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., similar to 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 >= similar to 10(5), and Gram-positive bacteria, such as S. aureus with a cell number of >= similar to 10(6), 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 similar to 30 min. To improve visual sensitivity, we dipped the swab carrying the bacteria in a vial containing a growth broth, incubated it for similar to 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 >= similar to 34 were obtained, whereas air bubbles were visible in the samples containing S. aureus with initial cell numbers of >= similar to 8.5 x 10(3).