Mapping bacteria on filter membranes, an innovative SERS approach.

The existence of pathogenic bacteria in drinking water has been a threat to the safety of human well-being. Traditional methods to detect bacteria are standard plate counts or rapid methods such as ELISA and PCR. However, those methods can be time-consuming or require pre-enrichment when detecting low concentrations of bacteria. Filter membrane has been used for bacteria collection and culturing when monitoring environmental water samples. In this study, we applied surface enhanced Raman spectroscopy (SERS) to rapidly screen bacteria cells on a filter membrane based on the unique signal from an indicator molecule, 4-mercaptophenylboronic acid (4-mpba), that can specifically bind to the surface of bacteria through diol group in its structure and give off consistent SERS signal. With a pore size of 0.22 μm, the filter membrane functions as both a concentrating device and a separation mechanism that eliminates molecules smaller than the pore size. With SERS mapping, 4-mpba gives characteristic signal, indicating the presence of bacteria, whereas no 4-mpba signal was observed in the absence of bacteria. The developed method can detect Escherichia coli, Salmonella enterica, and Listeria monocytogenes on a filter membrane non-selectively in 80 min. Application of the method in pond water was demonstrated. However, this method may not be able to discriminate between live and dead bacterial cells and further development is needed.