Enumeration of Bacteria in Suspensions Using Time Domain Reflectometry

Microbial detection techniques, such as bacterial counting, are essential in all aspects of environmental monitoring and analysis. However, the standard plate count method for bacterial enumeration with colony-forming units is time-consuming and labor-intensive. In this study, we present a fast and accurate method to count bacteria cells using the technique of time-domain reflectometry (TDR) based on the electrical properties of bacterial cell suspensions. A series of suspensions with various bacterial concentrations were used as the test materials, and the electrical conductivity (σ_a) was determined using the TDR method. The TDR measured-σ_a value was converted to the concentration of bacterial suspension using a pre-established standard curve on three types of bacteria, i.e., Bacillus subtilis (B. subtilis), Pseudomonas fluorescens (P. fluorescens), and Escherichia coli (E. coli). The σ_a values of suspensions increased exponentially with bacteria concentrations, mainly due to the release of Cl^- and extracellular polymeric substances from the cells that were electrically conductive. For the three types of bacterial strains, the lower detection limits were 6 log CFU mL^-1 for B. subtilis, and 7 log CFU mL^-1 for P. fluorescens and E. coli. Independent evaluation showed that values from the TDR based method matched well with those obtained with the traditional plate count method, with RMSEs of 0.260, 0.166, and 0.198 log CFU mL^-1 for B. subtilis, P. fluorescens, and E. coli, respectively. The TDR based approach provides a fast and accurate means for detecting bacterial cell numbers in suspensions.