Surface-enhanced Raman Spectroscopy for the Detection of a Metabolic Product in the Headspace Above Live Bacterial Cultures

In-situ surface-enhanced Raman spectroscopy of the headspace above cultures of 6 bacterial species allowed sensitive and selective detection of characteristic bands from chemisorbed methyl sulphide. This marker compound is created by dissociation of adsorbed dimethyl disulphide (DMDS) on the surface of the Au or Ag nanoparticle (NP) films which are used as the enhancing media. DMDS is a well-known fermentative metabolite of bacteria and was also detected here by GC-MS of the headspace of bacteria cultures. Kinetic binding plots of media spiked with DMDS and of live cultures, both showed that the more rapid adsorption of DMDS onto Au based substrates made them more suitable for the rapid detection of bacteria than Ag based substrates. E. coli DH5α was chosen for the proof-of-principal experiments. For this micro-organism, the sensitivity limit was found to be the detection of bacteria in the headspace of a 1.5 x 10⁷ CFU/ml culture. Under our conditions, this corresponds to detection 15 minutes after inoculation of the growth medium, but it will depend on the CFU/ml of the initial inoculum used. Since the metabolites are only produced by viable bacteria, antibiotic (gentamicin) treatment stopped the normal signal growth of the marker peak. This work is a promising step towards rapid bedside detection of bacterial infections and rapid screening of antibiotics against model cultures.