Pneumococcal resistance to penicillin G is mainly acquired by:

Transformation: A resistant bacterium may release the resistance carrying DNA into the medium and this may be imbibed by another sensitive organism-becoming unresponsive to the drug. This mechanism is probably not clinically significant except isolated instances of pneumococcal resistance to penicillin G due to altered penicillin binding protein, and some other cases. Resistance once acquired by any of the above mechanisms becomes prevalent due to the selection pressure of a widely used AMA, i.e. presence of the AMA provides oppounity for the resistant subpopulation to thrive in preference to the sensitive population. ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:671