Question

how bacterial DNA is not harmed by the activity of restriction enzyme​

Answer 1

Bacteria prevent eating away their own DNA by masking the restriction sites with methyl groups ( CH3 ). Methylation of DNA is a common way to modify DNA function and bacterial DNA is highly methylated. In this case it functions to make the restriction sites unrecognizable for the restriction enzymes.

Restriction enzyme, also called restriction endonuclease, a protein produced by bacteria that cleaves DNA at specific sites along the molecule. In the bacterial cell, restriction enzymes cleave foreign DNA, thus eliminating infecting organisms.