Bio-cathode denitrification is a new and unique technology for autotrophic nitrogen removal, which uses electricity instead of chemical matter as electron donor. Functional microorganisms are the driver of bio-cathode denitrification and inocula are the origin of functional microorganisms, so three typical inocula, denitrifying, denitratating and anammox (anaerobic ammonia oxidizing) sludge, were chosen to investigate their effect on the performance of bio-cathode denitrification in this work. The results showed that the nitrogen removal rate of denitrifying, denitratating and anammox bio-cathodes was around 101.79 mg/L.d, 47.31 mg/L.d and 100.04 mg/L.d respectively. The output power was 24.01 mW/m(2), 14.11 mW/m(2) and 45.93 mW/m(2). The denitrifying, denitratating and anammox biofilms were observed to grow on cathodes, indicating that the functional microorganisms could use electricity as the sole electron donor and so they were named as electrotrophic microorganisms. The growth rate and the cell yield of denitrifying, denitratating and anammox biofilms were 24 g/m(2).d and 1.71 x 10(-3) g/g NO3--N, 7.67 g/m(2).d and 1.94 x 10(-3) g/g NO3--N, 41 g/m(2).d and 2.32 x 10(-3) g/g NO2--N, respectively. Pseudomonas, Glycocaulis and SM1A02 were found to be enriched in the three biofilms, and Pseudomonas stutzeri was deduced to be the key functional microorganism. Denitrifying sludge was the optimal inoculum when wastewater treatment was the main purpose; however, Anammox sludge turned to be the optimal inoculum when wastewater treatment and energy recovery were combined. The functional microorganisms and their metabolic characteristics were revealed to be the critical factor that determined the enrichment of electrotrophic microorganisms from inocula and the performance of bio-cathode denitrification. These findings are significant to get an insight into the new electrotrophic microorganism and to develop the novel bio-cathode denitrification technology.