Not all Microbes are Pathogens!
Let's first define the title.
A microbe is a tiny living thing that is not visible to the human eye.
A pathogen is a microbe that can cause disease (make us sick)!
Okay so now, let's go over the four types of microbes.
Viruses are the smallest microbe. They directly kill cells by rupturing inside their host cell when they replicate. This is called lysis.
Bacteria are somewhat larger than viruses. They kill or damage cells directly by producing toxins.
Fungi work similar to bacteria, by releasing toxins into our blood or tissues.
Parasites are the largest microbe. Think of parasitic worms, like helminths. They don't have a host cell like viruses for example. Instead, they injure tissues that end up damaging our cellular responses.
BUT remember the title? (Hint: not all microbes are pathogens).
Not all of these microbes actually make us sick!
In fact, many tissues like our skin and stomach are constantly colonized by microbes. All of the microbes make up what we refer to as our microbiome.
These microbes that are not pathogenic have a symbiotic relationship with our cells. Some of them are even helpful to us! For example, certain types of bacteria help us digest cellulose.
What defines whether or not a microbe is pathogenic is if it damages our cells and spreads into underlying tissue rather than stay confined within the intestinal lumen.
The lumen is the inside space of the intestine, where a protective layer of mucus exists. If the microbe penetrates this mucosal layer and spreads into underlying tissue, it is pathogenic.
In summary, microbes that have a symbiotic relationship with our cells cause little damage while microbes that damage our cells and tissues are pathogenic.
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