Viruses

Introduction to Viruses

• Today’s focus: an in-depth look at viruses in microbiology, contrasting with previous discussions on bacteria.

• Acknowledgment of prior talk’s lack of scientific focus.

Virus Shapes

• Discussion of various virus shapes:

  • Long thin tube shape

  • Short fat tube shape

  • Circular shape

  • Geodiscuit structure

  • Heads and tails

  • Airbrush-like appearance

• Major distinction: Naked vs. Enveloped viruses

  • Naked viruses: no biological membrane; consist solely of a nucleic acid core and protein coat.

  • Enveloped viruses: have a phospholipid bilayer membrane surrounding them.

Size of Viruses

• Comparison of sizes:

  • Human cell enlarged vs. bacteria vs. viruses.

• Highlighting that viruses are significantly smaller than bacteria; visible under electron microscopes but not with light microscopes.

• Notable examples:

  • Common cold virus

  • Parvovirus

Characteristics of Viruses

1. Nucleic Acid Content:

   • Viruses contain either DNA or RNA, NEVER both.

   • Can be single-stranded or double-stranded:

     • DNA can be either stranded.

     • RNA can also be either stranded.

2. Complexity and Examples:

   • HIV: Contains 9,000 nucleotides, codes for 9 genes (relatively simple).

   • Bacteriophage T4: Contains 169,000 bases of DNA, codes for over 300 genes (more complex).

   • Complexity explained: simpler viruses target complex hosts, relying on host cellular machinery.

3. Protein Coat:

   • Viruses consist of a protein coat that is a repeated single protein, assembled like Legos.

4. Sizes:

   • Typical virus size: 10 to 500 nanometers (10^-9 meters).

5. Replication:

   • Viruses can only replicate in actively growing cells (viable cells).

Types of Bacterial Viruses

• Focus on bacteriophages (viruses that infect bacteria).

• Division of bacteriophages by infection type:

  1. Productive infections: Virus invades, produces viral particles immediately.

     • Lytic:

     • Destroy the host bacteria from within after invasion.

     • Leaking:

     • Make bacteria sick but do not kill them.

  2. Latent infections: Virus enters bacteria, remains inactive until later.

Lytic Bacteriophages

Mechanism of Action

• Attachment:

  • Bacteriophage binds to bacterial surface; requires correct receptor on bacteria.

• Penetration:

  • Lysozyme from the phage tail punches a hole in the bacterial cell wall.

  • Only the viral DNA enters the bacterial cell; the protein coat remains outside (phage ghost).

• Replication:

  • Bacterial machinery replicates the viral DNA, producing viral parts and proteins.

• Assembly:

  • Parts reassemble into new viral particles, producing lysozyme to lyse the bacteria, resulting in release of new phages.

Summary of Lytic Cycle

• Step 1: Phage binds to specific bacteria.

• Step 2: Lysozyme punctures the cell wall.

• Step 3: Viral DNA is inserted, while the protein coat stays outside (phage ghost).

• Step 4: Viral DNA replicates; early proteins synthesized to inhibit bacterial defenses;

• Step 5: Late proteins assemble new phages; lysozyme destroys the host cell's wall; release of new phages.

Leaking Bacteriophages

Characteristics

• Contain single-stranded DNA; filamentous structure.

Mechanism of Action

• Attachment at the tip of bacterial pilus (not the membrane).

• Viral DNA travels through the pilus into the bacterial cell; replication occurs while the bacteria remains alive.

• Proteins produced are integrally part of the cell membrane.

• Results in a sick bacterium without immediate lysis.

• Adaptation in science: bacteriophages can potentially be used in antibiotic treatment against bacterial infections.

Latent Bacteriophages

Overview

• Also referred to as temperate/lysogenic bacteriophages, showing interchangeable terminology.

• Mechanism of Action:

  • Attach and insert DNA, which integrates into bacterial chromosome using the enzyme integrase.

  • Remains dormant until the bacterium is under stress.

  • Upon stress, the viral DNA can be excised (exquisase enzyme) and enter the lytic cycle.

Summary of Latent Cycle

• Step 1: Viral DNA is integrated into bacteria.

• Step 2: Bacteria divides, copies both its and the viral DNA.

• Step 3: Stress condition triggers excision of viral DNA.

• Step 4: Viral DNA induces production of new viral particles leading to lysis of the host.

Transduction

Generalized Transduction

• A phage mistakenly packages bacterial DNA instead of its own, potentially transferring genes to other bacteria.

Specialized Transduction

• Only lysogenic phages can transfer specific parts of bacterial DNA due to precise excision inaccuracies.

• Importance of understanding both forms for genetic exchange in bacteria.

Classification of Human Viruses

Categories for Human Infection

1. Enteric Viruses:

   • Transmitted via fecal-oral route; e.g., polio virus.

2. Respiratory Viruses:

   • Enter through respiratory routes; e.g., influenza and common cold viruses.

3. Zoonotic Viruses:

   • Cross species barriers; e.g., rabies and West Nile virus.

4. Sexually Transmitted Viruses:

   • E.g., herpes and HIV; impact on genital tract lesions and systemic disease progression.

Conclusion

• A promise to continue discussing animal viruses in the next session, diving into human pathogenic mechanisms.

• Review prompts for students on quiz tasks and feedback.