Counting bacteria using functionalized gold nanoparticles as the light-scattering reporter.

A simple and rapid bacterial counting method was developed based on dark-field light-scattering imaging of bacteria and gold nanoparticle (AuNP) reporter simultaneously. Commercially available DH5α E. coli strain was used as the model bacterium to demonstrate the feasibility of the proposed method. With antibody-conjugated AuNPs, the simple sample treatment and target E. coli strain recognition can be finished within 15-30 min, with a detection limit of 2-6 × 10(4) colony forming unit per milliliter (CFU/mL). By using 90 nm AuNPs as the light-scattering signal reporter, the bacterial recognition and counting can be easily performed with low-cost instrumentation such as an entry-level dark-field microscope setup and a common tungsten lamp as the light source. An automatic image analysis algorithm was also developed to facilitate robust and fast bacterial counting. The preliminary results of water, milk, and fruit juice sample analysis showed that this simple, fast, and cost-effective method can be easily adopted for routine bacterial detection.