Bacterial concentration and detection using an ultrasonic nanosieve within a microfluidic device

Standard methods, such as plate counting, to detect bacteria in samples where only small volumes and low concentrations are available, will result in a negative detection, unless additional enriching steps, such as culturing, are used. However, these are laborious and time consuming, which may prevent their effective application to time sensitive situations, for example in clinical settings or for food quality control. Microfluidic concentration of bacterial cells can address this issue, enabling accurate detection and quantification in low abundance samples even when only small sample volumes are used. In this work we use a packed bed of microparticles trapped in a microfluidic chip, that are activated with surface acoustic waves to periodically concentrate and detect bacteria from sample volumes below 10 µL. We demonstrate a bacterial capturing efficiency of 99% and further demonstrate that the concentrated bacteria can be recovered with an 80% efficiency. This highly concentrated recovered sample can then be successfully used in standard methods, such as plate counting and PCR, for the detection of the bacteria using just 1 µL of sample without the need for a culture-based enrichment process. When integrating our ultrasonic nanosieve with fluorescence sensing, it is possible to achieve rapid detection of a wide range of bacteria concentrations. The device enables the fluorescence detection of bacteria concentration of 4 × 10⁵ CFU/mL in only 15 s and achieved a limit of detection of 3.25 × 10² CFU/mL with just 32 min of ultrasonic actuation, requiring only 10 µL of sample. These results demonstrate that our device offers a scalable, portable, and affordable method for the monitoring of low bacterial concentration using small sample volumes.