Nonlabeled quartz crystal microbalance biosensor for bacterial detection using carbohydrate and lectin recognitions.

High percentages of harmful microbes or their secreting toxins bind to specific carbohydrate sequences on human cells at the recognition and attachment sites. A number of studies also show that lectins react with specific structures of bacteria and fungi. In this report, we take advantage of the fact that a high percentage of microorganisms have both carbohydrate and lectin binding pockets at their surface. We demonstrate here for the first time that a carbohydrate nonlabeled mass sensor in combination with lectin-bacterial O-antigen recognition can be used for detection of high molecular weight bacterial targets with remarkably high sensitivity and enhanced specificity. A functional mannose self-assembled monolayer in combination with lectin concanavalin A (Con A) was used as molecular recognition elements for the detection of Escherichia coli W1485 using a quartz crytsal microbalance (QCM) as a transducer. The multivalent binding of Con A to the E. coli surface O-antigen favors the strong adhesion of E. coli to the mannose-modified QCM surface by forming bridges between these two. As a result, the contact area between cell and QCM surface that increases leads to rigid and strong attachment. Therefore, it enhances the binding between E. coli and the mannose. Our results show a significant improvement of the sensitivity and specificity of the carbohydrate QCM biosensor with a experimental detection limit of a few hundred bacterial cells. The linear range is from 7.5 x 10(2) to 7.5 x 10(7) cells/mL, which is four decades wider than the mannose-alone QCM sensor. The change of damping resistances for E. coli adhesion experiments was no more than 1.4%, suggesting that the bacterial attachment was rigid, rather than a viscoelastic behavior. Little nonspecific binding was observed for Staphylococcus aureus and other proteins (fetal bovine serum, Erythrina cristagalli lectin). Our approach not only overcomes the challenges of applying QCM technology for bacterial detection but also increases the binding of bacteria to their carbohydrate receptor through bacterial surface binding lectins that significantly enhanced specificity and sensitivity of QCM biosensors. Combining carbohydrate and lectin recognition events with an appropriate QCM transducer can yield sensor devices highly suitable for the fast, reversible, and straightforward on-line screening and detection of bacteria in food, water, and clinical and biodefense areas.