Rapid enumeration of bacterial cells in drinking water using a microfluidic device

The abundance of bacterial cells should be routinely monitored to maintain microbiological quality control of drinking water. We investigated the ability of a newly designed microfluidic device to determine bacterial numbers in freshwater at a density in the order of 104 cells/ml. A microfluidic device was designed and fabricated by polydimethylsiloxane (PDMS) and a thin glass slide (size: 48 mm x 23 mm). Sheath fluid and fluorescently stained samples were placed in gastight syringes, and injected into the microfluidic device via Teflon tubes by syringe pumps at flow rates of 2.0 mu l/min. A fluorescent microscope-based system was used for enumeration of cells flowing rapidly through the microchannel. The numbers of fluorescent beads and E. coli cells at a density in the order of 10(4)/ml determined by this on-chip flow cytometer were similar with the microscopic counts. The time required for quantitation by on-chip flow cytometer was only 10 min per sample, which was 3- to 5- fold less than microscopy, which usually requires concentration of bacterial cells onto a membrane filter. This simple enumeration method will contribute to the technical progress in microbiological quality control of drinking water.