Progress of science from microscopy to microarrays (part 1): diagnosis of parasitic diseases.

Even though description of the magnifying glass goes back to 1021 by an Arabic physicist in his book, Antony van Leeuwenhoek was the first man to improve the then simple microscope for viewing biological specimens in 1674. This suggests that every discovery has scope for improvement, be it physics or be it biology. In the field of biology, scientists have long studied gene expression as a hallmark of gene activities reflecting the current cell conditions and response to host immune defense systems. These studies have been cumbersome, technically demanding and time-consuming. Application of microarrays has revolutionized this field and help understand the simultaneous expression of thousands of genes in a single sample put onto a single solid support. It is also now possible to compare gene expression in two different cell types, different stages of life cycle or two tissue samples, such as in healthy and diseased ones. Thus microarrays are beginning to dominate other conventional and molecular diagnostic technologies. The microarrays consist of solid supports onto which the nucleic acid sequences from thousands of different genes are immobilized, or attached at fixed locations. These solid supports themselves are usually glass slides, silicon chips or nylon membranes. The nucleic acids are spotted or synthesized directly onto the support. Application of microarrays is new for parasites. Most of these applications are done for monitoring parasite gene expression, to predict the functions of uncharacterized genes, probe the physiologic adaptations made under various environmental conditions, identify virulence-associated genes and test the effects of drug targets. The best examples are vector-borne parasites, such as Plasmodium, Trypanosoma and Leishmania, in which genes expressed, during mammalian and insect host stages, have been elucidated. Microarrays have also been successfully applied to understand the factors responsible to induce transformation from tachyzoite-to-bradyzoite and vice versa in Toxoplasma gondii. Thus microarrays provide a novel tool for diagnosis, prognosis and clinical management of infectious disease.