Question

6. The final step in the infectious process is disease production (or pathology). By what major mechanism can an infection with microorganisms result in disease? Give an example of each mechanism for the pathogens.

Answer 1

A pathogen can be defined as anything that can cause disease, like bacteria, fungi, virus, etc. Pathogenesis is the mechanism by which the disease caused by the pathogen will develop.

There are different pathogens today which can cause illness in humans, like bacteria, fungi, virus, protozoa, worms, prions, etc. They are all capable of establishing an infection in the human body. The pathogens have certain similarities and differences by which they can cause illness. The basic mechanism by which they act are:

a. Attachment - the pathogenic organism attaches to the outer membrane of the host. If there is a break in the skin due to a wound/cut, the microbe can then enter into the body of the host.

b. Host entry - After the attachment of the microbe, it tries to enter the host. The microbe may mediate an active penetration process/may enter through an opening on the surface of the host.

c. Colonization - Once the microbe has entered the host, it now starts to replicate and form colonies. The colonies can be formed intracellularly or extracellularly. The microbe absorbs nutrients from the host to provide the energy needed for its growth and replication. In most cases, the microbe lyses the cell to release the progenitor cells, causing destruction to the host. The cells mainly affected in any kind of disease will primarily be the immune cells which provide protection to the host like the macrophages, monocytes, dendritic cells, etc.

However, it is not necessary that every pathogen that invades the host will cause a disease. The manifestation of the disease will occur only if all the conditions are satisfied, i.e., compatibility is established between the pathogen, host and environmental conditions,

Bacterial Pathogenesis - The bacteria consists of a capsule, which is a layer of polysaccharide present outside the cell membrane, which protects the bacteria from the effect of various antibiotics as well as the host immune system. The capsule prevents the bacteria from being recognized by the phagocytic cells of the host immune system like the macrophages, monocytes and neutrophils. On being unable to clear the bacteria, these phagocytic cells produce more cytokines which cause an increased inflammatory response and damage to the host system.

The lysis of the bacterial cell wall after the death of the bacteria releases toxic components into the bloodstream which may negatively impact the circulatory system of the host, causing it to collapse and may cause septic shock. The release of cytokines and activation of the complement cascade plays a very important role in causing septic shock.

The bacteria also produce toxins which are damaging particularly to the immune cells of the host. The different toxins produced can be membrane disrupting toxins, pore-forming toxins, proteolytic toxins which affect the various tissues of the host. Damage to the immune cells of the host greatly reduces the body's ability to ward off these infection-causing agents, thus increasing the susceptibility of the body to succumb to the infectious agents.

The bacteria can replicate intracellularly or extracellularly. Once inside the cell, the bacteria proliferate, particularly inside the phagocytic cells that have engulfed the bacteria. The bacteria produce various intermediates which are reactive and may also lower the pH of the bacteria-containing vacuole, finally causing the death of the phagocytic cell. The lysis of the cell releases the replicated bacteria which are now free to further infect other tissues and organs of the body, causing the further spread of the infection.

Viral pathogenesis - As stated earlier, there are various factors which must wok in combination to allow for the pathogenic organism to cause an infection in the host. The virus first needs to gain access to the host body by overcoming various barriers, like the skin, mucus, etc. After gaining entry to the body, the virus must also be able to fight the body first line of defence - the 'soldiers' of the body. Only if all the conditions are favourable, then the virus can replicate inside the host and cause infection.

Like all other pathogens, the virus acts by infecting the cells and then lysing them to release the viral progeny. Once inside the cell, the viral DNA integrates with the cellular DNA and directs the cell machinery to synthesise proteins - the viral structural and functional proteins. After the completion of this process, the viral particles assemble themselves and release themselves from the cell by lysing the cell. This allows not only the new viral particle to infect other cells of the body, but it also brings about the destruction of the cells, particularly the immune cells of the body.

The viruses then affect various other organs of the body, like the lungs, respiratory tract, etc. The overall action is to lower the immune system of the host by damaging the immune cells to allow the viral particles to replicate and infect the various organs of the body.

Fungal pathogenesis - Generally, the fungi are not as infectious as bacteria and viruses. Fungi are mainly found as commensals in the microbial flora of the organism - like Candida. However, sometimes, some fungi are responsible for causing infection to humans.

The fungi enter the body through various sources and multiply in the body. Generally, the fungi are not capable of causing an infection who is immuno-competent. Only when the person has any comorbid disease or is immune-compromised for some reason or the other, then the fungi get a chance to proliferate and colonize and infect the host. For example, excessive use of antibiotics can lead to the alteration of the microbial flora of the intestine, killing the 'good bacteria', and allowing the Candida albicans to cause an opportunistic infection.