Probably iron supplementation is prone to development of pathogenic bacterial colonisation...

In breast-fed infants bifidobacteria is predominant, counts of Escherichia coli is low, and other bacteria is rarely present. Infants receiving Iron fortified cow-milk preparation have high counts of Escherichia coli, counts and isolation frequency of bifidobacteria  are low and other bacteria are frequently isolated. In those on unfortified cow-milk preparation isolation frequency of Escherichia coli, bifidobacteria and bacteroides is comparable with that in breast-fed infants; however, counts of Escherichia coli is high.

 Mohammad, 

Thank you for your valuable input.  Do you have a reference for the differences in gut flora.