Virulence Factor - Bacterial and Viral


Microorganisms are organisms that cannot be seen through the naked eye and require a special aid to observe them. They are cosmopolitan in nature that is are found everywhere. Most of the microorganisms are harmless to us but some cause serious diseases.

The microorganisms that cause diseases are called pathogens and the etiology and progression of the disease is known as pathogenesis. Pathogens cause diseases due to certain factors produced by them these factors are known as virulence factors which can be certain chemicals, enzymes, toxins, etc. in this section we will be briefing about virulence factors that are produced by bacteria and viruses and their mode of action.

Virulence Factors - An Overview

Microorganisms have the ability to cause infection in other organisms, this virulence factor play a major role by helping microorganism in three ways-

  • By helping them invade the host.

  • By causing the disease.

  • By inactivating or escaping the immune system of the host.

Not only do virulence factors help the microorganism to cause disease, but it also determines the severity of the disease and there are certain genes that encode these virulence factors, and as the genes are inactivated the virulence of the pathogen also decreases considerably.

To summarize this, virulence factors determine the severity of an infection caused by a pathogen.

Different microorganisms produce different virulence factors which include adhesins, bacterial toxins, capsules, enzymes, etc. We will be focusing on only the virulence factors produced by bacteria and viruses.

Bacterial Virulence Factors


As the name suggests adhesins are a class of proteins produced by bacteria for adhering to the surface of the host cells. They are generally found on the surface of the bacterial membrane, and they specifically bind to the receptors present on the surface of the host cell.

For example- one of the most common adhesins produced by Vibrio cholerae is N-methyl phenylalanine which is produced by the tips of the pili and specifically attaches to intestinal epithelial cells.

Exoenzymes and Toxins

Once pathogens adhere to the surface of the host cell, the next thing they do is the invasion into the body of the organism. This is achieved by producing certain extracellular enzymes also called exoenzymes.

  • These exoenzymes are highly specific for the tissues and help in the invasion as well as escaping the immune system.

Example- Staphylococcus aureus produces an exoenzyme called Hyaluronidase S which helps in the degradation of Hyaluronic acid that acts as a binding agent between the cells and helps in their dissemination.


Certain bacteria, in addition to extracellular enzymes also produce certain poisonous substances that damage the host tissue and ease the process of invasion. This property of disease-causing bacteria to produce toxins is called toxigenicity. Two classes of toxins are produced by bacteria-

  • Endotoxin- Produced by Gram-negative bacteria and is found on their outer membrane. An example of endotoxin is Lipid A which causes a decrease in blood pressure, multiple organ failure, and ultimately death.

  • Exotoxin- They are produced mostly by Gram-positive bacteria and some Gram-negative bacteria. They are nothing but proteins produced by the bacteria itself. They are very specific to the type of cell they are interacting with and destroy it by specific molecular mechanisms. An example of exotoxin is Tetanus toxin which causes spastic paralysis.

Capsules and Proteases

Some bacteria survive inside the host by escaping or by developing protection against the immune system. In this, the most important one is the evasion against phagocytosis. For this certain bacterium produce capsules, that prevents the adherence of phagocytes to the bacteria and enlarges their size so that it becomes difficult for the phagocytes to engulf them.

Example- It has been observed that the strain of Streptococcus pneumoniae which produced capsule was more virulent as compared to non-encapsulated strain.

  • Protection by Proteases- Those varieties which cannot form capsule produces proteases that protect them against phagocytes.

  • Human immune system works by specifically recognizing bacteria with the help of antibodies that bind to their surfaces and initiate phagocytosis. This mechanism is known as antibody-mediated killing. Proteases overcome this problem by specifically digesting these antibodies.

  • There are other ways too by which bacteria escape the phagocytosis in addition to capsules and proteases, that is by altering the surface of the bacteria preventing them from being recognized, this mechanism is known as antigenic variation.

Viral Virulence Factors

Viruses are structurally different to those from bacteria, but they produce certain virulence factors that are common which are as follows-

Viral Adhesins

The outer protein layer of viruses called capsid produces certain factors called as adhesins that specifically bind to the receptors on the host cell surface. These adhesins target specific cells and enter by breaking the membrane wall.

Example- HIV contains adhesin gp120 which binds to CD4 of immune cells.

Antigenic variation

They are observed in viruses due to mutations, that alter the surfaces of the viruses due to which immune cells fail to recognize them and by this way they can easily enter the host body.

  • Antigenic variation is the main reason for the emergence of new viral strains that are resistant to vaccinations. Due to this, some vaccinations have to be repeated annually.

  • Apart from antigenic variations antigenic shift and antigenic drift also help in protecting viruses.


Developing medicines and vaccinations against bacteria and viruses has always been challenging due to recurrent mutations occurring in these microorganisms. But understanding various mechanism taken up by the pathogens and virulence factors produced by them provide better insights into the recognition and development of effective medicines against these intruders.

Updated on: 17-May-2023


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