Midterm Short Answers

Question 1

What are Koch’s postulates, and how do they contribute to identifying the causative agents of infectious diseases?

Answer 1

1. Suspected microbes are always present in diseased hosts and absent in healthy hosts.
2. Suspected microbe is grown in a pure culture outside the host.
3. Cultured microbes are introduced into healthy hosts and the same disease occurs.
4. The same microbial suspect is re-isolated from the newly diseased hosts.

Koch’s postulates were established to identify the specific pathogen for causing a disease.

Question 2

Describe the phases of a bacterial growth curve in a closed system.

Answer 2

1. Lag phase: bacteria are preparing their cell machinery for growth.
2. Log phase: bacteria population grows at an exponential rate.
3. Stationary phase: cells stop growing and shut down their growth machinery while turning on stress responses to help retain viability.
4. Death phase: cells dies with a “half-life” similar to that of a radioactive decay, a negative exponential curve.

Question 3

What is the theory of spontaneous generation, and how was it disproved?

Answer 3

Spontaneous generation is the theory that microbes arise spontaneously without parental organisms. Louis Pasteur produced data that refuted spontaneous generation. He showed that a broth boiled in a swan-neck flask remained free of microbial growth, despite being exposed to air.

Question 4

What is a Winogradsky column, and how does it reveal the complexity of bacterial communities?

Answer 4

A Winogradsky column is a model microbial ecosystem containing regions of enrichment where different microbes thrive. It shows the importance of bacteria and the unique roles microbes play in geochemical cycling.

Question 5

What is the plate count anomaly, and what implications does it have for understanding microbial communities?

Answer 5

The plate count anomaly refers to the fact that direct microscopic count of natural samples reveal far more organisms than those recoverable on plates. This implies the the diversity of microbial communities is underestimated. It highlights the limitation in culture-based methods for studying microbial communities

Question 6

How have next-generation sequencing technologies influenced the discovery of Archaea?

Answer 6

NGS technologies have accelerated the discover of Archaea by facilitating metagenomic studies revealing nover species in diverse habitats and providing genomic insights into their biology and evolution.

Question 7

What are the key differences between the cell envelopes of Gram-negative and Gram-positive bacteria?

Answer 7

Gram-positive:
- thick cell wall
- have glycosyl chains and S-layer
- multiple layers of peptidoglycan which are threaded by teichoic acids
- have one cell membrane

Gram negative:
- thin cell wall
- have LPS and porins
- have an inner and outer cell membrane
- thin layer (1-2 sheets) of peptidoglycan

Question 8

What are the fundamental differences between DNA and RNA?

Answer 8

DNA:
- is a double helix
- adenine pairs with thymine

RNA
- single stranded
- contains ribose sugar
- adenine pairs with uracil
- often folds back on itself to form complex hairpins and other secondary structures.

Question 9

How do environmental factors influence and control microbial growth?

Answer 9

environmental factors like temperature, pH, moisture, nutrient availability, oxygen levels, and osmotic pressure influence microbial growth. These factors determine whether microorganisms thrive, survive, or are inhibited in a given environment.
- Temperature: microorganisms have temperature ranges where they grow optimally. Psychrophiles grow in cold temps, and thermophiles in hot. Temperatures outside their ranges can inhibit microbial growth or even lead to death.
- pH: most microbes grow in neutral pH. Those that thrive in acidic environments are acidophiles, and those in alkaline are alkaliphiles. pH outside their optimal ranges can denature proteins and disrupt cellular processes
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Question 10

What are the risks associated with oxygen for organisms, and how do they employ defence strategies against these risks?

Answer 10

Oxygen is toxic to anaerobes, and to all cells that do not have the enzymes capable of efficiently destroying oxygen’s breakdown products (ROS).
some defence strategies employed by organisms include:
- antioxidant enzymes: catalase and peroxidase detoxify ROS
- anaerobic organisms use anaerobic (without oxygen) pathways to avoid ROS production

Question 11

Why is the 16S rRNA gene utilized as a key molecular tool for studying bacterial communities?

Answer 11

16S rRNA is utilized because the gene highly conserved, meaning certain regions of the gene are shared among diverse bacterial species.