Pathogenesis of infectious disease

9. Pathogenesis of infectious disease




Questions


image What are the basic determinants of pathogenicity?

image What are Koch’s postulates?

image Compare the mechanisms whereby microorganisms evade host defences.

Contamination of the body occurs when a microorganism becomes in contact with a host and this becomes colonization when the organism multiplies. Only a minority of microorganisms are in contact with humans, and most of these are commensal flora within their own ecological niche. However, viruses and other pathogenic microorganisms characteristically cause disease; they may have highly specific adhesins and toxins, the genes for which may be grouped together under the control of a single promoter in pathogenicity islands. The net result of the meeting between a human and any microorganism depends on the balance between host immunity and the virulence of the infectious agent. Commensalism occurs when there is a neutral balance between the two parties, symbiosis when both benefit and parasitism when the relationship is uneven. However, interaction with microorganisms occurs continually, the relationship can change and disease is common. So, how do we know if the two are linked? Koch’s postulates are an important set of criteria that can be used to judge whether a microorganism is the cause of a disease. They can be summarized (with a slight change in emphasis from the original treatise of Koch) as:


1. The organism occurs in every case of the disease and under circumstances which account for the pathological changes and clinical course of the disease

2. The organism occurs in other diseases as a fortuitous and non-pathogenic finding

3. After being isolated from the body and grown in pure culture, the organism can repeatedly produce exactly the same disease when inoculated into new, susceptible hosts

4. The organism can then be isolated from the new hosts.

These postulates are still relevant today, although it is difficult to apply them to poorly demarcated clinical syndromes such as diseases where the pathogen that initiated a disease process is no longer present, as occurs in some autoimmune diseases; malignancies; multifactorial diseases; serious diseases without an animal model; infections in the immunocompromised; and uncultivatable microorganisms. An understanding of molecular mechanisms has also shown that the same bacterium can be pathogenic and non-pathogenic depending on differential gene expression.


In order to cause an infection, a microorganism has to make contact with the host (Fig. 3.9.1), multiply within it and then be transmitted to another host/area.


image

Fig. 3.9.1 Methods of attachment used by microbes.

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Related posts:

Epidemiology of infectious diseases Infections of the immunocompromised host Principles of hospital infection control Infections of bone, joints and muscle Tuberculosis and other mycobacterial diseases Innate host defences to infectious disease

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Jul 3, 2016 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on Pathogenesis of infectious disease

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