Abstract The aim of this study the effect of H2O2 produced by E. faecalis in causing genomic instability by using comet assay and visualizing the histological alterations in rats colon tissues. The results showed a significant decrease correlated with the decrease of the concentration of E. faecalis in each of following characters: the mean % of the length of tail, the mean % of the DNA damage and the mean % of the tail lengthening. These differences were observed in all the cells, exposed to both concentrations of E. faecalis and were then compared with that of the control (without E. faecalis –treated cells).To determine whether dietary iron causes any genomic instability associated with adenomas or colorectal cancer, the comet assay has been used to detect DNA damage. All intestinal epithelial cells showed an increase in DNA damage after co-incubation with E. faecalis (EFCS).This increase was higher in rats which were fed dietary iron for 4 months. The role of catalase in preventing DNA –damage mediated by H2O2 was also studied .The results showed a significant decrease (P≤0.01) in the mean percentage of DNA damage in the treated cells.