Bag having slider-actuated complementary closure strips and a leakproofing structure

A bag having two generally parallel sheets forming the main walls of the bag, complementary closure strips attached to the sheets, and a slider provided to open and close the closure strips mounted thereon, the bag comprising, parallel to the closure strips, leakproofing means disposed on the insides of the closure strips, the leakproofing means designed to provide a leakproof barrier forming a barrier-type seal between the closure strips when the closure strips are in a closed position, the leakproofing means being adapted to be urged towards the opposing strip by the slider when the slider is moved towards a bag closing position, wherein the leakproofing means comprises at least one curved, flexible, cylindrically shaped section whose outside surface is in contact with the surface of the opposing strip to provide the leakproof barrier when the closure strips are closed.