These microvilli bathe in our saliva. Under the microvilli are the sense receptor cells. Closer to the inside of the tongue, there are thread-like fibers that have synapses on the ends. Through this process an individual can experience the bitter taste. As mentioned before, taste buds are bathed in saliva.
Saliva contains water, amino acids, proteins, sugars, organic acids, salts, etc.
Taste buds are fed by blood, which is an even more complex solution. Because there are so many substances passing over the tongue, often what one perceives is the change in concentration of a stimulus. Each person has varying degrees of sensitivity and different threshold levels to substances.