Chapter 3 More Medical Microbiology Specialties

Virology

Virus Evolution video

Viral Structure and Function

Viruses are acellular, meaning they are biological entities that do not have a cellular structure. Therefore, they lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane.

A virion consists of a nucleic acid core, an outer protein coating or capsid, and sometimes (notably among viruses that infect animal cells) an outer envelope made of protein and phospholipid membranes derived from the host cell.

Viruses may also contain additional proteins, such as enzymes. The most obvious difference between members of viral families is their morphology, which is quite diverse. 

Structure and Function of Viruses

Viruses are tiny and smaller in its size, ranging between 30-50 nm. Viruses do not contain cells and usually lack a cell wall but are surrounded by a protective protein coating called the capsid. It can be seen as a genetic element and is characterized by the combined evolution of the virus and the host. They contain either RNA or DNA as the genetic material.

Viruses mainly depend on a host to deliver the complex metabolic machinery of prokaryotic or eukaryotic cells for propagation. The main task of the virus is to carry its DNA or RNA genome to the host cell, which then can be transcribed by the host cell. The viral genome structure is packed in a capsulated symmetric protein. The protein associated with nucleic acid (also known as nucleoprotein) produces the nucleocapsid with the genome.

Bacteriophage and HIV

These microbes belong to the family viridae and genus virus.  Viruses could not be placed in any of the kingdoms because they are practically neither living nor dead. 

Once a susceptible cell is infected, a virus can start the cell machinery to generate more virus. Viruses are composed of a core of DNA or RNA surrounded by a protein coat. They are very small and their size ranges from 20 nanometers to 250 nanometers. Therefore, they can only be seen with an electron microscope

Many viruses have either DNA or RNA as the genetic element and the nucleic acid with single or double strands. The whole infectious virus, called as virion has nucleic acid and an outer shell of proteins. The simplest virus includes DNA or RNA for encoding four proteins and the most complex encodes 100-200 proteins.

Properties of Viruses

1. They are non-cellular organisms, which is enclosed in a protective envelope.

2. The presence of spikes helps in attaching the viruses to the host cell.

3. These viruses do not grow, neither respire nor metabolize, but they reproduce.

4. They are surrounded by a protein coat – capsid and have a nucleic acid core comprising DNA or RNA.

5. They are considered both as living and non-living things. These viruses are inactive when they are present outside of host cells, but become active within host cells. These viruses cause several infections and reproduce within the host cell by using the enzymes and raw materials.

Classification of Viruses

Viruses can be classified primarily on their phenotypic characteristics, core content, chemical composition, capsid structure, size, shape, modes of replication and other viral genome structures.

The below virus information describes the classification of viruses based on their different criteria.

Classification based on the presence of nucleic acid

DNA virus

The virus, having DNA as its genetic material. There are two different types of DNA virus

Single-stranded (ss) DNA virus: e.g. Picornaviruses, Parvovirus,  etc.

Double-stranded (ds) DNA virus: e.g. Adenovirus, Herpes virus, etc.

RNA virus

The virus, having RNA as its genetic material. There are two different types of RNA virus

Double-stranded (ds) RNA virus: e.g. Reovirus, etc.

Single-stranded (ss) RNA virus. It is further classified into two Positive sense RNA (+RNA) and negative sense RNA (-RNA).

Poliovirus, Hepatitis A, Rabies virus, Influenza virus are examples of single-stranded RNA virus.

Classification based on the replication properties and site of replication

Here, viruses invade into the host cell, where it replicates and assembly within the cell organelles.

1. Replication within the cytoplasm of the host cell.
   E.g. All RNA viruses except the Influenza virus.

2. Replication within the nucleus and the cytoplasm of the host cell.
   E.g. Influenza virus, Poxvirus, etc.

3. Replication within the nucleus of the host cell.
   All DNA viruses except Pox virus.

4. Replication of the virus through the double-stranded DNA intermediate.
   E.g. All DNA viruses, Retrovirus and some tumour causing RNA virus.

5. Replication of the virus through a single-stranded RNA intermediate.
   E.g. All RNA viruses except Reovirus and tumour-causing RNA viruses.

Classification based on the host range

Based on the type of host, there are four different types of viruses:

Animal viruses

These viruses infect by invading the cells of animals, including humans. Prominent examples of animal viruses include the influenza virus, mumps virus, rabies virus, poliovirus,  Herpes virus, etc.

Plant viruses

These viruses infect plants by invading the plant cells. Replication of plant viruses is obligate and does not happen without a host. Well-known examples of plant virus include the potato virus, tobacco mosaic virus (TMV), beet yellow virus, and turnip yellow virus, cauliflower mosaic virus, etc.

Bacteriophage

The virus which infects bacterial cells is known as bacteriophage. There are many varieties of bacteriophages, such as DNA virus,  MV-11, RNA virus, etc.

Bacteriophage

Virus Reproduction

Lytic infection is the method used by the majority of viruses to reproduce. A virus penetrates the host cell during lytic infection, replicates, and causes the cell to lyse or explode.

It follows similar infection stages to bacteriophages: attachment, penetration, biosynthesis, maturation, and release

Overview of the lytic cycle:

• Attachment: Virus binds to host cell during attachment.

• Entry: Injection of genetic material into the host cell.

• Replication: The virus takes control of the host cell’s metabolism, leading the organelles to produce new proteins and nucleic acids.

• Assembly: Nucleic acids and proteins are put together to form new viruses during assembly.

• Release: Viral enzymes induce the host cell to burst, releasing viruses into the surrounding environment. These novel viruses are capable of infecting other cells.

Persistent Infections

Persistent infection occurs when a virus is not completely cleared from the system of the host but stays in certain tissues or organs of the infected person. The virus may remain silent or undergo productive infection without seriously harming or killing the host.

Mechanisms of persistent infection may involve the regulation of the viral or host gene expressions or the alteration of the host immune response. The two primary categories of persistent infections are latent infection and chronic infection.

Examples of viruses that cause latent infections include herpes simplex virus (oral and genital herpes), varicella-zoster virus (chickenpox and shingles), and Epstein-Barr virus (mononucleosis). Hepatitis C virus and HIV are two examples of viruses that cause long-term chronic infections.

Latent Infection

Not all animal viruses undergo replication by the lytic cycle. There are viruses that are capable of remaining hidden or dormant inside the cell in a process called latency. These types of viruses are known as latent viruses and may cause latent infections. Viruses capable of latency may initially cause an acute infection before becoming dormant.

For example, the varicella-zoster virus infects many cells throughout the body and causes chickenpox, characterized by a rash of blisters covering the skin. About 10 to 12 days post-infection, the disease resolves and the virus goes dormant, living within nerve-cell ganglia for years.

During this time, the virus does not kill the nerve cells or continue replicating. It is not clear why the virus stops replicating within the nerve cells and expresses few viral proteins but, in some cases, typically after many years of dormancy, the virus is reactivated and causes a new disease called shingles. While chickenpox affects many areas throughout the body, shingles is a nerve cell-specific disease emerging from the ganglia in which the virus was dormant.

Chronic Infection

A chronic infection is a disease with symptoms that are recurrent or persistent over a long time. Some viral infections can be chronic if the body is unable to eliminate the virus.

HIV is an example of a virus that produces a chronic infection, often after a long period of latency. Once a person becomes infected with HIV, the virus can be detected in tissues continuously thereafter, but untreated patients often experience no symptoms for years.

However, the virus maintains chronic persistence through several mechanisms that interfere with immune function, including preventing expression of viral antigens on the surface of infected cells, altering immune cells themselves, restricting expression of viral genes, and rapidly changing viral antigens through mutation.

Eventually, the damage to the immune system results in progression of the disease leading to acquired immunodeficiency syndrome (AIDS). The various mechanisms that HIV uses to avoid being cleared by the immune system are also used by other chronically infecting viruses, including the hepatitis C virus.

Antivirals 

Target a variety of viruses, including those that cause herpes, hepatitis, and influenza

Can treat viral infections like herpes and shingles

Antiretrovirals

Treat retrovirus infections, such as HIV

Different classes of antiretrovirals target different stages of the HIV life cycle

Antiretroviral therapy (ART) is a combination of HIV medicines that helps people with HIV live long, healthy lives

ART cannot cure HIV, but it can reduce the risk of HIV transmission

Antivirals and antiretrovirals can prevent viruses from creating more viruses

What are Viral Diseases?

Viral diseases are infections caused by viruses. Different types of viruses cause different infections. Common cold is the most common type of viral infection that is caused by infections in the respiratory tract. Other viral diseases include:

• Chickenpox

• Herpes

• Influenza

• AIDS

• Mumps

• Measles

• Viral Hepatitis

Viruses also infect plants. Let us have an overview of the different types of viral diseases, their causes, symptoms and prevention methods.

Types of Viral Diseases

There are different types of viral diseases. These include:

Causes of Viral Diseases

Viral diseases are mainly caused when a virus enters the human body and uses the host machinery to reproduce. If the body’s immune system fails to fight against viruses, it multiplies and spreads to other cells causing infections.

Symptoms of Viral Diseases

Following are the important symptoms of viral diseases:

1. Flu-like symptoms such as fever, fatigue, sore throat, headache.

2. Irritability

3. Rashes

4. Malaise

5. Sneezing

6. Swollen tonsils

7. Excessive weight loss

Treatment of Viral Diseases

Viral diseases can be treated in the following ways:

• Proper nutrition

• Medication for fever, body ache and pain

•  Proper rest

• Drinking more fluids

Human papillomavirus that leads to cervical dysplasia can be treated by removing the abnormal cells on the cervix of a female, surgically.

List of Viral Diseases

Following is a list of virus diseases that have made a significant socioeconomic impact in the last few decades.

• AIDS (Acquired Immunodeficiency Syndrome)

• Ebola

• Influenza

• SARS (Severe Acute Respiratory Syndrome)

• Chikungunya

• Small Pox (Now eradicated)

Why antibiotics do not work on viruses?

Antibiotic target cellular structures and proteins present in the bacteria.

E.g. antibiotics target bacterial cell wall, cellular enzymes, metabolic pathways, ribosomes, etc. Since viruses do not contain cellular structure, antibiotics do not affect them.