Rapid Detection of Pathogenic Bacteria by the Naked Eye

Escherichia coli O157:H7 and Staphylococcus aureus are common pathogens. Gram-negative bacteria, such as E. coli, contain high concentrations of endogenous peroxidases, whereas Gram-positive bacteria, such as S. aureus, possess abundant endogenous catalases. Colorless 3,5,3',5'-tetramethyl benzidine (TMB) changes to blue oxidized TMB in the presence of E. coli and a low concentration of H₂O₂ (e.g., ~11 mM) at pH of 3. Moreover, visible air bubbles containing oxygen are generated after S. aureus reacts with H₂O₂ at a high concentration (e.g., 180 mM) at pH of 3. A novel method for rapidly detecting the presence of bacteria on the surfaces of samples, on the basis of these two endogenous enzymatic reactions, was explored. Briefly, a cotton swab was used for collecting bacteria from the surfaces of samples, such as tomatoes and door handles, then two-step endogenous enzymatic reactions were carried out. In the first step, a cotton swab containing bacteria was immersed in a reagent comprising H₂O₂ (11.2 mM) and TMB for 25 min. In the second step, the swab was dipped further in H₂O₂ (180 mM) at pH 3 for 5 min. Results showed that the presence of Gram-negative bacteria, such as E. coli with a cell number of ≥ ~10⁵, and Gram-positive bacteria, such as S. aureus with a cell number of ≥ ~10⁶, can be visually confirmed according to the appearance of the blue color in the swab and the formation of air bubbles in the reagent solution, respectively, within ~30 min. To improve visual sensitivity, we dipped the swab carrying the bacteria in a vial containing a growth broth, incubated it for ~4 h, and carried out the two-stage reaction steps. Results showed that bluish swabs resulting from the presence of E. coli O157: H7 with initial cell numbers of ≥ ~34 were obtained, whereas air bubbles were visible in the samples containing S. aureus with initial cell numbers of ≥ ~8.5 × 10³.