I thought the slugs were implanted in the Little Sisters to assist their recycling ADAM from the dead of the civil war. You see the Little Sisters extracting blood from corpses and drinking it. Since they held such valuable cargo, the ADAM, Dr. Suchong created the Big Daddies to protect them as they went about their grisly work.

I don't think the slugs in the Little Sisters actually created the ADAM.

The slugs were the source of ADAM in the first place. initially Rapture's plasmid industry harvested the slugs but they just couldn't produce the volume needed to support a plasmid industry.

Tenenbaum figured out a process to graft the slugs into the digestive tracts of little girls. Initially they were kept in confinement as their bodies produced massive amounts of ADAM, but during the Rapture civil war they were repurposed to extract ADAM from the dead by ingesting their blood, and given Big Daddy bodygaurds. Since the slugs were embeded in ttheir digest tracts, when the Little Sisters drank the blood of people with ADAM in their systems, the slugs were capable of processing the ADAM from the blood. The Little Sisters' bodies were saturated with ADAM from this process.

This is why when you save the Little Sisters, you're using a Plasmid created by Tenenbaum that kills the slug and siphons the ADAM from the girl's systems. Alternatively by killing the Little Sisters you're basically ripping the slugs from their bodies to get at the more potent ADAM reserve.