Overview of Microbe-Host Interactions

Normal Microbiota and Host Relationships

Normal microbiota are the microorganisms that reside in the bodies of all humans.

Learning Objectives

Explain the relationship between the normal microbiota and the host upon infection of a pathogen

Key Takeaways

Key Points

  • The phrase “normal microbiota ” refers to the microorganisms that reside on the surface and deep layers of skin, in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts of every human being.
  • These microbiota are not harmful to humans; some are even beneficial and most help maintain our health.
  • Our normal microbiota consists of various bacteria, fungi, and archaea.
  • While our bodies are happy to host the array of microbiota that are considered “normal,” the human body does not take a back seat when infection tries to use it as a host.
  • Resistance to and recovery from viral infections depends on the interactions that occur between virus and host. The host has a variety of barriers that it uses to prevent infection. One of the first lines of defense is mucus, which has a range of normal microbiota.
  • There are a number of other humoral components of the nonspecific immune system as well.

Key Terms

  • host: A cell or organism which harbors another organism or biological entity, usually a parasite.
  • microorganism: An organism that is too small to be seen by the unaided eye, especially a single-celled organism, such as a bacterium.
  • interferon: Any of a group of glycoproteins, produced by the immune system, that prevent viral replication in infected cells.

Normal Microbiota

The phrase “normal microbiota” refers to the microorganisms that reside on the surface and deep layers of skin, in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts of every human being. These microorganisms are not harmful to humans; in fact, some are even beneficial and all help maintain our health. Our normal microbiota consists of various bacteria, fungi, and archaea. An example of our bacterial microbiota is E. coli . Many people think of E. coli as the bacteria that makes you sick; however while it has that capacity, it can also remain dormant and benign in your gastrointestinal tract for your entire life. All humans actually acquire E. coli shortly after birth with the intake of food or water. Other forms of bacteria present in the human gut are necessary for proper digestion of carbohydrates.

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Escherichia coli: This is a magnified view of Escherichia coli (or E. coli).

Host Relationships

While our bodies are happy to host the array of microbiota that are considered “normal,” the human body does not take a back seat when infection tries to use it as a host. Interestingly, normal microbiota can be key players helping the body fight off infection. Resistance to and recovery from viral infections depends on the interactions that occur between the virus and its host. The host has a variety of defenses that it uses to prevent infection. One of the first lines of defense is mucus, which has a range of normal microbiota that compete with and may even attack invading bacteria and virae.

Once a virus or bacteria makes its way past the skin and mucosa, there may be changes that occur in the host to diminish the invader’s effectiveness. An example of such a change is a fever. There are a number of other humoral components of the nonspecific immune system as well. Specific immune responses are produced by antibodies. Different interferons (IgA, IgG, IgM, etc. ) play roles in defeating viruses located in our membranes. The body does not easily become a host to infection; it has a line up of defenses to try to protect you from harm.

Opportunistic Microorganisms

Opportunistic microorganisms lay dormant until the hosts’ immune system is suppressed and then they seize the opportunity to attack.

Learning Objectives

Describe the traits of an opportunistic microorganism

Key Takeaways

Key Points

  • A microorganism is a microscopic organism that can either be a single cell, cell cluster, or multicellular. Microorganisms are very diverse and include bacteria, fungi, algae, and protozoa.
  • Opportunistic microorganisms are typically non-pathogenic microorganisms that act as a pathogen in certain circumstances. They lay dormant for long periods of time until the host ‘s immune system is suppressed and then they take that opportunity to attack.
  • Haemophilus ducreyi, a microorganism, infects its host through broken skin or epidermis. Without the open wound, this sexually transmitted disease would be unable to use the human body as a host.

Key Terms

  • microorganism: An organism that is too small to be seen by the unaided eye, especially a single-celled organism, such as a bacterium.
  • Opportunistic: Taking advantage of situations that arise.
  • immunodeficiency: A depletion in the body’s natural immune system, or in some component of it.

In the general realm of biology, an opportunist is an organism that is able sustain its life from a number of different sources, but when favorable conditions arise, the organism immediately takes advantage of the opportunity to thrive. When the focus is turned more specifically to microbiology, scientists call organisms that behave this way opportunistic microorganisms. A microorganism is a microscopic organism that can either be a single cell, cell cluster, or multicellular. Microorganisms are very diverse and include bacteria, fungi, algae, and protozoa. Opportunistic microorganisms are typically non-pathogenic microorganisms that act as a pathogen in certain circumstances. They lay dormant for long periods of time until the hosts’ immune system is suppressed and then they seize the opportunity to attack.

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HIV: This is a magnified view of HIV budding from a lymphocyte.

Patients with Human Immunodeficiency Virus (HIV) are particularly susceptible to opportunistic infections. HIV can develop into Acquired Immune Deficiency Syndrome ( AIDS ), which infects and destroys helper T cells (specifically CD4+ T cells). When the number of CD4+ T cell numbers fall below a critical level, cell-mediated immunity is lost. When immunity is lost, the opportunistic microorganisms can easily infect the AIDS patient without being destroyed by the immune system. These opportunistic pathogens thrive while the human body slowly deteriorates.

An example of an opportunistic microorganism is Haemophilus ducreyi. This microorganism infects its host through broken skin or epidermis. In other words, without an open wound, this sexually transmitted disease would be unable to use the human body as a host. It takes advantage of the opportunity to infect the lymphocytes, macrophages and granulocytes as soon as it enters the area of broken skin.

Cooperation Among Microorganisms

Cooperative behavior, includes mutualism and altruism, benefits one party while the other performs a certain behavior.

Learning Objectives

Compare and contrast the following cooperative behavior: mutalism and altruism

Key Takeaways

Key Points

  • In microbial systems, there are two main types of cooperation, altruism and mutualism.
  • Mutualism is a relationship between microorganisms that is mutually beneficial (+/+). This means that both parties are receiving positive things from their interaction.
  • Altruism is a relationship between microorganisms that is beneficial to one party, but harmful to the the (+/-). Scientists believe that the individual that is at a loss performs the action because they believe it will ultimately benefit others whom they share a relationship with (like family).

Key Terms

  • Cooperation: Association for mutual benefit.
  • mutualism: A relationship between individuals of different species in which both individuals benefit
  • altruism: Regard for others, both natural and moral; devotion to the interests of others; brotherly kindness; – opposed to egoism or selfishness.

Microbial Cooperation

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Methanogenic Bacteria in Termites: Methanogenic bacteria have a syntrophic relationship with protozoans living in the guts of termites. The protozoans break down cellulose, releasing H2 which is then used in methanogenesis.

A cooperative behavior benefits one party while the other performs a certain behavior or takes a particular action. In microbial systems, there are two main types of cooperation, altruism and mutualism. It is important to remember that microorganisms include bacteria, archaea, fungi, and protists. They are too small to be seen with the naked eye, but they play a huge role in the world as we know it and have a great deal of biological diversity.

Mutualism

Mutualism is a relationship between microorganisms that is mutually beneficial (+/+). This means that both parties benefit from their interaction. A microbial example is the interaction between protozoa and archaea in the digestive tracts of some animals. These animals eat cellulose which is broken down by the protozoa to obtain energy. This process releases hydrogen as a waste product, which in turn reduces energy production. Specialized archaea convert the hydrogen (which they need) to methane, which allows energy production to increase. Both the protozoa and archaea benefit from this relationship.

Altruism

Altruism is a relationship between microorganisms that is beneficial to one party, but harmful to the the (+/-). Most scientists believe that the individual that is harmed, or at a loss, performs the action because they believe it will ultimately benefit others whom it is close to or share a relationship with (like family). On a microscopic level, this happens with programmed cell death, or apoptosis. Although it does not seem like it would be beneficial for the cell to die, it has been suggested that the resources it was using could be better utilized by other cells for growth and survival.