Bacterial measurements using microscopic techniques
It is frequently necessary to accurately measure the size of the microorganism one is viewing. For example, size determinations are often indispensable in the identification of an unknown bacteria. The size of microorganisms is generally expressed in metric units and is determined by the use of a microscope equipped with an ocular micrometer. An ocular micrometer is a small glass disk on which uniformly spaced lines of unknown distance, ranging from 0 to 100, are etched. The ocular micrometer is inserted into the ocular of the microscope and then calibrated against a stage micrometer, which has uniformly spaced lines of known distance etched on it. The stage micrometer is usually divided into 0.01 millimeter and 0.1 millimeter graduations. The ocular micrometer is calibrated using the stage micrometer by aligning the images at the left edge of the scales. The dimensions of microorganisms in dried, fixed, or stained smears tend to be reduced as much as 10 to 20% from the dimensions of the living microorganisms. Consequently, if the actual dimensions of a microorganism are required, measurements should be made in a wet-mount.