The enzyme which cuts down the viral dna with no harm to bacterial chromosme is

Bacteria and restriction enzymes

,use enzymes to cut (and thereby destroy) foreign DNA (such as viral DNA), which would restrict the growth of the virus; own DNA is protected in some way (often by addition of methyl group CH3) to the sequence recognized by enzyme (thus preventing enzyme from binding there)

The answer would be a single celled organism.

Explanation:

Even though transforming genetically any organism is not an easy task, as it requires the whole organism to be changed, it's more practical to do in a unicellular organism. For example, bacteria mutate very easily due to its ability to incorporate new genes by horizontal transfer. Anyway, unicellular organisms have mechanisms to avoid strange genetic material to get into their DNA, that's why scientists have to use such tools as vectors and more.