What is Pathogenicity?

It is the ability or quality of pathogenic agents to generate disease. It is also known by the capacity of a microorganism to affect the host cell or damage any other cells adjacent to it. For example, when a microbial species like Mycobacterium tuberculosis enters the human body and results in TB or Tuberculosis. The ability of that species is known as its pathogenicity. It is the qualitative term where the qualitative factors may be responsible for producing the disease in the first place.

Types of Pathogens


These are some microscopic creatures, which reproduce rapidly after entering the body. It releases a toxin that affects the tissue and generates disease. Mainly antibiotics are prescribed to build resistance against these microbes. All bacteria are not pathogenic. Some helpful bacteria are also found to perform a certain essential bodily function. For example, ‘E. coli help in building immunity of the body.


They are smaller in size. Their nature is to enter a host body and increase their number through replication. In this way, numerous viruses arise and are ready to launch themselves in more host cells. They can enter the body through respiratory droplets, through contact with the blood or bodily fluids of an infected person. For example, COVID-19 (Coronavirus disease) has resulted from the virus SARS-CoV-2 or severe acute respiratory syndrome coronavirus 2.


Some fungi cause serious health issues in humans. General skin problems involve athlete's foot and ringworm. These severely contagious diseases and can spread from one person to another.


These are some single-celled organisms that damage the host cells. It generates infection in other living bodies so as to develop the ability to survive and reproduce. They result in various kinds of diseases like dysentery, diarrhea, malaria, and many more.

Parasitic worms

These are some multicellular organisms that damage the host cells. It generates infection in other living bodies to develop the ability to survive and reproduce. They result in various kinds of diseases like dysentery, diarrhea, malaria, and many more.

Stages of Pathogenesis

A pathogen goes through four steps of mechanism to cause an illness. At first, it enters the host cell, travels to the preferable location, overcomes the host cell's immune defense, and ultimately causes the damage. In the majority of the  cases, the cycle completes itself when the microbe leaves its host and is transferred to a new one.

1. Exposure

In this stage, a possible pathogen gets access to a host cell. The common ways to connect to them are eating contaminated food, handling an infected non-living object, and coming in contact with infected air droplets. Some contacts cannot generate infection or disease. The potential pathogen must enter the host tissue to cause harm. The three major portal entries are mucous layers, skin epithelium, and parenteral channels. Through these points, the host cell is directly connected with the surroundings. Mucous membranes are the most sensitive route for pathogens. These contain the GI (gastrointestinal) or alimentary tract, respiratory vessels, and genitourinary tract. Most of these surfaces are internally located but some are connected with external openings like the mouth, eyes, urethra, nose, and anus.

The portal specificity of a microbe is decided by the organism's adaptations to the environment. This is mainly controlled by the enzymes and toxins secreted by them. They are also able to breach the protective layers of the skin and mucous membranes. It is regarded as the parenteral route. Skin is considered a good barrier towards the internal organs, but several skin diseases can break through this protection and harm the organism. In a pregnant woman, the placenta act as a preventive measure for the baby. Still, some microbes can cause a spontaneous abortion by crossing the mother-fetus barrier.

2. Adhesion

It is the capability test of pathogenic microbes to create linkage with the body using certain factors. The factors are called adhesion factors. Carbohydrate or protein molecules called adhesins are discovered on the surface of some pathogens and create bonds to certain receptors on host cells. The receptors are glycoproteins in composition. These are discovered in the Protozoan cilia, bacterial flagella and fimbriae, and the viral capsids or coverings. Protozoans have additional adhesive factors like hooks and barbs. Viruses exhibit spike proteins to enhance the adhesion. 

3. Invasion

The pathogens secrete exoenzymes or toxins as virulence factors to damage the host tissues together. The virulence factors also assist in protecting the microorganisms from the host's natural defense. The invasion is activated through the reproduction process of intracellular pathogens, from which some are obligatory (can reproduce only inside the host cells) and some are facultative (can duplicate in number either inside as well as outside of the host cell). They can enter the cell through endocytosis. The endocytosis is carried through two mechanisms. In the first one, the pathway relies on the nature of the effector proteins of the pathogen. This is utilized by Salmonella and Shigella sp. The process is often called membrane ruffling. Otherwise, the surface proteins of the pathogen attach to the receptors on the host cell. 

4. Infection

Some successful multiplication leads to the next stage called infection. This can be of three types. The first one spreads in a small region of the body, almost near to the portal of entry. This is called the Local infection. The second one is known to be a localized pathogen that can infect another location along with the primary. It is regarded as a Focal infection. The infection that escalates throughout the body is called Systemic infection. In a few cases, an initial infection occurred by one pathogen, leads to another infection caused by another pathogen. Here the first one would be primary and the second one would be the secondary infection.

Koch's postulates

The German physician Robert Koch set some criteria to judge whether a certain microorganism can generate a certain disease in the year 1890. His postulates are-

  • The bacteria must be found in every case of that certain disease.
  • It is needed to be isolated from the human host with the disease and grown in a pure nutrient resource medium.
  • A certain illness should be reproduced when pure culture is inoculated in a healthy susceptible host body.
  • The microbe should be recoverable from the experimentally infected organism.

Measurement of Pathogenicity

It can be measured through the utilization of highly regulated experiments with laboratory live specimens. The significant indicators of virulence are ID50 (Median infectious Dose) and LD50 or (Median lethal dose).

The ID50 is the number of pathogens or virions needed to instigate active infection in 50% of inoculated animals.

The LD50 pathogenic cells or amount of toxins required to destroy 50% of the infected animals.

Graphical representation of the virulence measurement with the help of lethal  and infectious dose.

Concept of pathogenicity island and its importance

Pathogenicity islands (PAIs) are a certain class of genome sequences exhibited by microorganisms through horizontal gene transfer. These are assimilated in the genome of the pathogenic bacteria, but generally are absent from non-pathogenic organisms or closely related species. These motile genetic elements are of the size of 10-200kb (kilobase) and encode virulence genes of the respective bacteria. Some examples are adherence factors, toxins, iron uptake systems, invasion factors, and secretion systems. If this island is added to a non-pathogenic species, it can transform that certain species into pathogenic or disease-causing organisms.

Context and Applications

This certain part of biology is studied in several sources for various purposes. This is a part of-

  • Microbiology
  • Biotechnology
  • Molecular Biology
  • Medical Science

Practice Problems

  1. Through what pathogenicity island can be transferred?
    1. Reproduction
    2. Duplication
    3. Conjugation
    4. Transcription

Correct Answer: c- Conjugation

2. What is a pathogen regarded as when it can infect a secondary location?

    1. Local Infection
    2. Focal Infection
    3. Secondary Infection
    4. Systemic Infection

Correct Answer: d- Systemic Infection

3. What is membrane ruffling known as?

    1. The building up of toxicity level in the host body.
    2. Killing of viruses through enzymes.
    3. Formation of actin-rich membrane protrusions.
    4. Creating an additional plasma membrane.

Correct Answer: c- Formation of actin-rich membrane protrusions

4. What are Pathogenicity islands?

    1. Microorganisms
    2. Genome sequences
    3. Proteins
    4. Antibodies

Correct Answer: b- Genome sequences

5. What are viral capsids made up of?

    1. Keratin
    2. Kinetochore
    3. Capsomeres
    4. Protease

Correct Answer: c- Capsomere

Common mistakes

Students often misjudge pathogenicity with virulence. Both are different. Pathogenicity may be understood as the ability of a microbe to cause or be the reason for a specific disease, whereas virulence represents the severity of the host-pathogen interaction. Koch's postulates also have some limitations, because it does not explain the harmless bacteria transforming into disease-causing ones by gaining extra virulence factors. Physical trauma can also force a microbe to gain access to deep tissues like medical surgery, an IV (Intravenous) line, and so on.

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