Straighter line BIO250L Lab 4 Selective
Media & Agar Exam Actual 2026/2027 –
Complete Questions with Detailed
Rationales | 100% Verified Answers –
Pass Guaranteed – (GRADED A+)
1. What is the primary difference between chemically defined media
and complex media?
A) Complex media are liquid; defined media are solid.
B) Defined media contain inhibitors; complex media do not.
C) Defined media have an exact chemical composition; complex media
contain extracts with unknown exact amounts.
D) Complex media are only used for fungi.
Answer: C
Rationale: Chemically defined media (e.g., glucose salts broth) have every
ingredient precisely measured. Complex media (e.g., Nutrient Agar, Tryptic
Soy Agar) use digests of animal or plant products (peptone, beef extract)
where the exact chemical composition varies batch to batch.
2. A medium that contains bile salts to inhibit Gram-positive bacteria
while allowing Gram-negative bacteria to grow is considered:
A) Differential only
B) Selective only
C) Both selective and differential
D) Enriched
, Answer: B
Rationale: Because it contains an inhibitor (bile salts) that suppresses one
group to favor another, its primary function is selective. If it also has a
color-changing indicator, it becomes both, but the action described here is
purely selective.
3. Which of the following best defines "differential media"?
A) Media that kills all non-target organisms.
B) Media that allows multiple organisms to grow but causes them to
appear visibly different from one another.
C) Media that only grows fastidious organisms.
D) Media that requires incubation in an anaerobic jar.
Answer: B
Rationale: Differential media contain substrates and pH indicators that
allow the observer to distinguish between species based on metabolic
reactions (e.g., color change, clearing zones).
4. A general-purpose medium used to grow a wide variety of non-
fastidious organisms is:
A) MacConkey Agar
B) Mannitol Salt Agar
C) Tryptic Soy Agar (TSA)
D) Starch Agar
Answer: C
Rationale: TSA is a complex, non-selective, non-differential medium used
simply to propagate many types of bacteria. The other options are
specialized.
, 5. In the lab, why is it critical to streak for isolated colonies before
inoculating differential media?
A) To increase the pH of the agar.
B) To ensure you are testing a pure culture so the metabolic result is
attributable to a single organism.
C) To activate the antibiotics in the media.
D) To evaporate excess moisture.
Answer: B
Rationale: If you inoculate a mixture of organisms, you won't know which
organism caused the color change or clearing zone. Isolated colonies
guarantee a pure culture for accurate interpretation.
6. Which of these is an example of a selective agent?
A) Phenol red
B) Neutral red
C) Sodium chloride (at 7.5%)
D) Starch
Answer: C
Rationale: High salt (7.5% NaCl) is the selective agent in MSA because it
inhibits most bacteria except staphylococci. Phenol red and neutral red are
pH indicators (differential agents). Starch is a substrate.
7. What does the term "fastidious" mean in microbiology?
A) The organism grows extremely fast.
, B) The organism has complex nutritional requirements and requires special
growth factors.
C) The organism is pathogenic.
D) The organism produces endospores.
Answer: B
Rationale: Fastidious organisms (e.g., Haemophilus or Neisseria) struggle to
grow on simple media and require enriched media with specific vitamins,
amino acids, or blood.
8. Agar is used to solidify media because:
A) Microbes can digest it for energy.
B) It melts at 100°C and stays liquid until 45°C, allowing us to pour plates
without killing microbes.
C) It is a synthetic polymer.
D) It turns pink when bacteria grow.
Answer: B
Rationale: Agar is a seaweed polysaccharide that most microbes cannot
degrade. Its melting and solidification temperatures make it ideal for
microbiology—liquid enough to pour at ~50°C (safe for microbes) and
solid at incubator temperatures (37°C).
9. Which medium is both selective AND differential?
A) Nutrient Agar
B) Starch Agar
C) Mannitol Salt Agar
D) Tryptic Soy Broth
Media & Agar Exam Actual 2026/2027 –
Complete Questions with Detailed
Rationales | 100% Verified Answers –
Pass Guaranteed – (GRADED A+)
1. What is the primary difference between chemically defined media
and complex media?
A) Complex media are liquid; defined media are solid.
B) Defined media contain inhibitors; complex media do not.
C) Defined media have an exact chemical composition; complex media
contain extracts with unknown exact amounts.
D) Complex media are only used for fungi.
Answer: C
Rationale: Chemically defined media (e.g., glucose salts broth) have every
ingredient precisely measured. Complex media (e.g., Nutrient Agar, Tryptic
Soy Agar) use digests of animal or plant products (peptone, beef extract)
where the exact chemical composition varies batch to batch.
2. A medium that contains bile salts to inhibit Gram-positive bacteria
while allowing Gram-negative bacteria to grow is considered:
A) Differential only
B) Selective only
C) Both selective and differential
D) Enriched
, Answer: B
Rationale: Because it contains an inhibitor (bile salts) that suppresses one
group to favor another, its primary function is selective. If it also has a
color-changing indicator, it becomes both, but the action described here is
purely selective.
3. Which of the following best defines "differential media"?
A) Media that kills all non-target organisms.
B) Media that allows multiple organisms to grow but causes them to
appear visibly different from one another.
C) Media that only grows fastidious organisms.
D) Media that requires incubation in an anaerobic jar.
Answer: B
Rationale: Differential media contain substrates and pH indicators that
allow the observer to distinguish between species based on metabolic
reactions (e.g., color change, clearing zones).
4. A general-purpose medium used to grow a wide variety of non-
fastidious organisms is:
A) MacConkey Agar
B) Mannitol Salt Agar
C) Tryptic Soy Agar (TSA)
D) Starch Agar
Answer: C
Rationale: TSA is a complex, non-selective, non-differential medium used
simply to propagate many types of bacteria. The other options are
specialized.
, 5. In the lab, why is it critical to streak for isolated colonies before
inoculating differential media?
A) To increase the pH of the agar.
B) To ensure you are testing a pure culture so the metabolic result is
attributable to a single organism.
C) To activate the antibiotics in the media.
D) To evaporate excess moisture.
Answer: B
Rationale: If you inoculate a mixture of organisms, you won't know which
organism caused the color change or clearing zone. Isolated colonies
guarantee a pure culture for accurate interpretation.
6. Which of these is an example of a selective agent?
A) Phenol red
B) Neutral red
C) Sodium chloride (at 7.5%)
D) Starch
Answer: C
Rationale: High salt (7.5% NaCl) is the selective agent in MSA because it
inhibits most bacteria except staphylococci. Phenol red and neutral red are
pH indicators (differential agents). Starch is a substrate.
7. What does the term "fastidious" mean in microbiology?
A) The organism grows extremely fast.
, B) The organism has complex nutritional requirements and requires special
growth factors.
C) The organism is pathogenic.
D) The organism produces endospores.
Answer: B
Rationale: Fastidious organisms (e.g., Haemophilus or Neisseria) struggle to
grow on simple media and require enriched media with specific vitamins,
amino acids, or blood.
8. Agar is used to solidify media because:
A) Microbes can digest it for energy.
B) It melts at 100°C and stays liquid until 45°C, allowing us to pour plates
without killing microbes.
C) It is a synthetic polymer.
D) It turns pink when bacteria grow.
Answer: B
Rationale: Agar is a seaweed polysaccharide that most microbes cannot
degrade. Its melting and solidification temperatures make it ideal for
microbiology—liquid enough to pour at ~50°C (safe for microbes) and
solid at incubator temperatures (37°C).
9. Which medium is both selective AND differential?
A) Nutrient Agar
B) Starch Agar
C) Mannitol Salt Agar
D) Tryptic Soy Broth
