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Pathogen Virulence Factors Definition and Pathogenicity

Virulence Definition

Virulence define the intensity of a pathogen to cause disease. The Virulence varies among different microbial species. Virulence enables the microorganism to carry a specific character which can damage the tissue of the host cell.

Virulence helps the microorganism to better survive its residency in the host. The virulence property of an organism varies from species to species.

The term virulence is originated from the Latin word vīrulentus, which means “full of poison”, “toxin”.  It is used to define that the pathogen is extremely toxic.

Virulence factor definition

The virulence factor is a factor that helps an organism to invade a host and cause disease. The extent of damage to the host is determined by the virulence factor. The virulence factors of an organism can be secretory, membrane-associated, or cytosolic in nature.

In microbiology when we track any novel pathogenic strain, these virulence factors play a vital role in epidemiology. Researchers also looking for new virulent factors such as route of entry, different pathobiological machinery, and its effects on the host’s immune system.

Therefore virulence factors are the major topic in medical research which will help to create new treatments and vaccines.

Before we know about the virulence factors first of all we need to know how a pathogen infects a host or how the pathogenicity of a pathogen works. During infection, a pathogen follows the following steps;

1 Adherence and Colonization

In this step, the pathogen gets attached to the host cell surface.

2. Invasion

Enter into the host cell cytoplasm and started to multiply.

3 Toxin Synthesis

Synthesis of different types of Toxins that alter the function of the host.

For More detail study about the Mechanism of Infection Follow My Previous Article, “What is Infection?”, here i have already discussed this points in detail.

Bacterial Virulence

Proteins or other molecules that are synthesized by the enzymes are responsible for bacterial virulence or are function as virulence factors. Different genes in chromosomal DNA, bacteriophage DNA or plasmids are coded in these proteins.

Mechanism by which bacteria cause disease

  • Adhesion: The first step of bacterial infection is adhesion where the bacteria first binds on the host cell surface by different receptor proteins. Some host cell contains mucous lining and of anti-microbial substances on their surface which prevents certain pathogens to bind on them.
  • Colonization: The colonization of bacterial cell within the host body is accomplished with the production of certain special proteins. These proteins help them to colonize within-host body. For example, Helicobacter pylori produce the enzyme urease which helps them to survive in the acidic environment of stomach.
  • Invasion: In invasion, the bacterial cell enters within the host cell by penetrating the epithelial tissue layers at the body surface. They penetrate into the host cell with the production of certain proteins which either disrupt host cell membranes or encourage their own endocytosis or macro-pinocytosis within the host cells. 
  • Immune response inhibitors: In this step, the pathogen inhibits the host’s immune system defenses by producing some virulence factors. They basically produce a special protein that binds host antibodies.
  • Toxins: Bacterial cells produce many virulence factors or proteins that are poisonous to host cells and cause tissue damage. For example, food poisoning toxins which are produces by certain food poisoning bacteria.

Virulence Factors of Viruses

Virus virulence is followed by the following steps such as replication, modification of host defenses, spread within the host, and finally production of toxins within the host cell.

The occurrence of infections and the degree of viral infection’s symptoms are determined by them. They bind to the host cell with the help of a specific receptor protein which is located on the host cell surface. These proteins are endocytosed and the bound virus then enters the host cell.

HIV is a virulent virus. They have a special mechanism by which they can escape from the host defenses. An immunocompromised state occurs when the HIV infects T-Helper Cells which results in a reduction of the adaptive immune response of the host.

Rabies and herpes simplex are known as the  “neurovirulent” they cause disease by invading the nervous system.

Virulence vs Pathogenicity

The most asked question among the students is how virulence is different from pathogenesis? So here are the answers;

  • Pathogenicity define as the potential of a pathogen to cause disease whereas virulence defines as the ability of the pathogen to infect or damage a host.
  • Pathogenicity follows the virulence whereas virulence represents the initial stage of host-pathogen interaction.
  • Pathogenicity determined by the virulence factors whereas the virulence are proteins or other molecules.

Pathogenic Factors/Virulence Factors

There are present different factors which determine the virulence or pathogenicity of a pathogen, such as;

1. Capsule

Capsule play an vital role in the pathogenicity of a pathogen. Capsules surround many bacterial cells that protect the bacteria from the immune and phagocytic response.

Capsule inhibits phagocytosis by preventing interaction between antibody and C3 bond to the outer membrane of the bacterial capsule also help in the attachment of the host cell.

2. Adherence and Colonization

The first and most important step of an infectious disease process is the entrance and attachment of a pathogen to a host body. Its one of the most important virulence factors.

Some of the pathogens are entering the host body through the skin, respiratory system, gastrointestinal system, conjunctiva of the eye. There are other pathogens which can enter the host body by sexual contact, needle sticks, blood transfusion, and organ transplant.

The pathogenicity of many microorganisms depends on the ability to addhere with the mucosal cell as a first step. With the help of the adhesion factor, many bacteria adhere to the epithelial cell line of the blunder intestine and blood vessel.

There are present several factors that help in the adhesion of a pathogen to the host surface these factors are known as adhesion factors, such as the surface like structure a pill, lipopolysaccharide side Chain, and M protein of Streptococcus progenies.

3. Invasion

The pathogenicity depends on the infectivity and invasiveness of a pathogen. Infectivity refers to the ability of pathogens to establish a discrete focal point of infection. The invasiveness refers to the ability of the pathogen to spread to adjacent or other tissues.

Invasiveness is a multifactorial and complex process. Invasive bacteria either destroy the cell wall or penetrate the cells of the barrier. A pathogen can penetrate the host mucosa membrane by two methods, such as;

  1. Actively penetrate
  2. Passively penetrate

A. Actively penetrate

A pathogen accomplished her active penetration through the production of a lytic substance that alters the host tissue. This produced substance can alter the tissue function by this following process

  • By attacking the extracellular matrix and basement membranes of integuments and intestinal linings.
  • degrading carbohydrate-protein complexes between cells or on the cell surface (the glycocalyx).
  • disrupting the host cell surface.

B. Passively penetrate

Passive mechanisms of penetration are not related to the pathogen itself. Examples include

  • wounds, abrasions, or burns on the skin’s surface;
  • arthropod vectors that create small wounds while feeding; and
  • tissue damage caused by other organisms (e.g., a dog bite).

Once the pathogen can penetrate to the mucosa membrane, it penetrates to the deeper tissue and continuities disseminating throughout the body of the host.

4. Co-enzyme

Many bacteria release enzyme that can damage hostess in a variety of machines such as;

  1. The pathogen will release some enzyme which will help to break down the collagen and fibrin, which help in the beta penetration of microorganisms in the tissue.
  2. They will release a few enzymes (Proteases, etc) that will help in the background of cellular material.
  3. Pathogens will release enzymes which will modify and inactivate the antibiotics. For example, beta-lactamase hydrolyze the beta lectum ring in the beta-lactam cross of antibiotic and inactive at the antibiotic.

5. Toxins

The virulence factors of a pathogen also depends on the toxin production by the pathogen. After entering the host cell, the microorganism started to synthesize different types of toxins which will alter the normal functions of the host body cell. Bacterial toxins directly harm tissues or Trigger destructive biological activities. There are present different types of toxins such as Exotoxin and Endotoxin.

All these virulence factors are responsible for the pathogenicity of a pathogen. The pathogenicity varies in different species of pathogens.


  • Prescott’s Microbiology by Joanne Willey (Author), Linda Sherwood (Author), Christopher J. Woolverton (Author)
  • Microbiology Principles and Explorations by Jacquelyn and Laura Black
  • wikipedia,”Virulence factors

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What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting? What is Southern Blotting?
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