Identification and quantitation of pathogenic bacteria via in-situ formation of silver nanoparticles on cell walls, and their detection via SERS

The authors present a fast and sensitive surface-enhanced Raman spectroscopy (SERS) method for the determination of Mycobacterium smegmatis. It is based on the formation of silver nanoparticles (AgNPs) directly on the surface of bacteria via the silver mirror reaction. To achieve this, the bacteria are mixed with silver nitrate, treated with NaOH, and then with ammonium hydroxide until all silver oxide precipitate is completely dissolved. Treatment of the reaction mixture with glucose at 55 °C results in the formation of AgNPs on the surface of the bacteria. The detection of M. smegmatis by SERS is simple and straightforward in that 4 μL of a suspension of Ag-coated M. smegmatis are pipetted onto a polypropylene surface and SERS spectra are acquired. Quantitative evaluation is done best by using the distinct vibrational band at 731 cm⁻¹. Routine detection and identification of M. smegmatis thereafter required ∼1000 bacilli per laser spot area and a limit of detection below 100 bacilli, using a cheap, dispersive Raman spectrometer. Our method was also applied to detect M. bovis BCG, M. tuberculosis, Staphylococcus aureus, S. epidermis, Bacillus cereus and two laboratory strains of Escherichia coli, thus demonstrating the wider applicability of this approach. (author)