Ordinary Raman spectroscopy could be used to detect bacteria, in principle. In practise the concentrations of the associated molecules are so low, the signals overshadowed by other effects.
This is where SERS and the nano particles come in. When struck with light of certain frequencies, nanoparticles can induce a plasmon resonance. This resonance can give rise to a strong magnification (1000–100 000x) of parts of the Raman spectrum.
Several academic papers describe how they used Surface Enhanced Raman Spectroscopy to distinguish bacterial species from colonies grown on a Petri dish. The nature of SERS enhancement depends however on the characteristics of the nano particles than are being used.