Infection

Infection

What is it?

A Microorganism is any organism that is too small to be seen by the naked eye and a Pathogen is a particular microorganism that causes disease.

An infection is the presence of microorganism that cause damage to body tissues.

Relationships between Humans and Microbes

There are several different relationships that humans have with microbes:

1.       Symbiosis benefits the human but doesn’t harm the microorganism.

2.       Mutualism benefits human and microorganism.

3.       Commensalism benefits the microorganism but doesn’t harm the human.

4.       Pathogenicity benefits the microorganism but harms the human.

5.       Opportunism is when a benign microorganism becomes pathogenic because of decrease human host resistance.

We have physical protective barriers and normal flora that act as our first line of defence and are non-specific. If compromised, normally non-infectious material can gain access to our tissues resulting in infection. This can occur both in response to cuts in the skin allowing infectious agents to enter from the eternal environment as well as internal damage in our organs causing a release of bacteria into the bloodstream.

We then have immune and inflammatory systems to maintain much of the symbiotic relationship. Compromise in these systems can mean that many microorganisms can leave where they are normally found and cause infection elsewhere in the body. Patients with an immune deficiency can easily become infected by opportunistic microorganisms as defence systems are weakened.

The Stages of Progression (Microbe's POV)

There are 4 stages of progression for the microbe:

1.       Colonisation

2.       Invasion

3.       Multiplication

4.       Spread

Infectious microorganisms exist in reservoirs (environment, animals or infected human) and can be transmitted through direct contact, indirectly by vectors or direct exposure through food, water and soil. There is also human-human transmission by aerosolized microorganisms in droplets and physical contact such as sexual contact, blood transfusion or contact with contaminated materials. After an infectious microorganism is deposited in a receptive environment it will colonise the host by adhering to the cell through specific surface receptors. (COLONISATION)

When the infectious agent has created this colony on the surface of the cell, they will begin invading the cell, surrounding tissues and other sites. These infectious agents are stealthy and can develop mechanisms to avoid the host’s inflammatory and immune defence. (INVASION)

Once they’ve invaded, they thrive in the warm, nutrient filled host environment and will begin rapid multiplication. Viral pathogens will replicate within the infected cells and bacterial pathogens will replicate in macrophages and other cells. (MULTIPLICATION)

Finally, the pathogen will spread: some producing localised infections staying within the affected region of the body and others being highly invasive and spreading to other regions of the body through the lymphatics, blood or internal organs. The success of a spread depends on the functionality of the person’s immune system as well as virulence factors (a pathogens ability to cause disease. (SPREAD)

 The Stages of Progression (Host's POV)

There are also 4 stages of infection…