We develop an analytical formulation to calculate the induced resonance frequency shifts of an electrostatically actuated torsional microsensor due to an added bio-mass (E. coli). Based on a lumped-parameter model, we investigate the static and the linear eigenvalue problem of the torsional microsensor. The added mass is modeled as a point mass load placed on different locations along the x-axis of the microsensor. A closed-form expression is determined to calculate the frequency shift under a DC voltage as a function of the added mass. Finite Element Analysis (FEA) results show good agreement with the analytical approach.