,BIOL 271 Module 4 Exam | Microbiology w/Lab
Q&A | Portage Learning
1. Which type of media is designed to encourage the growth of specific microbes
while restricting the growth of others?
A) Enriched media
B) Differential media
C) Selective media
D) LB agar
Correct Answer: C) Selective media
Rationale: Selective media contain inhibitors like antibiotics, dyes, or high salt
that suppress unwanted organisms while allowing the target microbe to
flourish. Differential media distinguish species, enriched media provide extra
nutrients, and LB agar is non‑selective.
2. A culture medium that distinguishes between two similar bacterial species
based on metabolic differences is called:
A) Selective media
B) Enriched media
C) Nutrient broth
D) Differential media
Correct Answer: D) Differential media
,Rationale: Differential media use indicators such as pH dyes to reveal
biochemical differences between microbes. For example, MacConkey agar
differentiates lactose fermenters (red) from non‑fermenters (colorless),
whereas selective media would eliminate one group entirely.
3. MacConkey agar is selective for which group of bacteria?
A) Gram‑positive bacteria
B) Gram‑negative bacteria
C) All bacteria
D) Fungi
Correct Answer: B) Gram‑negative bacteria
Rationale: Crystal violet and bile salts in MacConkey agar inhibit Gram‑positive
organisms, making it selective for Gram‑negative species. It also differentiates
lactose fermenters (pink/red) from non‑fermenters (colorless).
4. Mannitol Salt Agar (MSA) is selective for which genus?
A) Staphylococcus
B) Escherichia
C) Bacillus
D) Pseudomonas
Correct Answer: A) Staphylococcus
, Rationale: MSA contains a high concentration of sodium chloride (7.5%) that
most bacteria cannot tolerate, but staphylococci can grow. It also differentiates
species that ferment mannitol, turning the medium yellow.
5. Blood agar is classified as which type of medium?
A) Selective and differential
B) Enriched and selective
C) Enriched and differential
D) Enriched only
Correct Answer: C) Enriched and differential
Rationale: Blood agar is enriched with sheep blood to support fastidious
organisms and is differential based on hemolysis patterns (alpha, beta, gamma).
It is not selective, as it supports growth of many different bacteria.
6. The primary purpose of enriched media is to:
A) Inhibit the growth of Gram‑positive bacteria
B) Distinguish between Gram‑positive and Gram‑negative
C) Provide nutrients for fastidious microorganisms
D) Select for antibiotic‑resistant strains
Correct Answer: C) Provide nutrients for fastidious microorganisms
Q&A | Portage Learning
1. Which type of media is designed to encourage the growth of specific microbes
while restricting the growth of others?
A) Enriched media
B) Differential media
C) Selective media
D) LB agar
Correct Answer: C) Selective media
Rationale: Selective media contain inhibitors like antibiotics, dyes, or high salt
that suppress unwanted organisms while allowing the target microbe to
flourish. Differential media distinguish species, enriched media provide extra
nutrients, and LB agar is non‑selective.
2. A culture medium that distinguishes between two similar bacterial species
based on metabolic differences is called:
A) Selective media
B) Enriched media
C) Nutrient broth
D) Differential media
Correct Answer: D) Differential media
,Rationale: Differential media use indicators such as pH dyes to reveal
biochemical differences between microbes. For example, MacConkey agar
differentiates lactose fermenters (red) from non‑fermenters (colorless),
whereas selective media would eliminate one group entirely.
3. MacConkey agar is selective for which group of bacteria?
A) Gram‑positive bacteria
B) Gram‑negative bacteria
C) All bacteria
D) Fungi
Correct Answer: B) Gram‑negative bacteria
Rationale: Crystal violet and bile salts in MacConkey agar inhibit Gram‑positive
organisms, making it selective for Gram‑negative species. It also differentiates
lactose fermenters (pink/red) from non‑fermenters (colorless).
4. Mannitol Salt Agar (MSA) is selective for which genus?
A) Staphylococcus
B) Escherichia
C) Bacillus
D) Pseudomonas
Correct Answer: A) Staphylococcus
, Rationale: MSA contains a high concentration of sodium chloride (7.5%) that
most bacteria cannot tolerate, but staphylococci can grow. It also differentiates
species that ferment mannitol, turning the medium yellow.
5. Blood agar is classified as which type of medium?
A) Selective and differential
B) Enriched and selective
C) Enriched and differential
D) Enriched only
Correct Answer: C) Enriched and differential
Rationale: Blood agar is enriched with sheep blood to support fastidious
organisms and is differential based on hemolysis patterns (alpha, beta, gamma).
It is not selective, as it supports growth of many different bacteria.
6. The primary purpose of enriched media is to:
A) Inhibit the growth of Gram‑positive bacteria
B) Distinguish between Gram‑positive and Gram‑negative
C) Provide nutrients for fastidious microorganisms
D) Select for antibiotic‑resistant strains
Correct Answer: C) Provide nutrients for fastidious microorganisms
