Optimization of an efficient solid-phase enrichment medium for Salmonella detection using response surface...

Salmonella is a pathogenic bacterium contributing to food poisoning and acute infectious intestinal disease. The traditional standard detection method is based on the principle of liquid phase enrichment and has a low sensitivity on targeted bacteria. We previously developed a visual immunosensor technique for efficient detection and isolation of Salmonella by applying fluorescent nanobioprobes on a specially-designed cellulose-based swab. In this study, a whole-sample solid-phase enrichment assay (WSEA) was established by optimization of the enrichment medium using response surface methodology (RSM), a powerful statistical tool for regression analysis. The optimal formula was determined as: 0.60% polyvalent poly peptone, 0.40% buffered peptone water, 0.09% ferric citrate amine, 0.24% sodium hyposulfite, 0.035% cystine, 0.01 µg mL^-1 super absorbing polymer, 0.011% sodium deoxycholate, 15.00 µg mL^-1 ethyl green and 30.00 µg mL^-1 sodium selenite. Using this formula, Salmonella was visualized with naked eyes by relying on the indication of black spots formed on the swab. The analytic sensitivity of the assay was determined as 10¹ cells mL^-1 with a concentration of interfering bacteria (Escherichia coli) at 10⁵ cells mL^-1. This optimized formula was confirmed with 4006 patients' fecal samples, in which the positive rate was 0.42% by the conventional culture-based method and 2.12% by WSEA. The optimized formulation on solid phase enrichment by RSM allows relatively quick, low-cost, and large-scale detection of Salmonella, and could be used in grassroots medical institutions.