The best caulking to prevent mildew inside a shower stall depends on what the shower stall and caulking are made from. Almost all caulking contains mildewcide, an additive in caulks and paints that deters the growth of fungus, mold and mildew on the dried caulk film. However, some caulks resist mildew better than others.
In his article, "Mildew-Proof Caulk Manufacturers," Tim Carter explains that to make a mildew-proof caulk, the manufacturer would have to use enough mildewcide in the caulk to ensure that over the projected lifespan of the caulk seam, the mildewcide would remain active and prevent mildew from occurring.
Most mildewcides are water soluble, meaning they will dissolve with constant use of water over time. Since caulk is designed to stop water infiltration, and your shower is used daily, the mildewcide is exposed to water all the time. Therefore, mold will grow eventually.
According to Natural Handyman, Polyseamseal Tub and Tile Adhesive Caulk is not quite mildew-proof, but he highly recommendeds it as the most mildew-resistant caulk, lasting up to five years or more with no mildew.
Other types of caulking include polyvinyl acetate (PVA) caulks, acrylic latex based caulks, siliconized acrylic latex caulks and 100 percent silicone caulk. The use of 100 percent silicone caulk is recommended by manufacturers for fiberglass showers or bath enclosures. You also can use it with a three- or five-piece tub surrounded with caulked joints. Home Additions Plus recommends using acrylic latex caulk or PVA if the tub or shower surface area is ceramic or cast-iron based. Make sure the caulk says it's for bathroom showers and tubs.
Natural Handyman's advice on the best way to prevent mildew on, around or underneath caulking in a shower stall or enclosure is to apply the caulking correctly. Remove the old caulking, clean the area with alcohol, and if there is mold remaining, use a concentrated mildew killer to clean away the mold. Let it dry, and apply the new caulk.
Silicone caulk is hard to remove from your hands, walls or floors. Keep your hands clean, and wipe up any spills immediately.