Pathogenicity, Virulence And Infection

Pathogenicity of any microbe is determined by the chances of developing a diseases. Various strains of same pathogen may have different chances of pathogenicity. I.e. with regards to their virulence. For instance, some strains of pathogen microorganisms are highly virulent, i.e. only a few cells of pathogen can develop disease in host. Other strains may be less virulent i.e. larger number of cells are required to develop disease in the host. Some strains of same pathogen may be avirulent, i.e. incapable of causing disease even when large number of cells are inoculated into the host.

Virulent strains of many pathogen, when grown on artificial media repeated generation after generation or cultured in animal other then host, may lose their pathogenicity. Such avirulent strains are called attenuated strains and are widely used as vaccine to elicit immunity to various disease.

The virulence of a pathogen is usually measured by determining it’s LD50 dose for a particular type of laboratory animal. The LD50 dose is defined as that number of organisms which, when inoculated to a number of laboratory animal, will kill 50 percent of them. For example, an LD50 dose of 10 cells of strain X compared with 100,000 cells of strain Y would indicate that X is 10,000 times more virulent then Y. The LD50 does can be determined more precisely than other endpoints such as the dose that kill 100 percent of animals. (LD100 dose, sometimes also termed minimum lethal dose or MLD) because the rate of change in mortality verses change in dose is greatest around the point of 50 percent mortality.


Infection represent the most intimate way in which a microorganisms may cause disease. The host is invaded by the microorganisms which subsequently multiply in close association of host tissue. Most but not all microbially caused disease are infectious. Food poisoning is the disease that is non infectious, it occur due to the ingesting the poison (toxin) in a food.

In order to cause a infectious disease, a pathogen must have following qualities:

  1. It must enter the host
  2. It must metabolize and multiply on or in the host tissues
  3. It must resist host defences
  4. It must damage the host

Each process is complex and all four process must be fulfilled by microorganisms in order to cause infectious disease to the host. Some infection may result in only a minor amount of damage to the host, so minor that there are no detectable clinical symptoms of infection. Such infections are called non clinical infection.

Microbial Adherence

Unless a pathogen is directly introduced to the host tissue (as by wound, injection by an arthropod, or other similar means), the first step of infection is usually adherence or attachment of the pathogen to some surface of host. Such surfaces represent hostile environment and the microorganisms must compete with normal flora organisms for surface attachment. Later pathogen should invade to reach host tissues and can continue its growth against the various defence mechanisms of host.

Penetration Of Epithelial Cell Layer

Although penetration of the epithelial layer follows adherence in most infections, this is not always a prerequisite to infection. The pathogen may merely multiply on the epithelial surface and can cause damage to host without penetration in to the body. For example, V. Cholerae, the causative agent of severe diarrheal disease known as cholera, multiply on the surface of epithelial cells of small intestine and produce a toxin that causes the loss of fluid from the epithelial cells and kills the cell.

Passive Penetration Into The Body

It should be noticed that penetration of body surface may be achieved by passively, not actively all the time. Passively it occur by mechanisms having nothing to do with the properties of the microorganisms. Any mechanically caused breach in the body surface may penetrate microorganisms to the underlying tissues. Wound or burns represent such passive mechanisms.

Another mode of passive penetration is by arthropod. For example, Borrelia species causes relapsing fever in humans when the spirochetes are introduced through the bite of a tic or a body louse.

Active Penetration Into The Body

Some pathogenic microorganisms are capable to penetrate the epithelial layer of host to which they are attached. For example, in bacillus dysentery, Shigella bacteria penetration into and kill the epithelial cells of host colon, then spread to adjacent cells. The result in formation of lesions known as ulcers, i.e. areas of the intestinal wall which have disintegration or necrotic (dead) tissue.

After penetration to the epithelial cells, some pathogens may penetrate into the deeper tissue of the body and may even become widely disseminated throughout the body, particularly if the organism obtain access to the lymphatic vessels or the blood vascular system.

Reference: Microbiology Prescott

Nihal Sharma

Assistant Editor

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