Sunday, September 11, 2011

Types of Microbiological stains

nullGram stain: The Gram stain classifies bacteria according to whether they retain crystal violet stain (gram-positive—blue) or not (gram-negative—red) and highlights cell morphology (eg, bacilli, cocci) and cell arrangement (eg, clumps, chains, diploids). Such characteristics can direct antibiotic therapy pending definitive identification. To do a Gram stain, technicians heat-fix specimen material to a slide and stain it by sequential exposure to Gram's crystal violet, iodine, decolorizer, and counterstain (typically safranin).
Acid-fast and moderate (modified) acid-fast stains: These stains are used to identify acid-fast organisms (Mycobacterium sp) and moderately acid-fast organisms (primarily Nocardia sp). These stains are also useful for staining Rhodococcus and related genera, as well as oocysts of some parasites (eg, Cryptosporidium).
Although detection of mycobacteria in sputum requires only about 5, 000 to 10, 000 organisms/mL, mycobacteria are often present in lower levels, so sensitivity is limited. Usually, several mL of sputum are decontaminated with Na hydroxide and concentrated by centrifugation for acid-fast staining. Specificity is better, although some moderately acid-fast organisms are difficult to distinguish from mycobacteria.
Fluorescent stains: These stains allow detection at lower concentrations (1 × 104 cells/mL). Examples are acridine orange (bacteria and fungi), auramine-rhodamine and auramine O (mycobacteria), and calcofluor white (fungi, especially dermatophytes).
Coupling a fluorescent dye to an antibody directed at a pathogen (direct or indirect immunofluorescence) should theoretically increase sensitivity and specificity. However, these tests are difficult to read and interpret, and few (eg, Pneumocystis and Legionella direct fluorescent antibody tests) are commercially available and commonly used.
India ink (colloidal carbon) stain: This stain is used to detect mainly Cryptococcus neoformans and other encapsulated fungi in a cell suspension (eg, CSF sediment). The background field, rather than the organism itself, is stained, which makes any capsule around the organism visible as a halo. In CSF, the test is not as sensitive as cryptococcal antigen. Specificity is also limited; leukocytes may appear encapsulated.
Wright's stain and Giemsa stain: These stains are used for detection of parasites in blood, Histoplasma capsulatum in phagocytes and tissue cells, intracellular inclusions formed by viruses and chlamydia, trophozoites of Pneumocystis jiroveci, and some intracellular bacteria.
Trichrome stain (Gomori-Wheatley stain) and iron hematoxylin stain:
These stains are used to detect intestinal protozoa.
The Gomori-Wheatley stain is used to detect microsporidia. It may miss helminth eggs and larvae and does not reliably identify Cryptosporidium. Fungi and human cells take up the stain.
The iron hematoxylin stain differentially stains cells, cell inclusions, and nuclei. Helminth eggs may stain too dark to permit identification.

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