A simple stain is a laboratory technique in which a single dye is applied to a specimen to enhance contrast and allow basic cell morphology to be observed with a light microscope.
Explanation
Microorganisms are nearly transparent under bright‑field microscopy. Applying a basic dye, such as methylene blue, crystal violet or safranin, adds colour to cells so that their shape, size and arrangement can be distinguished. These dyes carry a positive charge and bind electrostatically to negatively charged components of the cell, such as nucleic acids and proteins. To perform a simple stain, a thin smear of the sample is spread on a slide, air‑dried and heat‑fixed to adhere the cells and kill living organisms. The dye is then flooded over the smear for a minute or two, rinsed with water and gently blotted dry. Because only one stain is used, all organisms on the slide take up the same colour, so the technique does not differentiate between cell types. Simple staining is fast and requires minimal reagents, making it a common first step when examining unknown specimens or assessing the quality of a culture.
Uses and illustrative examples
Simple stains are routinely used to reveal the coccus clusters of Staphylococcus, the rod‑shaped cells of Bacillus subtilis or the spiral forms of Spirillum volutans. Medical technologists may apply methylene blue to sputum smears to visualize leukocytes and bacteria in suspected pneumonia. In teaching laboratories, a simple stain helps students learn to recognize basic bacterial morphologies before attempting more complex differential stains. Yeast cells, protozoa and cultured mammalian cells can all be quickly assessed with a single dye to check for contamination or count cells. However, because simple staining colours all cell types the same, it cannot distinguish between Gram‑positive and Gram‑negative bacteria or identify acid‑fast organisms.
Despite its limitations, the simple stain remains a fundamental tool for rapid morphological assessment of microorganisms and other cells.
Related Terms: Staining, Gram stain, Heat fixation, Microscopy, Morphology.