Quite a bit of bacteria is resistant to Maracyn Two now; it is minocycline and it's been overused in the aquarium world and often to treat the wrong diseases so yes, a lot of bacteria is resistant. In addition, it has potential to harm the kidneys and it is not effective in high pH waters.
There are only two antibiotics that are good for treating internal infections. They are Kanaplex (kanamycin) and Maracyn Two (minocycline). Both absorb easily through the gills whereas other antibiotics do not and are only effective for topical infections. But again, in the cases of dropsy symptoms where a kidney infection is a possible cause, Kanaplex is preferred.