Optical detection and enumeration of Escherichia coli and Salmonella enterica using a low-magnification light microscope

A rapid and cost-effective method for detecting bacterial cells from surfaces is critical to food safety, clinical hygiene, and pharmacy quality. Herein, we 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×) objective lens. Then, integrate the developed optical detection method with swab sampling to detect bacterial cells loading on stainless-steel surfaces. Using Salmonella enterica (SE1045) and Escherichia coli (E. coli OP50) as model bacterial cells, we achieved a capture efficiency of up to 76.0 ± 2.0 % for SE1045 cells and 81.1 ± 3.3 % for E. coli OP50 cells at 10³ CFU/mL upon the optimized conditions, which slightly decreased with the increasing bacterial concentrations. Our assay showed good linear relationships between the concentrations of bacterial cells with the cell counting in images in the range of 10³ -10⁷ CFU/mL for SE1045, and 10³ -10⁸ CFU/mL for E. coli OP50 cells. The limit of detection (LOD) was 10³ CFU/mL for both SE1045 and E. coli OP50 cells. A further increase in sensitivity in detecting E. coli OP50 cells was achieved through a heat treatment, enabling the LOD to be reduced as low as 10² CFU/mL. Furthermore, a preliminary application succeeded in assessing bacterial contamination on stainless-steel surfaces following integration with the approximately 40 % recovery rate, suggesting prospects for evaluating the bacteria from surfaces. The entire process was completed within around 2 h, costing merely a few dollars per sample. Considering the low cost of standard light microscopes, our method holds significant potential for practical industrial applications in bacterial contamination control on surfaces, especially in low-resource settings.