Skills to Develop
- State why the ability to compete for iron and other nutrients is important for bacteria to cause disease and describe briefly three ways bacteria may accomplish this as part of their pathogenicity.
Often the ability to be pathogenic is directly related to the bacterium's ability to compete successfully with host tissue and normal flora for limited nutrients. One reason the generation time of bacteria growing in the body is substantially slower than in lab culture is because essential nutrients are limited. In fact this is a major reason why the overwhelming majority of bacteria found in nature are not harmful to humans.
To be pathogenic, a bacterium must be able to multiply in host tissue. The more rapid the rate of replication, the more likely infection may be established. Pathogens, therefore, are able to compete successfully for limited nutrients in the body. Generally bacteria compete for nutrients by synthesizing specific transport systems or cell wall components capable of binding limiting substrates and transporting them into the cell. A good example of this is the ability of bacteria to compete for iron.
As we will see later in Unit 5 under innate immunity, the body makes considerable metabolic adjustment during infection to deprive microorganisms of iron. Iron is essential for both bacterial growth and human cell growth. Bacteria synthesize iron chelators - compounds capable of binding iron - called siderophores. Many siderophores are excreted by the bacterium into the environment, bind free iron, and then re-enter the cell and release the iron. Other siderophores are found on the cell wall where they bind iron and transport it into the bacterium.
Meanwhile, the body produces iron chelators of its own (transferrin, lactoferrin, ferritin, and hemin) so the concentration of free iron is very low. The ability of bacterial iron chelators to compete successfully with the body's iron chelators as well as those of normal flora may be essential to pathogenic bacteria. In addition to their own siderophores, some bacteria:
- Produce receptors for siderophores of other bacteria in this way take iron from other 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 influenzae are 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.
- Produce proteases that degrade human lactoferrin, transferrin, or heme to release the bound iron for capture by bacterial siderophores.
- Do not use iron as a cofactor. Borrelia burgdorferi instead uses manganese as a cofactor.
- Are able to produce exotoxins that kill host cells only when iron concentrations are low. In this way the bacteria can gain access to the iron that was in those cells.
Staphylococcus aureus, on the other hand, produces surface adhesins that bind to extracellular matrix proteins and polysaccharides surrounding host cell tissue, including fibronectin, collagen, laminin, hyaluronic acid, and elastin. S. aureus proteases and hyaluronidase then dissolve these components of the extracellular matrix providing food for the bacteria and enabling the bacteria to spread.
The ability to be pathogenic is directly related to the bacterium's ability to compete successfully with host tissue and normal flora for limited nutrients. Bacteria compete for nutrients by synthesizing specific transport systems or cell wall components capable of binding limiting substrates and transporting them into the cell. Iron is an essential nutrient for both bacterial growth and human cell growth. Both bacteria and their host synthesize compounds capable of binding iron for their use. Certain bacteria have developed means of “stealing” iron from other bacteria and/or from host cells.
Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)