Introduction to Microbiology SCIN202 Assignment 2 (100 points)
Student Name:
Student ID: 7337264
Course ID: SCIN202 I001 Fall 2025
Professor Stahl
Part 1:
1. What were the two strains of bacteria used in Griffith’s experiments? Describe them.
The two strains that Griffith used for his experiment were Streptococcus pneumoniae:
the smooth (S) strain and the rough (R) strain. The S strain had a smooth, polysaccharide
capsule that protected it from the host immune system, making it virulent. On the other hand,
the R strain did not have this capsule, appearing rough under the microscope and was non
virulent, meaning it could not cause disease in mice (Cold Spring Harbor Laboratory, 2002;
OpenStax, 2024).
2. Which of the two bacterial strains in Griffith’s experiments has the ability to cause an
infection? Explain why.
Virulent indicates whether a microorganism is highly infectious and causes diseases
versus non virulent which means they do not cause any harm. The virulent S strain caused
infection because its polysaccharide capsule shielded it from phagocytosis by the host’s
immune cells. This capsule allowed the bacteria to survive and multiply in the host, leading to
pneumonia and death in Griffith’s experimental mice. The R strain is non virulent and lacks this
capsule. It was easily destroyed by the immune system, preventing infection (Cold Spring
Harbor Laboratory, 2002).
3. What was Avery’s tube assay in his expansion of Griffith’s experiment? Describe it in
detail.
Oswald Avery and his team built on Griffith’s findings by performing a tube assay to
identify the “transforming principle.” He treated the heat killed S strain extracts with enzymes
that destroyed proteins, RNA, or DNA before mixing them with live R strain cells.
Transformation occurred in all cases except when DNA was destroyed. This proved that DNA
was the hereditary material responsible for transferring genetic traits (Cold Spring Harbor
Laboratory, 2002; OpenStax, 2024).
Part 2:
1. After isolating the plasmids from bacteria, what did they add to each tube in order to
cut the plasmids into pieces?
Explain what happened to the areas that were cut.
Student Name:
Student ID: 7337264
Course ID: SCIN202 I001 Fall 2025
Professor Stahl
Part 1:
1. What were the two strains of bacteria used in Griffith’s experiments? Describe them.
The two strains that Griffith used for his experiment were Streptococcus pneumoniae:
the smooth (S) strain and the rough (R) strain. The S strain had a smooth, polysaccharide
capsule that protected it from the host immune system, making it virulent. On the other hand,
the R strain did not have this capsule, appearing rough under the microscope and was non
virulent, meaning it could not cause disease in mice (Cold Spring Harbor Laboratory, 2002;
OpenStax, 2024).
2. Which of the two bacterial strains in Griffith’s experiments has the ability to cause an
infection? Explain why.
Virulent indicates whether a microorganism is highly infectious and causes diseases
versus non virulent which means they do not cause any harm. The virulent S strain caused
infection because its polysaccharide capsule shielded it from phagocytosis by the host’s
immune cells. This capsule allowed the bacteria to survive and multiply in the host, leading to
pneumonia and death in Griffith’s experimental mice. The R strain is non virulent and lacks this
capsule. It was easily destroyed by the immune system, preventing infection (Cold Spring
Harbor Laboratory, 2002).
3. What was Avery’s tube assay in his expansion of Griffith’s experiment? Describe it in
detail.
Oswald Avery and his team built on Griffith’s findings by performing a tube assay to
identify the “transforming principle.” He treated the heat killed S strain extracts with enzymes
that destroyed proteins, RNA, or DNA before mixing them with live R strain cells.
Transformation occurred in all cases except when DNA was destroyed. This proved that DNA
was the hereditary material responsible for transferring genetic traits (Cold Spring Harbor
Laboratory, 2002; OpenStax, 2024).
Part 2:
1. After isolating the plasmids from bacteria, what did they add to each tube in order to
cut the plasmids into pieces?
Explain what happened to the areas that were cut.
