Bitterness can be detected in a solution as weak as one part per 2 million, sourness one part per 130,000, and saltiness one part per 400. It takes much more sweetness to register a sweet sensation - one part per 200.
However, taste buds can be tricked. After you brush your teeth, the usually sweet taste of orange juice seems bitter because of the chemicals left behind by your toothpaste. Conversely, certain chemicals in artichokes make almost anything you put in your mouth for a few minutes afterward seem sweet.