Development of a gold nanoparticle-based colorimetric assay for rapid detection of total bacterial count and coliform bacteria using the molecular hybridization approach

Total bacterial counts (TBC) and coliforms bacteria have been used as the index of microbial quality and fecal contamination for foods, the detection of them is involved in microbiological criteria of most countries to guarantee food safety. In this study, a gold nanoparticle (AuNPs) based colorimetric assay for rapid detection of TBC and coliform bacteria was developed by using the molecular hybridization approach. Thiol modified primers SH-27F and SH-lacZF were designed and conjugated with AuNPs, which was acted as both hybridization probe and signal probe, and salt induced aggregation of AuNPs was indicator of the hybridization and detection. Escherichia coli was studied as model strain in this study. The result showed that under the optimal conditions, the limit of detection for TBC and coliforms bacteria were 10² CFU/mL and 10¹ CFU/mL by visual detection, and 10¹ CFU/mL by UV–vis spectrum analysis, a smart phone application named ColorPicker was used for the quantitative analysis of the colorimetric assay. The colour signal had good linear relationship of E. coli concentration in the range of 10¹–10⁴ CFU/mL. The detection is rapid, simple and sensitive, without the needing of culture enrichment and precious equipment, has good application potential for the on-site detection of TBC and coliforms bacteria. In addition, the proposed strategy provides a direction for the design of highly sensitive colorimetric signal probes for other pathogenic microorganisms.