Discussion
Our goal is to develop rapid molecular diagnostic approaches to the identification of uropathogens involving techniques that can be applied in a variety of settings and platforms, including a point-of-care microfabricated device. To achieve this goal, the effects of time and temperature on the specificity of oligonucleotide probe binding to uropathogen 16S rDNA sequences were examined using both a rapid dot-blot hybridization method and a novel electrochemical sensor array platform. The rapid dot-blot hybridization method described here involves 10min each for the prehybridization, hybridization, and washing steps, or a total of only 30min for all three steps. In the standard dot-blot method, hybridization typically involves an overnight incubation. Our rapid approach demonstrates that if rRNA is used rather than PCR-generated rDNA, then a much shorter analytical time would be sufficient to generate a robust signal, allowing an approximately 24to 36-fold reduction in procedural time (from approximately 12–18h to 40min). The success of the rapid dot-blot method is critically dependent on probe characteristics. Oligonucleotide probes were designed by comparing the 16S gene sequences of eleven uropathogenic species to locate variable regions containing signature sequences for four uropathogens: E. coli, P. mirabilis, P. aeruginosa, and K. oxytoca. Probes were selected that contained at least Wve mismatches per 20bp region in each of the nontarget sequences. Probes with fewer than Wve mismatches per 20bp region were not considered robust, since they yielded false-positive results and therefore they were redesigned (data not shown). Probes with mismatches located in the center of the sequence had greater speciWcity than probes with mismatches concentrated at the ends. This degree of sequence mismatching was suYcient to obtain target speciWcity not only at 55°C but also in
the much less stringent condition of room temperature hybridization. The dot-blot experiments were informative regarding probe design, kinetics, and temperature-dependence of hybridization using a well-established technique. We examined whether the results of the rapid dot-blot hybridization studies could be applied to a novel electrochemical sensor array platform. There is enormous interest in development of DNA biosensors that are able to detect nucleic acid hybridization events without the need for target ampliWcation [8,20]. An electrochemical sensor provides instantaneous and quantitative readout of nucleic acid hybridization, which oVers tremendous advantages for clinical microbiology and genetic diagnostics. The sensor strategy combines approaches from diverse disciplines including microfabrication technology, self-assembled monolayer (SAM) generation, DNA/RNA hybridization, and enzyme signal ampliWcation. SAMs allow immobilization of biological molecules such as streptavidin onto microfabricated devices to provide surface functionalization