A novel method for determination of trace amounts of microorganism populations in solution was developed by using a multiwalled carbon nanotube (MWNT) modified glassy carbon electrode and square wave voltammetry (SWV). The simultaneous combination of MWNT and SWV allowed the electrochemical signal of electro-active materials in Escherichia coli O157:H7 (E. coli) to be dramatically amplified. Compared with a bare glassy carbon electrode, the MWNT-modified glassy carbon electrode showed catalytic properties in the oxidation of electro-active materials on cell surfaces. Moreover, SWV was proved to be more sensitive than cyclic voltammetry (CV) for investigation of the electrochemical behavior of cells. In this paper, a linear relationship was obtained between the SWV peak current and the cell concentration in the range 2×102–2×108 cellmL−1 with a detection limit of 2×102 cellmL−1. The effect of antibiotic drug Gentamycin Sulfate injection (GSI) on the growth of E. coli was also investigated.