Microbiology - Microscopes

1.What are the advantages and disadvantages of the different types of light and electron microscopes discussed in Chapter 3 that are used to study microorganisms? Focus your response in terms of the following parameters: ◦Range of magnification

◦Resolving ability

◦Sample preparation

◦Possible states of sample (e.g. whole organism, part of, living, non-living, etc.)

Brightfield illumination uses visible light to observe stained specimens and count microbes. The specimen appears dark against a bright background. Brightfield microscopes have a magnification up to 2000X. Though they are inexpensive and easy to use they cannot resolve very small specimens such as viruses. Specimens are non-living.

Darkfield microscopes are similar to brightfield but an opaque disk is used. These microscopes are used when specimens cannot be stained. The specimens are living and appear dark against a light background. Commonly used to identify Treponema pallidum, which causes syphilis. Because the darkfield microscope uses scattered light, dust may obscure the image as it is easily observed.

Phase-contrast microscopes use direct and reflected or refracted light rays to form an image of a specimen. These microscopes give a detailed examination of the internal structures in living microorganisms; it is possible to see the nuclei and organanelles of cells. There is little preparation required as it is not required to fix or stain a specimen.

Differential interference microscopes (DIC) are similar to phase-contrast in that they both provide high contrast images, the difference is that a DIC microscope uses a prism and images appear in color and are three dimensional. The resolution of a DIC microscope is higher than that of the phase-contrast.

Fluorescence microscopes use fluorescent dyes called fluorochromes. Specimens appear luminescent and bright against a dark background. Fluorescence microscopes are used primarily for immunofluorescence which can detect...