Microscopy Types in Microbiology
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Direct answer
Microscopy types in microbiology are chosen to match the organism and the feature needed. Dark-field microscopy shows live spirochaetes such as Treponema pallidum and Leptospira, phase contrast reveals unstained living cells, fluorescence microscopy detects labelled antigens, and electron microscopy resolves viruses and ultrastructure. Light microscopy resolves about 0.2 micrometres, which is too coarse for most viruses.
What you must remember
- Bright-field microscopy: the routine method for stained smears such as Gram and Ziehl-Neelsen, viewed under oil immersion.
- Dark-field microscopy: oblique illumination shows bright, motile spirochaetes on a dark background, best seen in fresh specimens.
- Phase contrast: converts small differences in refractive index into visible contrast, so living, unstained cells and motility can be observed.
- Fluorescence microscopy: fluorochrome-labelled antibodies glow under ultraviolet light, as in the direct fluorescent antibody test for rabies.
- Auramine staining: a fluorescence technique in which acid-fast bacilli appear bright against a dark field.
- Transmission electron microscopy: passes electrons through thin sections or negatively stained particles to show virus morphology and internal structure.
- Scanning electron microscopy: shows the surface of a specimen in three dimensions and is rarely used for routine diagnosis.
- Resolution limit: light microscopy resolves about 0.2 micrometres, so viruses need electron microscopy or antigen tests.
Matching each microscope to the organism
Each technique changes how light or electrons interact with the specimen, and that change decides what becomes visible. Staining adds contrast to bright-field images but can also kill or distort delicate organisms, which is why living preparations are examined by other methods. The table sets out the principle and a typical use for each type examined.
| Type | Principle | Typical use |
|---|---|---|
| Bright-field | Transmitted light through a stained smear | Gram stain of cocci |
| Dark-field | Oblique light, bright objects on dark | Treponema pallidum |
| Phase contrast | Differences in refractive index | Motility of living cells |
| Fluorescence | Labelled antibodies emit light | Rabies antigen in brain smears |
| Transmission electron | Electron beam through thin sections | Virus particle morphology |
Dark-field is preferred when an organism cannot be stained, but it needs fresh specimens because spirochaetes are delicate and lose their motility as the specimen dries. Electron microscopy requires special preparation and is not a routine diagnostic tool in most laboratories, so virus detection usually depends on antigen or nucleic acid testing instead.
Where the exam tries to catch you
Microscopy questions usually give an organism and ask which technique makes it visible.
- Trap: choosing bright-field for Treponema pallidum Correct: the spirochaete is too thin for routine staining, so dark-field microscopy of a fresh specimen is used.
- Trap: using light microscopy to identify a virus Correct: viruses lie below the resolution of light microscopy, so electron microscopy or antigen detection is required.
- Trap: thinking phase contrast requires fixation and staining Correct: phase contrast is designed for living, unstained cells, which is its main advantage.
Frequently asked questions
Why is dark-field microscopy used to diagnose syphilis?
Dark-field microscopy is used to diagnose syphilis because Treponema pallidum is too thin to see with routine bright-field staining. Oblique illumination of a fresh specimen from a primary chancre makes the motile spirochaetes visible as bright objects on a dark background.
Which microscopy method shows rabies antigen in brain impression smears?
Fluorescence microscopy, used with a direct fluorescent antibody test, shows rabies antigen in brain impression smears. Antibody labelled with a fluorochrome binds the viral antigen in infected neurons, which then glow under ultraviolet illumination.
Why does light microscopy fail to show most viruses?
Light microscopy fails to show most viruses because its resolution of about 0.2 micrometres is too coarse for particles that are typically tens of nanometres across. Transmission electron microscopy is therefore needed to see virus morphology directly.
What does phase contrast microscopy allow that bright-field cannot?
Phase contrast microscopy allows unstained living cells to be seen with clear internal detail, which bright-field can show only after staining. It is useful for observing motility and living morphology without fixation or dye.
How does scanning electron microscopy differ from transmission electron microscopy?
Scanning electron microscopy shows surface morphology in three dimensions, whereas transmission electron microscopy passes electrons through thin sections to reveal internal ultrastructure. Transmission microscopy is the method more often used for virus particles and internal cell structure.