Are antibiotics the safest ways to treat ecthyma gangrenosum?

Yes: This is overwhelmingly a bacterial infection which should respond to antibiotics if aided by the host's immune system. If you have no white blood cells, however, response may be quite limited.

Infection: Ecthyma gangrenosum is an infection that is related to pseudomonas aeroginosa, and sepsis related to this bacteria the best mechanism to treat this infection is with antibiotics. Interestingly enough, hyperbaric oxygen therapy is also effective, since the high oxygen levels destroys the bacterial toxin.