How did subject Delta obtain the ability to save Little Sisters?

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In original Bioshock, the main character obtains the ability to
return Little Sisters back to their human state,
by getting special plasmid from
Brigid Tenenbaum
Yet in Bioshock 2, Subject Delta seems to have this ability from very beginning of game. Is it explained in any way how?
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How did Subject Delta survive?
In the end, Eleanor either preserves Delta's essence within herself by extracting his ADAM, or she allows him to die without doing so. The "Good" Ending (Sofia lives): Save all the Little Sisters. Spare at least one choice character (Grace, Stanley, Dr. Alexander).How do you rescue Little Sisters in Bioshock?
By rescuing a Little Sister, Jack runs his hand over the forehead of the angry Little Sister, and with a flash of light, they're saved and returned to a normal state.How many Little Sisters can you harvest and still get the good ending?
There are a total of 21 Little Sisters which can be rescued or harvested, disregarding any extra Little Sisters which may appear as a result of glitches. If the player harvests all 21 of these Little Sisters, they will gain a total of 3360 ADAM throughout the game.Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Kamaji Ogino, RODNAE Productions, Suzy Hazelwood, Julissa Helmuth