Viruses vs. Bacteria

Structural and Biological Differences

Viruses and bacteria are fundamentally different in their structure, replication, and biological properties. Understanding these differences is essential for microbiology students.

• Bacteria:

  • Living cells considered as living organisms.

  • Cell structure: Possess a cell wall, cell membrane, cytoplasm, and DNA that floats freely inside.

  • Extra parts: Flagella (for movement), Pili (for attachment).

  • Size: Larger than viruses (typically 0.2–10 μm).

  • Reproduction: Independent (binary fission).

  • Antibiotics: Often effective against bacteria.

• Viruses:

  • Not considered living cells; they are particles, not complete cells.

  • Structure: Very simple; consist of genetic material (DNA or RNA) and a protein coat called a capsid. Some have an outer envelope made of fats.

  • Size: Much smaller than bacteria (20–300 nm).

  • Reproduction: Cannot reproduce on their own; must infect a living cell and use the cell’s machinery.

  • Antibiotics: Ineffective against viruses.

Key Point: Bacteria are living cells; viruses are non-living particles that require a host to replicate.

Viral Structure

Capsid and Envelope

The structure of viruses is designed to protect their genetic material and facilitate infection of host cells.

• Capsid:

  • Protein coat surrounding the viral genome.

  • Made of repeating protein subunits called capsomeres.

  • Protects nucleic acid from degradation and helps attach to host cells.

• Envelope:

  • Lipid membrane derived from the host cell during viral release (budding).

  • Contains glycoprotein spikes for host cell attachment.

  • Not all viruses have an envelope; non-enveloped viruses rely on capsid proteins for attachment.

Morphology

• Helical: Rod-shaped or filamentous capsid (e.g., Tobacco mosaic virus).

• Icosahedral: Spherical, 20-sided polyhedral capsid (e.g., Adenovirus).

• Complex: Combination of shapes, often with additional structures like tails (e.g., Bacteriophage).

Glycoprotein Spikes

• Composed of carbohydrate-protein complexes.

• Functions:

  • Attachment to host receptors.

  • May induce hemagglutination (clumping of red blood cells).

Classification of Viruses

Criteria for Classification

Viruses are classified based on several structural and genetic features.

• Type of nucleic acid: DNA or RNA, single-stranded (ss) or double-stranded (ds).

• Capsid symmetry and morphology: Helical, icosahedral, or complex.

• Number of capsomeres: Structural subunits of the capsid.

• Size of the virion.

Not used for viral classification: Biochemical tests (used for bacteria).

Viral Species: Group of viruses with the same genetic material and ecological niche.

Nomenclature: Viruses are not named using binomial nomenclature.

Viral Replication

Steps of Viral Multiplication

Viral replication involves several key steps, which may differ between DNA and RNA viruses.

1. Attachment (Adsorption): Viral glycoprotein spikes bind to specific receptors on the host cell membrane.

2. Penetration: Virus enters the host cell via:

   • Endocytosis (host cell engulfs the virus)

   • Fusion (enveloped viruses merge with host membrane)

   • Injection (bacteriophages inject nucleic acid into bacteria)

3. Uncoating: Viral capsid is removed, releasing nucleic acid into the cytoplasm.

4. Biosynthesis: Host cell machinery synthesizes viral nucleic acids and proteins.

5. Maturation/Assembly: Viral components self-assemble into new virions.

6. Release:

   • Enveloped viruses: Bud from host membrane, acquiring lipid envelope.

   • Non-enveloped viruses: Cause cell lysis.

DNA Virus Replication

• Replicate in the nucleus using host enzymes for transcription and translation.

• Steps: DNA synthesis → transcription → translation → assembly.

• Examples: Herpesviridae, Adenoviridae, Hepadnaviridae.

RNA Virus Replication

• Positive-sense ssRNA (+): Acts directly as mRNA for protein synthesis.

• Negative-sense ssRNA (−): Complementary RNA (+) must first be synthesized for translation.

• Retroviruses: Carry reverse transcriptase to convert RNA → DNA, which is then integrated into the host genome.

• Examples: Picornaviridae (ssRNA +), Rhabdoviridae (ssRNA −), Retroviridae (ssRNA + with reverse transcription).

Bacteriophage Replication

• Lytic cycle: Virus replicates and lyses the host cell.

• Can confer immunity to reinfection by the same phage.

• May allow specialized transduction (transfer of host genes).

• Does not immediately kill host cells in some cases (lysogeny).

Viral Infections

Types of Infection

Viruses can cause different types of infections, each with distinct characteristics and outcomes.

Persistent Infections

• Virus persists in equilibrium with host without immediate disease.

• Slow replication, may eventually cause disease (e.g., HIV infection).

Oncogenic Viruses

• Contain oncogenes or viral genes that can transform host cells.

• Transforming properties include chromosomal rearrangements and tumor formation.

• Examples: HPV, Epstein-Barr Virus.

Special Viral Agents

Viroids

• Infectious RNA molecules without a protein coat.

• Primarily infect plants, causing stunted growth and disease.

• Enter cells via wounds or vectors like insects.

Prions

• Infectious proteins without nucleic acid (PrPSc).

• Cause neurodegenerative diseases like Creutzfeldt-Jakob disease and Bovine spongiform encephalopathy.

Immune Evasion and Variation

Antigenic Variation

Viruses can evade the host immune system through changes in their surface proteins.

• Antigenic drift: Small mutations cause minor changes in viral genome and surface proteins.

• Antigenic shift: Major changes due to genetic reassortment, leading to new viral strains (e.g., influenza).

Laboratory Techniques

Virus Cultivation

• Viruses are cultured in living cells (cell cultures, embryonated eggs, or laboratory animals).

• Methods not suitable: Standard bacterial media (viruses require living cells).

Plaques

• Clear zones on cell cultures where viruses have lysed host cells.

Key Terms

• Plaque: Area of cell destruction due to viral infection in a cell culture.

• Continuous cell line: Cell culture that can be maintained indefinitely.

• Prophage: Bacteriophage DNA integrated into the host genome.

• Viroid: Infectious RNA molecule without a protein coat.

• Prion: Infectious protein causing neurodegenerative disease.

• Transformation (viral): Genetic change in a cell due to uptake of viral DNA.

Comparison Table: Viruses vs. Bacteria

Important Equations and Processes

• DNA Virus Replication:

• RNA Virus Replication (Retrovirus):

Additional info: These notes expand on the provided study guide and notes, offering definitions, examples, and structured tables for clarity. The content is suitable for college-level microbiology exam preparation, focusing on Chapter 13: Viruses, as outlined in standard microbiology textbooks.