Which of the following staining is the most accurate technique to determine the exact morphology (shape and size) of a bacterial cell? A. Capsule stain B. Gram stain C. Simple stain D. Negative stain
The correct answer and explanation is:
Correct Answer: D. Negative stain
Explanation:
The negative stain is the most accurate technique to determine the exact morphology, including shape and size, of a bacterial cell. This is because negative staining does not involve heat-fixing or harsh staining procedures that might distort the bacterial cells. Instead, it stains the background while leaving the actual cells clear and intact. As a result, the cells retain their true dimensions and shapes, which are important when studying morphology.
In the negative staining method, acidic dyes such as nigrosin or India ink are commonly used. These dyes have negatively charged chromophores, which are repelled by the negatively charged bacterial cell surface. Therefore, the dye does not penetrate the cell but rather stains the surrounding area. The result is a dark background with clear, unstained bacterial cells that can be observed in their natural form under the microscope.
In contrast, Gram staining and simple staining require heat fixation of the bacterial smear, which can shrink or distort the cells. While these stains are useful for differentiating bacterial types (Gram stain) or for general visibility (simple stain), they are not ideal for accurate morphological measurements.
Capsule staining is a differential staining technique used specifically to visualize the capsule surrounding some bacteria, which does help in identifying virulence factors but does not provide the most accurate information about overall cell size and shape.
Therefore, when precise observation of a bacterium’s actual size and shape is needed, negative staining is preferred because it preserves cellular integrity, minimizes distortion, and gives a clear contrast that outlines the bacterial structure accurately. This makes it an essential tool in microbiology for initial morphological studies.