The Gram stain is commonly used in epidemiologic and clinical studies. It involves smearing a sample onto a slide, staining the material using dyes that bind to bacterial cells, and visually inspecting under a microscope. Many epidemiologic and clinical studies of pregnancy, sexually transmitted disease and lower respiratory infections include Gram stains in the study protocol. Clinically, Gram stains are performed routinely to guide diagnosis and treatment.
However, Gram stains are a potential source of bacterial DNA; using quantitative PCR, Gram stains can be interrogated to identify the presence and amount of specific bacterial genera and species. Although qPCR validation is required before testing each organism of interest, archived Gram stains can serve as a valuable resource for initial surveying of specific bacterial organisms and as preliminary resource to study bacterial community composition.
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