,WGU C157 Microbiology Exam 1 | Questions &
Verified Answers | 2026 Edition | Western
Governors University
1. Which of the following individuals is credited with first observing and describing microorganisms using
a simple microscope?**
A) Robert Hooke
B) Antonie van Leeuwenhoek
C) Louis Pasteur
D) Robert Koch
Correct Answer: B) Antonie van Leeuwenhoek
Rationale: Antonie van Leeuwenhoek, a Dutch draper, is known as the "Father of Microbiology" for his
pioneering work with simple microscopes. He was the first to observe and describe single-celled
organisms, which he called "animalcules," from various samples including rainwater, dental plaque, and
pond water. Robert Hooke first described cells, Pasteur disproved spontaneous generation, and Koch
established the germ theory of disease.
**2. What is the fundamental principle of the Germ Theory of Disease, as established by Louis Pasteur
and Robert Koch?**
A) All microorganisms cause disease.
B) Microorganisms are spontaneously generated from non-living matter.
C) Specific microorganisms cause specific diseases.
D) Diseases are caused by imbalances in the body's humors.
Correct Answer: C) Specific microorganisms cause specific diseases.
Rationale: The Germ Theory of Disease states that many diseases are caused by the presence and
actions of specific microorganisms within the body. This foundational concept was established through
the work of Pasteur, who linked microbes to fermentation and spoilage, and Koch, who developed a
,systematic method to prove the causative agent of a disease. Koch's postulates provided the framework
for linking a specific pathogen to a specific disease.
**3. A microbiologist is studying a microorganism that lacks a membrane-bound nucleus and other
organelles. This organism belongs to which domain?**
A) Eukarya
B) Bacteria
C) Archaea
D) Fungi
Correct Answer: B) Bacteria
Rationale: Bacteria are prokaryotic organisms, meaning they lack a membrane-bound nucleus and
membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Their
genetic material is located in the nucleoid region. Archaea are also prokaryotic but differ biochemically
from bacteria. Eukarya includes organisms with a true nucleus (e.g., fungi, protozoa, plants, animals).
**4. Which of the following bacterial structures is responsible for the ability to survive harsh
environmental conditions, including heat, radiation, and desiccation?**
A) Capsule
B) Flagella
C) Endospore
D) Pilus
Correct Answer: C) Endospore
Rationale: Endospores are highly resistant, dormant structures formed by certain genera of bacteria
(e.g., *Bacillus*, *Clostridium*) in response to unfavorable environmental conditions. They allow the
bacterium to survive extremes of heat, radiation, desiccation, and chemical disinfectants. When
conditions become favorable, the endospore germinates to form a vegetative cell.
, **5. In the Gram staining procedure, which reagent is used as the mordant to form a crystal violet-
iodine complex?**
A) Crystal violet
B) Gram's iodine
C) Decolorizer (ethanol/acetone)
D) Safranin
Correct Answer: B) Gram's iodine
Rationale: Gram's iodine acts as a mordant in the Gram staining procedure. After the primary stain
(crystal violet) is applied, iodine is added to form an insoluble crystal violet-iodine complex within the
cell wall. This complex is trapped in the thick peptidoglycan layer of Gram-positive bacteria but can be
washed out of Gram-negative bacteria during the decolorization step.
**6. Gram-positive bacteria appear purple after the Gram stain because:**
A) They have a thin peptidoglycan layer that retains the crystal violet-iodine complex.
B) They have an outer membrane that prevents the decolorizer from penetrating.
C) They have a thick peptidoglycan layer that retains the crystal violet-iodine complex.
D) They take up the safranin counterstain more readily.
Correct Answer: C) They have a thick peptidoglycan layer that retains the crystal violet-iodine complex.
Rationale: Gram-positive bacteria have a thick, multilayered peptidoglycan cell wall (comprising 60-90%
of the cell wall) that retains the crystal violet-iodine complex during the decolorization step. The
alcohol/acetone decolorizer dehydrates the thick peptidoglycan, trapping the dye complex, thus the
cells appear purple. In contrast, Gram-negative bacteria have a thin peptidoglycan layer and an outer
membrane that is dissolved by the decolorizer, allowing the dye complex to wash out.
**7. Which of the following is NOT a characteristic of prokaryotic cells?**
A) Presence of a nucleoid region
B) 70S ribosomes
C) Membrane-bound organelles
Verified Answers | 2026 Edition | Western
Governors University
1. Which of the following individuals is credited with first observing and describing microorganisms using
a simple microscope?**
A) Robert Hooke
B) Antonie van Leeuwenhoek
C) Louis Pasteur
D) Robert Koch
Correct Answer: B) Antonie van Leeuwenhoek
Rationale: Antonie van Leeuwenhoek, a Dutch draper, is known as the "Father of Microbiology" for his
pioneering work with simple microscopes. He was the first to observe and describe single-celled
organisms, which he called "animalcules," from various samples including rainwater, dental plaque, and
pond water. Robert Hooke first described cells, Pasteur disproved spontaneous generation, and Koch
established the germ theory of disease.
**2. What is the fundamental principle of the Germ Theory of Disease, as established by Louis Pasteur
and Robert Koch?**
A) All microorganisms cause disease.
B) Microorganisms are spontaneously generated from non-living matter.
C) Specific microorganisms cause specific diseases.
D) Diseases are caused by imbalances in the body's humors.
Correct Answer: C) Specific microorganisms cause specific diseases.
Rationale: The Germ Theory of Disease states that many diseases are caused by the presence and
actions of specific microorganisms within the body. This foundational concept was established through
the work of Pasteur, who linked microbes to fermentation and spoilage, and Koch, who developed a
,systematic method to prove the causative agent of a disease. Koch's postulates provided the framework
for linking a specific pathogen to a specific disease.
**3. A microbiologist is studying a microorganism that lacks a membrane-bound nucleus and other
organelles. This organism belongs to which domain?**
A) Eukarya
B) Bacteria
C) Archaea
D) Fungi
Correct Answer: B) Bacteria
Rationale: Bacteria are prokaryotic organisms, meaning they lack a membrane-bound nucleus and
membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Their
genetic material is located in the nucleoid region. Archaea are also prokaryotic but differ biochemically
from bacteria. Eukarya includes organisms with a true nucleus (e.g., fungi, protozoa, plants, animals).
**4. Which of the following bacterial structures is responsible for the ability to survive harsh
environmental conditions, including heat, radiation, and desiccation?**
A) Capsule
B) Flagella
C) Endospore
D) Pilus
Correct Answer: C) Endospore
Rationale: Endospores are highly resistant, dormant structures formed by certain genera of bacteria
(e.g., *Bacillus*, *Clostridium*) in response to unfavorable environmental conditions. They allow the
bacterium to survive extremes of heat, radiation, desiccation, and chemical disinfectants. When
conditions become favorable, the endospore germinates to form a vegetative cell.
, **5. In the Gram staining procedure, which reagent is used as the mordant to form a crystal violet-
iodine complex?**
A) Crystal violet
B) Gram's iodine
C) Decolorizer (ethanol/acetone)
D) Safranin
Correct Answer: B) Gram's iodine
Rationale: Gram's iodine acts as a mordant in the Gram staining procedure. After the primary stain
(crystal violet) is applied, iodine is added to form an insoluble crystal violet-iodine complex within the
cell wall. This complex is trapped in the thick peptidoglycan layer of Gram-positive bacteria but can be
washed out of Gram-negative bacteria during the decolorization step.
**6. Gram-positive bacteria appear purple after the Gram stain because:**
A) They have a thin peptidoglycan layer that retains the crystal violet-iodine complex.
B) They have an outer membrane that prevents the decolorizer from penetrating.
C) They have a thick peptidoglycan layer that retains the crystal violet-iodine complex.
D) They take up the safranin counterstain more readily.
Correct Answer: C) They have a thick peptidoglycan layer that retains the crystal violet-iodine complex.
Rationale: Gram-positive bacteria have a thick, multilayered peptidoglycan cell wall (comprising 60-90%
of the cell wall) that retains the crystal violet-iodine complex during the decolorization step. The
alcohol/acetone decolorizer dehydrates the thick peptidoglycan, trapping the dye complex, thus the
cells appear purple. In contrast, Gram-negative bacteria have a thin peptidoglycan layer and an outer
membrane that is dissolved by the decolorizer, allowing the dye complex to wash out.
**7. Which of the following is NOT a characteristic of prokaryotic cells?**
A) Presence of a nucleoid region
B) 70S ribosomes
C) Membrane-bound organelles
