Quartz crystal microbalance measurements suggest that a quorum-sensing LuxR homolog receptor SpnR in Serratia marcescens AS-1 dissociates from spn box after complexation with N-hexanoylhomoserine lactone (C6HSL). After reaching quorum of cells on increasing the C6HSL concentration, forming stable SpnR C6HSL complex probably leads to conformational changes in the DNA-binding domain, and its binding constant reduces by one order after complexation. The interaction analyses of immobilized spn box on a gold electrode with additive SpnR clearly shows the negative regulation for spn operon.