Microbiology Study Guide: Viruses and Bacteriophages

Study Guide - Smart Notes

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Q1. What are the living and non-living characteristics of viruses?

Background

Topic: Nature of Viruses

This question tests your understanding of the unique properties of viruses and how they differ from living organisms and non-living matter.

Key Terms:

Living characteristics: Traits that viruses share with living organisms (e.g., genetic material, ability to evolve).
Non-living characteristics: Traits that make viruses similar to non-living things (e.g., lack of metabolism, inability to reproduce independently).

Step-by-Step Guidance

List features that viruses share with living organisms, such as containing DNA or RNA and the ability to mutate and evolve.
Identify features that make viruses non-living, such as the absence of cellular structure and metabolism.
Consider how viruses replicate and whether they require a host cell to do so.
Think about whether viruses can carry out life processes independently.

Try solving on your own before revealing the answer!

Final Answer:

Living characteristics: Viruses contain genetic material (DNA or RNA), can evolve through mutation, and can reproduce, but only inside a host cell.

Non-living characteristics: Viruses lack cellular structure, do not have metabolism, cannot reproduce independently, and do not respond to stimuli outside a host.

Viruses are considered obligate intracellular parasites because they require a host cell to carry out life processes.

Q2. How big are viruses relative to bacteria?

Background

Topic: Size Comparison

This question tests your knowledge of the relative sizes of viruses and bacteria, which is important for understanding microscopy and filtration techniques.

Key Terms:

Micrometer (µm): Unit of length commonly used for bacteria.
Nanometer (nm): Unit of length commonly used for viruses.

Step-by-Step Guidance

Recall the typical size range for bacteria (in micrometers).
Recall the typical size range for viruses (in nanometers).
Convert units if necessary to compare (1 µm = 1000 nm).
Think about how much smaller viruses are compared to bacteria.

Try solving on your own before revealing the answer!

Final Answer:

Viruses are much smaller than bacteria. Most bacteria are 1–10 µm in size, while most viruses are 20–300 nm (0.02–0.3 µm).

This means viruses are typically 10 to 100 times smaller than bacteria.

Q3. What does the structure of viruses consist of? What are capsids, capsomers, nucleocapsids?

Background

Topic: Viral Structure

This question tests your understanding of the basic components that make up a virus and the terminology used to describe viral architecture.

Key Terms:

Capsid: Protein shell that surrounds the viral genome.
Capsomer: Subunit of the capsid.
Nucleocapsid: The combination of the viral genome and the capsid.

Step-by-Step Guidance

Identify the main structural components of a virus (genetic material and protein coat).
Define what a capsid is and its function.
Explain what capsomers are and how they relate to the capsid.
Describe what is meant by the nucleocapsid.

Try solving on your own before revealing the answer!

Final Answer:

The structure of viruses consists of genetic material (DNA or RNA) surrounded by a protein coat called the capsid. The capsid is made up of protein subunits called capsomers. The nucleocapsid refers to the combination of the viral genome and the capsid.

Q4. What type of nucleic acids do viruses have?

Background

Topic: Viral Genomes

This question tests your knowledge of the diversity of viral genetic material.

Key Terms:

DNA viruses: Viruses with DNA as their genetic material.
RNA viruses: Viruses with RNA as their genetic material.
Single-stranded (ss) or double-stranded (ds): Refers to whether the nucleic acid is single or double stranded.

Step-by-Step Guidance

Recall that viruses can have either DNA or RNA as their genetic material.
Consider whether the nucleic acid can be single-stranded or double-stranded.
Think about whether both DNA and RNA can be found in the same virus (hint: no).

Try solving on your own before revealing the answer!

Final Answer:

Viruses can have either DNA or RNA as their genetic material, but never both. The nucleic acid can be single-stranded or double-stranded, and can be linear or circular.

Q5. What are naked viruses, enveloped viruses, bacteriophages?

Background

Topic: Types of Viruses

This question tests your understanding of different virus categories based on structure and host specificity.

Key Terms:

Naked virus: Virus without an envelope; only has a nucleocapsid.
Enveloped virus: Virus with a lipid membrane surrounding the nucleocapsid.
Bacteriophage: Virus that infects bacteria.

Step-by-Step Guidance

Define what a naked virus is and what it lacks compared to enveloped viruses.
Describe what an envelope is and how enveloped viruses acquire it.
Explain what bacteriophages are and their host range.

Try solving on your own before revealing the answer!

Final Answer:

Naked viruses lack a lipid envelope and consist only of a nucleocapsid. Enveloped viruses have an outer lipid membrane derived from the host cell. Bacteriophages are viruses that specifically infect bacteria.

Q6. Know the general characteristics of bacteriophages, including replication through lytic cycle.

Background

Topic: Bacteriophage Biology

This question tests your understanding of bacteriophage structure, host specificity, and the steps of the lytic replication cycle.

Key Terms:

Bacteriophage: Virus that infects bacteria.
Lytic cycle: Replication process resulting in host cell lysis and release of new phages.

Step-by-Step Guidance

List the main structural features of bacteriophages (e.g., head, tail, tail fibers).
Describe the steps of the lytic cycle: attachment, penetration, biosynthesis, maturation, and release.
Explain what happens to the host cell during the lytic cycle.
Consider how the lytic cycle differs from the lysogenic cycle (if relevant).

Try solving on your own before revealing the answer!

Final Answer:

Bacteriophages have a head (capsid), tail, and tail fibers. In the lytic cycle, they attach to a bacterial cell, inject their DNA, hijack the host machinery to make new phages, assemble them, and then lyse the cell to release progeny phages.

Q7. What is phage typing? How can you demonstrate phage specificity?

Background

Topic: Phage Typing and Specificity

This question tests your understanding of how bacteriophages can be used to identify bacterial strains and the concept of host specificity.

Key Terms:

Phage typing: Method to identify bacteria based on susceptibility to specific phages.
Phage specificity: The ability of a phage to infect only certain bacterial strains.

Step-by-Step Guidance

Define phage typing and its purpose in microbiology.
Explain how phage specificity is determined experimentally.
Describe what results you would expect if a bacterium is sensitive or resistant to a phage.

Try solving on your own before revealing the answer!

Final Answer:

Phage typing is a technique to identify bacterial strains based on which bacteriophages can infect them. Phage specificity is demonstrated by exposing bacteria to different phages and observing which cause lysis (clear zones or plaques).

Q8. Be able to identify bacteria sensitive or insensitive to a particular phage based on a phage typing plate or picture.

Background

Topic: Interpreting Phage Typing Results

This question tests your ability to analyze experimental results from phage typing plates.

Key Terms:

Sensitive: Bacteria that are lysed by a phage (plaques form).
Insensitive (resistant): Bacteria that are not lysed (no plaques).
Plaque: Clear zone on a bacterial lawn where phages have lysed cells.

Step-by-Step Guidance

Examine the phage typing plate for clear zones (plaques) on the bacterial lawn.
Determine which areas correspond to phage application.
Identify which bacterial strains show plaques (sensitive) and which do not (insensitive).

Try solving on your own before revealing the answer!

Final Answer:

Bacteria that are sensitive to a phage will show clear plaques where the phage was applied. Insensitive bacteria will have no plaques and the lawn will remain intact.

Q9. How do you demonstrate phage typing in the lab? What results can you expect to see?

Background

Topic: Laboratory Techniques in Phage Typing

This question tests your understanding of the experimental procedure for phage typing and expected outcomes.

Key Terms:

Bacterial lawn: Even layer of bacteria grown on agar.
Plaque: Clear area where phage has lysed bacteria.

Step-by-Step Guidance

Describe how to prepare a bacterial lawn on an agar plate.
Explain how to apply different phages to designated areas of the plate.
Discuss incubation and observation of results (plaque formation).
Predict what you would see if bacteria are sensitive or insensitive to the phages.

Try solving on your own before revealing the answer!

Final Answer:

To demonstrate phage typing, spread bacteria on an agar plate, apply phage drops, incubate, and look for plaques. Plaques indicate sensitivity; no plaques indicate resistance.

Q10. What is a plaque? Be able to identify it on a lawn culture.

Background

Topic: Plaque Formation

This question tests your understanding of what a plaque is and how to recognize it in a laboratory setting.

Key Terms:

Plaque: Clear zone on a bacterial lawn caused by phage-induced lysis.
Lawn culture: Uniform growth of bacteria on an agar plate.

Step-by-Step Guidance

Define what a plaque is in the context of bacteriophage experiments.
Describe how plaques appear visually on a bacterial lawn.
Explain why plaques form where they do.

Try solving on your own before revealing the answer!

Final Answer:

A plaque is a clear, circular area on a bacterial lawn where phages have lysed the bacteria. It appears as a spot with no bacterial growth.

Q11. How did you perform a plaque count? Know the procedure.

Background

Topic: Plaque Assay Technique

This question tests your understanding of the steps involved in counting plaques to estimate phage concentration.

Key Terms:

Plaque assay: Method to quantify phage particles by counting plaques.
Serial dilution: Stepwise dilution of a phage sample to obtain countable plaques.

Step-by-Step Guidance

Describe how to prepare serial dilutions of a phage sample.
Explain how to mix diluted phage with host bacteria and pour onto agar plates.
Discuss incubation and counting of plaques on plates with 30–300 plaques.
Note the importance of recording the dilution factor for calculations.

Try solving on your own before revealing the answer!

Final Answer:

To perform a plaque count, make serial dilutions of the phage, mix with host bacteria, pour onto agar, incubate, and count plaques on plates with 30–300 plaques. Record the dilution used for calculations.

Q12. Know how to calculate PFUs from a given plate.

Background

Topic: Calculating Plaque-Forming Units (PFUs)

This question tests your ability to use plaque counts and dilution factors to calculate the concentration of phages in a sample.

Key Formula:

Step-by-Step Guidance

Count the number of plaques on a plate with 30–300 plaques.
Identify the dilution factor used for that plate.
Note the volume of diluted phage plated (in mL).
Set up the formula for PFU/mL using the values above, but do not calculate the final value yet.

Try solving on your own before revealing the answer!

Final Answer:

To calculate PFU/mL, use the formula:

Plug in your plaque count, dilution factor, and plated volume to get the phage concentration.