5.2 Parasitic Nature of Virus

Introduction

Viruses are obligate intracellular parasites. This means they cannot reproduce or carry out any metabolic activity outside a living host cell. They must infect a host cell and use its machinery to replicate. Viruses are highly specific to their hosts, and this specificity determines their host range. Understanding the parasitic nature of viruses is essential for understanding how they cause disease and how they can be controlled.

Definition

Parasitic Nature of Virus: Viruses are parasitic in nature. They are highly specific to their host. They are obligate intracellular parasites, which means they cannot multiply outside a living cell.

In simple words: Viruses are parasites that must live inside a host cell to survive and reproduce.

Core Concept

• Viruses are parasitic in nature.
• They are highly specific to their host.
• They are obligate intracellular parasites.
• They cannot multiply outside a living cell.
• They infect all sorts of cells, from bacterial cells to human cells.
• An isolated virus is unable to reproduce or do anything else except infect an appropriate host cell.
• Viruses lack metabolic enzymes, ribosomes, etc., for making proteins.
• Therefore, they need a host cell to complete their life cycles.

Detailed Explanation

Host Specificity of Viruses

Viruses are highly specific to their host. This means that a particular virus can only infect a specific type of host.

Examples of Host Specificity:

• Viruses have protein spikes on their surfaces.
• These spikes help them to attach to specific receptors on the host cells.
• The specificity of attachment determines the host range of the virus.
• Some viruses have a narrow range (e.g., poliovirus can enter only humans and other primates).
• Some viruses have a broad range (e.g., rabies virus can enter all mammalian cells).

Viruses Must Require Host Cell to Complete Life Cycle

Definition: A virus must have a host cell to parasitize in order to complete its life cycle because viruses are obligate intracellular parasites.

• Viruses are obligate intracellular parasites.
• They cannot multiply outside a living cell.
• Viruses infect all sorts of cells, from bacterial cells to human cells.
• An isolated virus is unable to reproduce or do anything else except infect an appropriate host cell.
• Viruses lack metabolic enzymes, ribosomes, etc., for making proteins.
• Therefore, they need a host cell to complete their life cycles.

Why Viruses Need a Host Cell:

Survival of Viruses Inside a Host Cell

Definition: When a virus infects an organism, it enters inside the host cell. It hides from the immune system of the host.

• The host immune system is unable to find the virus.
• The virus rapidly changes its own genetic constitution so the vaccines or antibodies of the host against them become ineffective.
• When the virus enters a cell, the capsid protects the viral genome from nucleases of the host cell.
• Nucleases are the enzymes found in all cells that break down DNA or RNA.

How Viruses Hide from the Immune System:

Viruses Can Pass Unfavourable Conditions Outside the Host

• Viruses, unlike other "living" organisms, do not need food to survive.
• They remain dormant or inactive outside the host body.
• Being tiny, they are not threatened by other microorganisms.
• Their only concern would be the pH and temperature, as these would denature the protein.
• The virulence of the virus outside the host is maintained for a certain period of time.
• The time period depends on what virus it is or the pH and temperature of the medium.
• Viruses outside the host are non-living.
• They go under a dormant period as they do not have any metabolic activity outside a living host.
• However, the genome of the virus remains viable for a long time (in inactive form) outside the host.

Survival of Viruses Outside the Host:

• Non-enveloped viruses have a tough capsid.
• Enveloped viruses have a lipid envelope.
• The lipid envelope is sensitive to degradation by sunlight and normal cleaning procedures.
• Therefore, enveloped viruses survive for shorter periods.

Summary of Parasitic Nature of Viruses

Intermediate Level

Host Specificity in Detail

Why Viruses Need a Host Cell

Survival Inside the Host Cell

Survival Outside the Host

Important Points

• 1. Viruses are parasitic in nature.
• 2. They are highly specific to their host.
• 3. Bacteriophage infects only bacteria, the tobacco mosaic virus infects only tobacco plants, and rabies virus infects only mammals.
• 4. Some human viruses even specialize in a particular tissue.
• 5. HIV will enter only certain types of white blood cells, the poliovirus reproduces in spinal nerve cells, and hepatitis viruses infect only liver cells.
• 6. Human cold viruses infect only the cells lining the upper respiratory tract.
• 7. Actually, viruses have protein spikes on their surfaces which help them to attach with specific receptors on the host cells.
• 8. The specificity of attachment determines the host range of the virus.
• 9. Some viruses have a narrow range, whereas others have quite a broad range.
• 10. For example, poliovirus can enter the cells of only humans and other primates whereas rabies virus can enter all mammalian cells.
• 11. Viruses must require host cell to complete life cycle:
• - A virus must have a host cell to parasitize in order to complete its life cycle because viruses are obligate intracellular parasites, which means they cannot multiply outside a living cell.
• - Viruses infect all sorts of cells, from bacterial cells to human cells.
• - An isolated virus is unable to reproduce or do anything else except infect an appropriate host cell.
• - Viruses lack metabolic enzymes, ribosomes, etc., for making proteins.
• - Therefore, they need host cell to complete their life cycles.
• 12. Survival of viruses inside a host cell:
• - When virus infects an organism, it enters inside host cell.
• - It hides from the immune system of host.
• - The host immune system is unable to find the virus.
• - Virus rapidly changes its own genetic constitution so the vaccines or antibodies of host against them become ineffective.
• - When virus enters a cell the capsid protects the viral genome from nucleases of the host cell.
• - Nucleases are the enzymes found in all cells.
• - The breakdown DNA or RNA.
• 13. Viruses can pass unfavourable conditions outside the host:
• - Viruses unlike other "living" organisms do not need food to survive.
• - They remain dormant or inactive outside the host body.
• - Being tiny, they are not threatened by the other microorganisms.
• - Their only concern would be the pH and temperature, as these would denature the protein.
• - The virulence of the virus outside the host is maintained for a certain period of time and the time period depends on what virus is or pH and temperature of the medium.
• - Viruses outside the host are non-living.
• - It can be said that they go under dormant period as they do not have any metabolic activity outside living host, however, the genome of the virus remains viable for long time (in inactive form) outside the host.
• - Non-enveloped viruses can in fact survive for long periods outside the host (up to several days) whereas enveloped viruses survive for shorter time periods.
• - This is because many enveloped viruses rely on the proteins on the surface of the membrane to attach to the host cell, this envelope is generally sensitive to degradation to sunlight and normal cleaning procedures.

Remember

• - Viruses = obligate intracellular parasites
• - Must have a host cell to reproduce
• - Host specificity = viruses infect specific hosts
• - Protein spikes = help attach to host receptors
• - Capsid = protects the viral genome
• - Dormant = viruses are inactive outside the host
• - Non-enveloped viruses = survive longer outside the host
• - Enveloped viruses = survive shorter outside the host
• - Examples: Bacteriophage (bacteria), TMV (tobacco), Rabies (mammals), HIV (helper T cells), Poliovirus (spinal nerve cells), Hepatitis viruses (liver cells)

Common Mistakes

• 1. Thinking viruses can reproduce outside a host – Viruses are obligate intracellular parasites; they cannot reproduce outside a host cell.
• 2. Thinking viruses are not specific – Viruses are highly specific to their host.
• 3. Confusing host range and tissue specificity – Host range = which organisms a virus can infect; Tissue specificity = which cells in the body a virus can infect.
• 4. Thinking all viruses survive equally outside the host – Non-enveloped viruses survive longer than enveloped viruses.
• 5. Forgetting the role of protein spikes – Protein spikes help in attachment to host receptors.
• 6. Thinking the immune system can always find viruses – Viruses can hide inside host cells and evade the immune system.
• 7. Confusing nucleases and capsid – Nucleases are host enzymes that break down DNA/RNA; Capsid protects the viral genome from nucleases.
• 8. Thinking viruses need food to survive – Viruses do not need food; they are dormant outside the host.
• 9. Forgetting the role of pH and temperature – pH and temperature can denature viral proteins.
• 10. Thinking all viruses infect all cells – Viruses are specific to certain hosts and certain cells.
• 11. Confusing the terms "host range" and "tissue specificity" – Host range = organism level; Tissue specificity = cell level.
• 12. Thinking viruses are always harmful – Some viruses are beneficial (e.g., in gene therapy).

Quick Revision

• Viruses are parasitic in nature.
• They are highly specific to their host.
• Bacteriophage infects only bacteria, the tobacco mosaic virus infects only tobacco plants and rabies virus infects only mammals.
• Some human viruses even specialize in a particular tissue.
• HIV will enter only certain types of white blood cells, the poliovirus reproduces in spinal nerve cells, and hepatitis viruses infect only liver cells.
• Human cold viruses infect only the cells lining the upper respiratory tract.
• Actually viruses have protein spikes on their surfaces which help them to attach with specific receptors on the host cells.
• The specificity of attachment determines the host range of the virus.
• Some viruses have a narrow range, whereas others have quite a broad range.
• For example, poliovirus can enter the cells of only humans and other primates whereas rabies virus can enter all mammalian cells.
• Viruses must require host cell to complete life cycle:
• A virus must have a host cell to parasitize in order to complete its life cycle because viruses are obligate intracellular parasites, which means they cannot multiply outside a living cell.
• Viruses infect all sorts of cells, from bacterial cells to human cells.
• An isolated virus is unable to reproduce or do anything else except infect an appropriate host cell.
• Viruses lack metabolic enzymes, ribosomes, etc., for making proteins.
• Therefore, they need host cell to complete their life cycles.
• Survival of viruses inside a host cell:
• When virus infects an organism, it enters inside host cell.
• It hides from the immune system of host.
• The host immune system is unable to find the virus.
• Virus rapidly changes its own genetic constitution so the vaccines or antibodies of host against them become ineffective.
• When virus enters a cell the capsid protects the viral genome from nucleases of the host cell.
• Nucleases are the enzymes found in all cells.
• The breakdown DNA or RNA.
• Viruses can pass unfavourable conditions outside the host:
• Viruses unlike other "living" organisms do not need food to survive.
• They remain dormant or inactive outside the host body.
• Being tiny, they are not threatened by the other microorganisms.
• Their only concern would be the pH and temperature, as these would denature the protein.
• The virulence of the virus outside the host is maintained for a certain period of time and the time period depends on what virus is or pH and temperature of the medium.
• Viruses outside the host are non-living.
• It can be said that they go under dormant period as they do not have any metabolic activity outside living host, however, the genome of the virus remains viable for long time (in inactive form) outside the host.
• Non-enveloped viruses can in fact survive for long periods outside the host (up to several days) whereas enveloped viruses survive for shorter time periods.
• This is because many enveloped viruses rely on the proteins on the surface of the membrane to attach to the host cell, this envelope is generally sensitive to degradation to sunlight and normal cleaning procedures.

60-Second Revision

• Viruses = obligate intracellular parasites
• Must have a host cell to reproduce
• Host specificity = viruses infect specific hosts
• Protein spikes = help attach to host receptors
• Capsid = protects the viral genome
• Dormant = viruses are inactive outside the host
• Non-enveloped viruses = survive longer outside the host
• Enveloped viruses = survive shorter outside the host
• Examples: Bacteriophage (bacteria), TMV (tobacco), Rabies (mammals), HIV (helper T cells), Poliovirus (spinal nerve cells), Hepatitis viruses (liver cells)
• Warning: Viruses cannot reproduce outside a host. They are highly specific. Non-enveloped viruses survive longer outside the host than enveloped viruses. Protein spikes help in attachment. Capsid protects the viral genome from nucleases. Viruses hide from the immune system. pH and temperature affect viral survival. Viruses do not need food. They are dormant outside the host.