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Why is iron essential?
Iron is essential for almost all forms of life, because it is a necessary component in for energy production, DNA replication, transportation of oxygen and protection against oxidative stress. Bacteria are not an exception in this respect and they have to be able to aquire iron in the animal hosts to multiply and cause disease.
How can the host animal utilize that iron is essential for bacteria?
One defense mechanism that higher organisms have against bacteria is to minimize the access to free iron. The main part of the iron in vertebrates is intracellular and bound to the iron binding protein ferritin or complexed to the porphyrin ring of heme, which is a cofactor to hemoglobin and myoglobin. Furthermore, in serum there is a iron transporting protein in serum, which is called transferrin, to which iron is bound extremely strong.
How can bacteria circumvent the animal host?
It wouldn't be possible for bacteria to survive within an animal host unless it has developed special mechanisms for extraction of the bound iron. Most pathogenic bacteria have mechanisms for uptake of iron with such high affinity for iron they can compete with the iron binding substanses of the host animal. These mechanisms can be based on siderophores, heme aquisitation or transferrin/lactoferrin receptors.
Borrelia burgdorferi, which causes Lyme disease, is an exception from the above rule since it utilizes manganese instead of iron. That is, enzymes which normally contain iron are modified and in B. burgdorferi they contain manganese. Members of the genus Lactobacillus have also been regarded as non-dependent of iron, but this has been questioned in recent studies.