Question

Bacteria in our normal microflora do not trigger immune response but, like all bacteria, need iron and therefore have iron acquisition systems. Why do the bacteria in our normal flora not trigger immune response if they are chelating iron from our cells?

Answer 1

My answer to this would be that it is evolutionary favorable for us to have microflora, although the bacteriamay leach iron from our system we have a mutualistic relationship with them. Some of their functions include,

-fermenting unused energy substrates
-training the immune system
-preventing growth of harmful pathogenic bacteria <- A very important point.
-regulating the development of the gut
-producing vitamins for the host (such as biotin and vitamin K), and producing hormones to direct the host to store fat
(This list is from wikipedia, but it sounds correct, I had to look it up due to possessing a fleeting memory)

It is pretty easy to see from this list that it would have been advantageous for us to develop an immune system that doesn't recognize specific bacteria as a threat, as this would waste precious resources and therefore make someone less likely to survive as long and be reproductively successful before modern medical advances.

Answer 2

They do trigger immune response, but not to the extent that they are wiped out. The "actual" correct answer is very close to this. The bacteria in our normal flora are not pathogenic although they do in some forms "attack" our cells, they chelate iron from our cells that we would otherwise use, they eat some nutrients that we would otherwise use, they occasionally do mean things to us but all in all, they are our "friends".
Mainly, bacterial cells need more than one virulence factor to be considered pathogenic, a lot more than one. One would be high affinity iron chelators. Another could be the ability to bind to epithelial cells in our gut, another could be forcing the epithelial cells to endycytose them, another could be preventing the primary lysosome from fusing with the phagosome. It needs all of these (and likely more) to be considered pathogenic.

As for the second half, our body doesn't "ignore" certain types of bacteria, for example, e coli is present in most everyone's small intestines, if we didn't have it, a lot of bad things would happen. When e coli gets a plasmid by horizontal gene transfer it can gain pathogenicity and infect us, it is the same bacteria, but it expresses some different proteins. Also, our body attacks those e coli cells as they are foreign but they don't attack them to the extent that they would attack Salmonella enterica that drops into our intestines.

In summation, you are mostly right, but I just asked this question so openstudy would stop telling me to ask a question. Sorry about that.

Answer 3

No problem, It has been a long time since i have answered such a question thanks for the specifics.