Label-free detection of live bacteria using growth medium enhanced with Au nanoparticles

We investigate whether incubating bacteria in a growth medium that contains gold nanoparticles can provide a more convenient platform for determining bacterial concentration than existing techniques. Citrate AuNPs were mixed with Luria-Bertani broth, seeded with known concentrations of live bacteria, and the mixture monitored for 12 h. Changes to the optical properties were detected as the bacteria proliferated. Peak wavelength of the longer wavelength particle-particle plasmon resonance, the inflection point associated with this peak, an increase in overall attenuance at 400 nm, spectral correlation to a reference spectrum and aggregation factor were each influenced by the starting bacterial concentration. The wavelength of the particle-particle plasmon resonance provided the most consistent results. Incubation times of 6 to 8 h provided optimum sensitivity using this signal alone, with a detection limit of 2.1 × 10⁴ CFU/mL at 95% confidence. Since each parameter extracted from the experimental spectra was subject to some scatter, a three-channel calibration based on the equally weighted average of the individual calibration curves based on attenuance, wavelength of the aggregation plasmon peak and spectral correlation was also investigated. This provided an even lower detection limit of 360 CFU/mL at 95% confidence.