Microorganisms play a vital role in human survival. In fact, life as we know it would be impossible without them! As a human fetus develops, it’s protected with in the sterile womb of its mother. As soon as it’s born, an infant is immediately exposed to microbes from both its mother and the surrounding environment.
Protective microbes begin to grow first on the infant’s skin, then their oropharynx, which is the back of the throat. The gastrointestinal tract is colonized next, followed by other mucosal surfaces. These populations of bacteria are called microbial flora, and they perform a wide variety of crucial tasks. They help protect us from infection by virulent microorganisms. They stimulate the immune response. They help us metabolize our food. And they provide essential growth factors.
As we age, the balance of microbes that are on us and in us are influenced by factors such as what we eat, our overall health, hormonal changes, and personal hygiene. Drastic changes in any of those factors can disrupt the intricately balanced ecosystem of microbes, which can in turn make us sick i.e. taking antibiotics can wipe out all of the good and bad bacteria in the gut, completely disrupting the equilibrium of microbial populations within.
Now let’s get a bit more specific. When you’re exposed to a microorganism,there are three possible outcomes. The first of these is called transient colonization. The second is called permanent colonization. And the third outcome is disease. The language here can be a little bit tricky, so let’s make sure we understand the difference between colonization and disease. Microorganisms that colonize humans, whether transiently or permanently, do not interfere with normal bodily functions. They’re just present.
Defination of Pathogen
However, disease occurs when microorganisms cause damage to a human host. A bacterium, virus, or other microorganism that can cause disease is called a pathogen.
For the most part, diseases are not simple. In fact, most organisms do not cause a single, well-defined disease. It’s far more common for a single bacterial pathogen to cause different types of disease depending on the setting. Staphylococcus aureus, can cause food poisoning, pneumonia, or a wound infection. In that same vein, some diseases can be caused by a wide range of organisms.
Interestingly, meningitis, which is an inflammation of the protective membranes covering the brain and spinal cord, can be caused by bacteria, viruses, fungi, or parasites. Additionally, there are relatively few organisms that can be classified as always being pathogenic. Some examples of always-pathogenic microorganisms are the rabies virus, plasmodium species, and bacillus anthracis.
For the most part, microorganisms are only able to cause disease under specific circumstances. Let’s make another distinction. Disease that arises from exposure to an external source is called an exogenous infection, i.e. pathogens in this category include the influenza virus, the bacteria that causes gonorrhoea (Neisseria gonorrhoeae), or the bacteria that causes tetanus (Clostridium tetani), disease that arises from a person’s own microbial flora spreading to inappropriate sites in the body is called an endogenous infection.
Incredibly, endogenous infections are to blame for the majority of human diseases.
Factors to consider are as follows.
- There is the site of exposure.
- There is the virulence, or the
degree of pathology, of the
organism.
- There is the host’s ability to
respond to the organism.
All of these determine whether the complex interaction between a human host and a microorganism will result in a long-term symbiotic relationship, a short-term colonization, or disease.
A strict pathogen is an organism that is always associated with human disease. Examples include Mycobacterium tuberculosis, which causes, as one might guess, tuberculosis, Plasmodium species, which cause malaria, and the rabies virus, which causes rabies. But most human infections are caused by opportunistic pathogens, which are typically part of the normal microbial flora. These organisms establish disease when they take advantage of an opportunity not normally available, like a weakened immune system, disrupted microbial flora, or breached barrier introduced by a wound.
