Optical detection of bacterial cells on stainless-steel surface with a low-magnification light microscope

A Rapid and cost-effective method for detecting bacterial cells on surfaces is critical to protect public health from various aspects, including food safety, clinical hygiene, and pharmacy quality. Herein, we first established an optical detection method based on a gold chip coating with 3-mercaptophenylboronic acid (3-MPBA) to capture bacterial cells, which allows for the detection and quantification of bacterial cells with a standard light microscope under low-magnification (10 fold) objective lens. Then, integrating the developed optical detection method with swab sampling to achieve to detect bacterial cells loading on stainless-steel surfaces. Using Salmonella enterica (SE1045) and Escherichia coli as model bacterial cells, we achieved a capture efficiency of up to 76.0 % for SE1045 cells and 81.1 % for E. coli cells at Log 3 CFU/mL upon the optimized conditions. Our assay showed good linear relationship between the concentrations of bacterial cells with the cell counting in images with the limit of detection (LOD) of Log 3 CFU/mL for both SE1045 and E. coli cells. A further increase in sensitivity in detecting E. coli cells was achieved through a heat treatment, enabling the LOD to be pushed as low as Log 2 CFU/mL. Furthermore, successful application was observed in assessing bacterial contamination on stainless-steel surface following integrating with swab collection, achieving a recovery rate of approximately 70 % suggests future prospects for evaluating the cleanliness of surfaces. The entire process was completed within around 2 hours, with a cost of merely 2 dollars per sample. Given a standard light microscope cost around 250 dollars, our developed method has shown great potential in practical industrial applications for bacterial contamination control on surfaces in low-resource settings.