Detection and Analysis of Bacillus subtilis Growth with Piezoelectric Quartz Crystal Impedance Based on Starch Hydrolysis

Abstract

A piezoelectric quartz crystal (PQC) impedance method based on the α-amylase-catalyzed hydrolysis of starch present in a culture medium has been developed for in situ monitoring of the whole growth process of Bacillus subtilis and the variation in the activity of α-amylase during bacterial growth. An S-shaped response behavior was observed for Δf0, and simultaneously inverse S-shaped responses were found for ΔR1 and ΔL1. The ratio of ΔR1 to Δf0 or ΔL1 coincided well with that calculated from Martin's equations reflecting the solution density–viscosity effect, suggesting that the continuing change in liquid loading onto the PQC surface causes significant variation in Δf0, ΔR1, and ΔL1. Bacterial growth equations were derived from the kinetics of the enzyme-catalyzed hydrolysis of starch, which fit well with the experimental responses of Δf0, ΔR1, and ΔL1. Kinetic parameters of bacterial growth, including the asymptote (A), the maximum specific growth rate (μm), and the lag time (λ), were obtained and were in good agreement with those obtained from the pour plate count method. The variation in the activity of α-amylase exhibited peak-type behavior with its maximum value at the later stage of the log phase. In addition, the influence of initial bacterial concentration was also investigated.