"lize college level grammar and professional forma4ng when comple"ng this
worksheet.
Submissions without proper forma4ng, all required photos or sufficient responses
will be rejected.
Pre-lab Ques>ons
1. In this lab, you will conduct three different experiments that each
cover the topic of selecFve media and agar. How are these
experiments different from one another? Explain how the different
media in each experiment are uFlized in the context of selecFve vs
differenFal media. In Experiment 1, we uFlized starch agar instead of
nutrient agar as well as iodine. The purpose of this experiment was to
test bacteria capable of digesFng starch and observe the bacteria
growth. The iodine helps detect the presence of starch since it turns
blue/black under these condiFons. In Experiment 2, we uFlized
mannitol salt agar (MSA) which is both selecFve and differenFal for
the gram-posiFve bacteria Staphylococcus. This species of bacteria is
commonly found on our skin and responsible for many skin
, FOR MORE EXAMS
EMAIL:
infecFons. However, most types of Staphylococcus are harmless to
humans. The purpose of this experiment was to use MSA to idenFfy
some microbes growing on our skin. In Experiment 3, we used
MacConkey agar which is both a selecFve and differenFal media. The
goal was to collect various liquid samples and test them for the
presence of coliform, a type of bacteria commonly found in
contaminated water. Although these three experiments were very
different from one another, they all had the goal of using different
types of selecFve media in idenFfying certain microbes.
2. What is the difference between chemically defined and chemically
complex media? Give an example (a clinical or environmental
research applicaFon) of when you would use each type of medium to
culture different microorganisms.
Chemically defined media are media in which the composiFons are
known. They are also made with purified ingredients. This type of
media is beneficial for microbiologists since it gives them more
control in research and medical labs. Some examples include water as
a diluent, sodium acetate for carbon source and energy, ammonium
sulfate for nitrogen and sulfur source, and potassium phosphate
mono basic for phosphorus and potassium. Chemically complex
media is composed of yeast, plant, and animal extracts, and refers to
media where the complete composiFon is not fully known or
idenFfied. This type of media can be beneficial since it is likely to
provide more essenFal nutrients and vitamins to a wide range of
microorganisms. However, it gives less control to microbiologists
since they cannot always restrict the nutrients that are present. Some
examples include beef extract for carbon, energy, vitamins, nitrogen,
, FOR MORE EXAMS
EMAIL:
and sulfur, lepton for pepFdes and single amino acids, and sodium
chloride to create an isotonic environment that prevents cell lysis.
3. Why do we grow microorganisms on general growth media first,
before inoculaFng differenFal media?
It is important to have a pure, isolated colony that has been able to
grow without a medium affecFng its growth. This allows us to grow
and study microorganisms in controlled condiFons, yielding more
precise results. By growing microorganisms on general growth media
first, we are ensuring that the targeted bacteria is isolated and
capable of growing in a neutral environment.
4. What changes might you see on a differenFal plate that helps idenFfy
the bacteria? DifferenFal media allows microbiologists to idenFfy
microorganisms more easily due to their unique composiFons. This
type of media contains compounds that allow certain microbes to be
disFnguished on a culture plate. For example, we could potenFally
see change in the color of microbial colonies. Some differenFal media
can contain the pH indicator phenol red that changes color from red
to yellow as acidity increases. This would allow us to idenFfy a
microorganism based on their ability to produce acid during
fermentaFon.
