Rapid Microbiological Methods for a New Generation

By Michael J. Miller, Ph.D., Senior Research Fellow, Eli Lilly and Co. Microarrays are collections of miniaturized test sites, arranged on solid substrates, that permit many tests to be performed at the same time. They are composed of an orderly arrangement of protein or thousands of DNA or RNA fragments on glass, silicon or nylon substrates. This technology evolved from Southern Blotting technology, in which fragmented DNA is attached to a substrate and then probed with a known gene or DNA fragment, using fluorescent tags to allow visual detection. Microarrays are usually fabricated using a variety of technologies, including printing with fine-pointed pins onto glass slides, photolithography, ink-jet printing or electrochemistry. Other methods may also be used, such as in situ synthesis, whereby the probes are synthesized directly on the chip instead of spotting them on the array. An example would be CombiMatrix’s (Mukilteo, Wash.) CustomArray. Applications for microarrays include nucleic acid sequence identification and measuring expression levels of genes. For example, Affymetrix’s (Santa Clara, Calif.) GeneChip contains the entire human genome (~50,000 known genes and gene variants) on a single chip, while CombiMatrix’s CustomArray contains probes that can detect influenza A and Avian H5N1 (bird flu) strains. Both of these platforms are commercially available. A biosensor can detect an analyte that is comprised of a biological component combined with a physicochemical detector component. Nineteenth-century miners used a canary in a cage as a biosensor to detect lethal concentrations of gas. Today, the most widespread example of a commercially available biosensor is the blood glucose monitor, but future uses will include remote sensing of airborne bacteria and detection of pathogens. The biological component of a biosensor may contain tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids or whole cells. The detector element works in a physicochemical manner and may include