Conductometric sensor for viable Escherichia coli and Staphylococcus aureus based on magnetic analyte separation via aptamer

A method is described to determine viable populations of Escherichia coli ( E. coli ) and Staphylococcus aureus ( S. aureus ). The method employs aptamer-magnetic separation combined with resistivity based detection. The bacteria were separated by means of aptamer-functionalized magnetic beads. They were then quantified by measuring their growth kinetics through time-dependent conductivity changes of culture media. The time-course of growth was logged by real-time and contactless measurements that yielded starting concentrations from the duration of lag intervals prior to the log phase of growth. In pure water samples, the linear ranges for measuring E. coli and S. aureus cells are 2.5 × 10 3 –2.5 × 10 8  CFU·mL −1 and 4.1 × 10 3 –4.1 × 10 8  CFU·mL −1 , respectively. In spiked tap water samples, the lower limits of detection are 2.3 × 10 4  CFU·mL −1 and 4.0 × 10 3  CFU·mL −1 for E. coli and S. aureus , with recoveries of 87.0–108.7% and 92.5–105.0%, respectively. The relative standard deviation of these measurements (10.0%) is below that of plate counting method (13.9%). The presence of micro/nanoparticles such as magnetic beads or selenium nanoparticles in the culture media does not interfere, unlike in case of automatted optical density monitoring. The E. coli and S. aureus cells captured on the aptamer-functionalized magnetic beads can be directly tested for their susceptibility to antibiotics. The process of magnetic separation and determination of load burden requires neither bulky, sophisticated equipment nor expensive reagents. Aptamer-functionalized magnetic beads are used to selectively capture and separate E. coli and S. aureus cells in aqueous samples. They are directly transferred to a multichannel conductometric sensor for the quantification of viable bacteria via automated monitoring of their growth kinetics.