11.3H: Nutritional Immunity

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Learning Objectives

Describe at least four ways the body deprives microorganisms of iron.

We will now take a closer look at nutritional immunity. Iron is needed as a cofactor for certain enzymes in both bacteria and humans. Both bacteria and human cells produce iron chelators that trap free iron from their environment and transport it into the cell. During infection, the body makes considerable metabolic adjustment in order to make iron unavailable to microorganisms. Much of this is due to production of a defense chemical called leukocyte-endogenous mediator (LEM). As a result of infection, there is:

decreased intestinal absorption of iron from the diet;
a decrease of iron in the plasma and an increase in iron in storage as ferritin;
increased synthesis of the human iron-binding proteins (iron chelators) such as lactoferrin, transferrin, ferritin, and hemin that trap iron for use by human cells while making it unavailable to most microbes;
coupled with the febrile response, decreased ability of bacteria to synthesize their own iron chelators called siderophores;
prior stationing of lactoferrin at common sites of microbial invasion such as in the mucous of mucous membranes, and the entry of transferrin into the tissue during inflammation.

This lack of iron, which is needed as a cofactor for certain enzyme reactions, can inhibit the growth of many bacteria.

As seen in Unit 3, some bacteria produce in addition to their own siderophore, receptors for siderophores of other bacteria in this way take iron from other bacteria. Furthermore, a number of pathogenic bacteria are able to bind human transferrin, lactoferrin, ferritin, and hemin and use that as their iron source. For example, Neisseria gonorrhoeae, Neisseria meningitidis, and Haemophilus influenzaeare able to use iron bound to human transferrin and lactoferrin for their iron needs, while pathogenic Yersinia species are able to use transferrin and hemin as iron sources. Borrelia burgdorferi doesn't even use iron as a cofactor, but instead uses manganese. Furthermore, a number of bacteria are able to produce exotoxins that kill host cells only when iron concentrations are low. Perhaps in this way the bacteria can gain access to the iron that was in those cells.

Summary

Iron is needed as a cofactor for certain enzymes in both bacteria and humans.
Both bacteria and human cells produce iron chelators that trap free iron from their environment and transport it into the cell.
During infection, the body makes considerable metabolic adjustment in order to make iron unavailable to microorganisms.
The lack of iron can inhibit the growth of many bacteria.
Some bacteria in addition to their own siderophores, produce receptors for iron chelators of other bacteria and/or human cells and in this way take iron being trapped for use by other organisms.
A number of bacteria are able to produce toxins that kill host cells only when iron concentrations are low and in this way gain access to the iron that was in those cells.