How would the defensin mimic be used by itself? Wouldn't a patient still have their other naturally occurring defense mechanisms?
Theoretically possible - but what would be the point. As a virtual certainty it will actually be used:
a) in places it doesn't normally appear (see b)
OR
b) in much greater concentration (tanks are great when supported by the correct proportion of infantry - but often suicidal when inadequately supported)
OR
c) where, clearly, the rest of the immune system is not working
Or some combo of the above. (And clearly just being used in a being with an immune system is not particularly helpful in preventing adaption of the micro-organisms to the drugs.)