The spontaneous formation of sub-nanometer scale electrical double layer (EDL) makes water an ideal media to make transistors. Owing to the strong interaction with high charge density in EDL, metals are more suitable than semiconductors provided there conductivity can be significantly modulated by gating. A novel principle of an electrochemical field effect transistor (eFET) made from noble metal operating in water is developed by self-assembling 1D chains of 10 nm Au particles based on an age-old concept of percolation. These nanoparticle necklace network (N3) exhibit a strong conduction gap due to the percolation threshold that can be tuned by gating the eFET leading to a switching gain of 104. The high gain and facile interface with metallic channel with naturally nano-rough interface, a “living transistor” gated by cell membrane potential of microorganisms has become possible while semiconducting materials eFETS (made from graphene, CNT, and nanowires) are limited to mammalian cells (that have thin cell walls). The low power and small format of these eFET microsensor has potential applications in combinatorial synthetic biology and multianalyte detection. Physics, engineering, and bio-application of living transistor will be described.