Study Guide: Viruses and Bacteriophages in Microbiology

Viruses: Structure, Classification, and Comparison

Comparison of Viruses, Eukaryotic Cells, and Prokaryotic Cells

Understanding the differences between viruses, eukaryotic cells, and prokaryotic cells is fundamental in microbiology. These entities differ in cellular structure, metabolic capabilities, genetic material, and replication mechanisms.

• Cellular Structure: - Viruses are acellular; they lack cellular organization and do not possess organelles. - Eukaryotic cells have a nucleus and membrane-bound organelles. - Prokaryotic cells lack a nucleus and membrane-bound organelles.

• Considered Alive? - Viruses are not considered alive; they require host cells for replication. - Eukaryotes and prokaryotes are living organisms.

• Size: - Viruses: 20–300 nm (nanometers) - Prokaryotes: 0.5–5 μm (micrometers) - Eukaryotes: 10–100 μm

• Structure: - Viruses: Composed of genetic material (DNA or RNA), a protein capsid, and sometimes an envelope with spikes. - Prokaryotes/Eukaryotes: Have cell membranes, cytoplasm, and genetic material (DNA).

• Replication: - Viruses: Replicate only inside host cells using host machinery. - Cells: Replicate independently via cell division.

• Metabolism: - Viruses: No metabolism. - Cells: Exhibit metabolism.

• Genome Composition: - Viruses: DNA or RNA, single or double-stranded. - Cells: DNA, double-stranded.

Example: Influenza virus (RNA virus) vs. Escherichia coli (prokaryote) vs. human cell (eukaryote).

Virus Structure and Components

Main Components of a Virus

Viruses are composed of several key structural elements that determine their infectivity and host range.

• Genetic Material: DNA or RNA, single or double-stranded.

• Capsid: Protein shell that encases the genetic material; made of capsomeres.

• Envelope (optional): Lipid membrane derived from host cell; present in some viruses.

• Spikes: Glycoproteins protruding from the envelope or capsid; facilitate attachment to host cells.

Example: HIV has an RNA genome, a capsid, an envelope, and spike proteins (gp120).

Antigenic Shift and Drift

Impact on Influenza Virus Evolution and Outbreaks

Antigenic shift and drift are mechanisms by which influenza viruses evolve, leading to new strains and outbreaks.

• Antigenic Drift: Gradual accumulation of mutations in viral genes, especially those encoding surface proteins (e.g., hemagglutinin, neuraminidase).

• Antigenic Shift: Abrupt, major change due to reassortment of gene segments between different viral strains, resulting in new subtypes.

• Impact: Antigenic drift causes seasonal flu epidemics; antigenic shift can lead to pandemics.

Example: The 2009 H1N1 pandemic resulted from antigenic shift.

Host Range and Tropism

Significance of a Virus's Host Range and Tropism

Host range refers to the spectrum of hosts a virus can infect, while tropism describes the specificity for particular cell types or tissues.

• Host Range: Determined by the ability of the virus to attach and enter host cells.

• Tropism: Dictated by the presence of specific receptors on host cells and viral surface proteins.

• Significance: Influences disease spread, severity, and zoonotic potential.

Example: Rabies virus has a broad host range; HIV exhibits tropism for CD4+ T cells.

Bacteriophage Replication

Features of Bacteriophage Lytic Replication

The lytic cycle is a replication pathway in which bacteriophages destroy the host cell.

• Attachment: Phage binds to specific receptors on bacterial surface.

• Penetration: Phage injects its genetic material into the host.

• Biosynthesis: Host machinery synthesizes viral components.

• Assembly: New phage particles are assembled.

• Release: Host cell lyses, releasing new phages.

Example: T4 bacteriophage infecting E. coli.

Features of Bacteriophage Lysogenic Replication

The lysogenic cycle allows the phage genome to integrate into the host genome and replicate passively.

• Integration: Phage DNA incorporates into bacterial chromosome (prophage).

• Replication: Prophage replicates with host DNA during cell division.

• Induction: Environmental triggers can activate the lytic cycle.

Example: Lambda phage in E. coli.

Lytic vs. Latent Infections in Animal Viruses

Comparison and Examples

Animal viruses can cause either lytic (acute) or latent infections, affecting disease progression and transmission.

• Lytic (Acute) Infection: Rapid virus replication, cell lysis, and symptom onset. Example: Influenza virus infection.

• Latent Infection: Virus remains dormant in host cells, reactivating periodically. Example: Herpes simplex virus (HSV) infection.

Evolution Driven by Viruses

Selective Pressure and Innovation

Viruses drive evolution in host populations by exerting selective pressure and facilitating gene transfer.

• Selective Pressure: Hosts evolve resistance mechanisms; viruses adapt to overcome them.

• Innovation: Viruses can transfer genes between organisms (horizontal gene transfer), promoting genetic diversity.

Example: CRISPR-Cas systems in bacteria evolved as defense against phages.

Phage Conversion and Bacterial Pathogenicity

Role of Phage Conversion

Phage conversion occurs when a bacteriophage introduces new genes into a bacterium, altering its phenotype.

• Pathogenicity: Phage genes can encode toxins or virulence factors, making bacteria pathogenic.

Example: Corynebacterium diphtheriae produces diphtheria toxin only when infected by a specific phage.

Laboratory Growth of Bacteriophages and Animal Viruses

Methods and Plaque Assay

Bacteriophages and animal viruses are grown in the lab using specific host cells and techniques.

• Bacteriophages: Grown on bacterial lawns; plaques indicate areas of cell lysis.

• Animal Viruses: Cultured in cell lines, embryonated eggs, or live animals.

• Plaque Assay: Quantitative method to measure virus concentration; plaques are clear zones formed by virus-induced cell lysis.

Example: Counting plaques to determine phage titer in a sample.

Antiviral Drugs

Example and Mode of Action

Antiviral drugs inhibit various stages of the viral life cycle to prevent infection and replication.

• Example: Acyclovir (used against herpesviruses).

• Mode of Action: Acyclovir is a nucleoside analog that inhibits viral DNA polymerase, preventing viral DNA synthesis.

Additional info: Other antiviral drugs target entry, uncoating, or release of viruses (e.g., oseltamivir for influenza).

Summary Table: Comparison of Viruses, Prokaryotes, and Eukaryotes

Feature	Viruses	Prokaryotes	Eukaryotes
Cellular Structure	Acellular	Cellular (no nucleus)	Cellular (nucleus)
Considered Alive?	No	Yes	Yes
Size	20–300 nm	0.5–5 μm	10–100 μm
Genome	DNA or RNA	DNA	DNA
Metabolism	No	Yes	Yes
Replication	Host-dependent	Binary fission	Mitosis/meiosis