Observing Microorganisms Through a Microscope: Microscopy and Staining Techniques

Observing Microorganisms Through a Microscope

Introduction

Microscopy is fundamental in microbiology, allowing scientists to observe microorganisms that are otherwise invisible to the naked eye. This chapter covers the principles of microscopy, types of microscopes, and staining techniques essential for visualizing microbial cells and their structures.

Units of Measurement in Microbiology

Metric Units

• Microorganisms are measured in micrometers (μm) and nanometers (nm).

• 1 μm = $1 \times 10^{-6}$ meters

• 1 nm = $1 \times 10^{-9}$ meters

• 1000 nm = 1 μm

Metric Unit	Meaning of Prefix	Metric Equivalent	U.S. Equivalent
Meter (m)	—	1	39.4 in. = 1.09 yd
Millimeter (mm)	1/1,000	0.001 m	0.0394 in.
Micrometer (μm)	1/1,000,000	0.000001 m	0.000039 in.
Nanometer (nm)	1/1,000,000,000	0.000000001 m	0.000000039 in.

Microscopy: The Instruments

Simple Microscope

• Has only one lens, similar to a magnifying glass but with higher magnification.

Compound Light Microscope

• Uses two sets of lenses: objective and ocular (eyepiece).

• Total magnification is calculated as: $\text{Total Magnification} = \text{Objective Lens Magnification} \times \text{Ocular Lens Magnification}$

• Often called brightfield microscopy because the background is light.

Resolution and Magnification

• Resolution (resolving power): The ability of the lenses to distinguish two points as separate entities.

• A microscope with a resolving power of 0.4 nm can distinguish between two points at least 0.4 nm apart.

• Shorter wavelengths of light provide greater resolution.

• The limit of resolution for a compound light microscope is about 0.2 μm, limiting magnification to approximately 1500x.

Refractive Index and Immersion Oil

• Refractive index: A measure of the light-bending ability of a medium.

• Immersion oil is used with the 100x objective to prevent light from refracting away from the objective lens, improving resolution.

Types of Light Microscopy

Brightfield Microscopy

• Dark objects are visible against a bright background.

• Light reflected off the specimen does not enter the objective lens.

• Unstained cells may be difficult to view due to low contrast.

Darkfield Microscopy

• Light objects are visible against a dark background.

• An opaque disk in the condenser blocks direct light; only light reflected by the specimen enters the objective lens.

• Useful for viewing live, unstained microorganisms, such as Treponema pallidum (causative agent of syphilis).

Phase-Contrast Microscopy

• Allows detailed examination of living organisms and internal cell structures without staining or fixation.

• Combines direct and diffracted light rays to form an image.

Differential Interference Contrast (DIC) Microscopy

• Similar to phase-contrast but uses two light beams and prisms to split light, providing more contrast and color.

• Produces brightly colored, three-dimensional images.

Fluorescence Microscopy

• Uses ultraviolet (UV) or visible light.

• Fluorescent substances absorb light and emit longer wavelength (visible) light.

• Cells may be stained with fluorescent dyes (fluorochromes) if they do not naturally fluoresce.

• Example: Auramine O stains Mycobacterium tuberculosis bright yellow.

Fluorescent-Antibody (FA) Technique (Immunofluorescence)

• Antibodies specific to a microbial pathogen are tagged with a fluorochrome.

• If the pathogen is present, the antibody binds and the microbe fluoresces under the microscope.

• Allows rapid and specific detection of pathogens in clinical specimens.

Confocal Microscopy

• Cells are stained with fluorochrome dyes.

• Short-wavelength (blue) light excites a single plane of the specimen.

• Exceptionally clear two-dimensional images are obtained; three-dimensional images can be constructed by computer.

Electron Microscopy

Principles

• Uses electrons instead of light, providing much greater resolution due to the shorter wavelength of electrons.

• Used to visualize structures too small for light microscopes, such as viruses and internal cellular components.

• Images are black and white but can be digitally colored.

Transmission Electron Microscopy (TEM)

• A beam of electrons passes through ultrathin sections of a specimen, then through electromagnetic lenses.

• Specimens are stained with heavy-metal salts for contrast.

• Magnification: 10,000–10,000,000x; limit of resolution: 0.2 nm.

• Used to view internal structures (interior) of cells.

• Preparation involves fixation, dehydration, and slicing into thin sections; viewed under high vacuum.

• Preparation kills the specimen and may cause shrinkage or artifacts.

Scanning Electron Microscopy (SEM)

• An electron gun scans the surface of an entire specimen with a beam of electrons.

• Secondary electrons emitted from the specimen are collected and used to produce a three-dimensional image.

• Magnification: 1,000–500,000x; limit of resolution: 0.5 nm.

• Used to view the exterior surface of specimens.

Staining Techniques

Preparing Smears for Staining

• Staining: Coloring microorganisms with a dye to emphasize certain structures.

• Smear: A thin film of material containing microorganisms spread over a slide.

• Fixing: Attaches microorganisms to the slide, kills them, and preserves their structure with minimal distortion.

• Fixing can be done by heat (passing through a flame) or chemically (using methanol).

Types of Stains

• Basic dye: Chromophore is a cation (+); binds to negatively charged bacterial cells (e.g., crystal violet, methylene blue, safranin).

• Acidic dye: Chromophore is an anion (−); stains the background, not the cell (negative staining; e.g., eosin, acid fuchsin, nigrosin).

Simple Stains

• Use a single basic dye to highlight the entire microorganism, making cell shapes and structures visible.

• A mordant may be used to help the stain bind better or enlarge the specimen.

Differential Stains

• Used to distinguish between different types of bacteria.

• Examples: Gram stain and acid-fast stain.

Gram Stain

• Classifies bacteria as gram-positive (thick peptidoglycan cell wall, stains purple) or gram-negative (thin peptidoglycan, outer membrane, stains pink/red).

• Steps: Primary stain (crystal violet), mordant (iodine), decolorizer (alcohol/acetone), counterstain (safranin).

Step	Color of Gram-Positive	Color of Gram-Negative
Primary Stain: Crystal Violet	Purple	Purple
Mordant: Gram's Iodine	Purple	Purple
Decolorizing Agent: Alcohol/Acetone	Purple	Colorless
Counterstain: Safranin	Purple	Pink/Red

• Gram stain is crucial in medical microbiology for detecting bacteria in clinical specimens and guiding treatment.

• Most reliable on young, actively growing bacteria.

Acid-Fast Stain

• Binds only to bacteria with waxy cell walls (e.g., Mycobacterium species).

• Used to identify acid-fast organisms, which retain the primary stain even after acid-alcohol decolorization.

Summary Table: Types of Microscopy

Microscope Type	Distinguishing Features	Typical Use	Stained/Unstained
Brightfield	Light background, dark specimen	General observation	Stained
Darkfield	Dark background, light specimen	Live, unstained cells	Unstained
Phase-Contrast	Enhances contrast in transparent specimens	Internal structures, live cells	Unstained
DIC	3D, colored images	Detailed cell structure	Unstained
Fluorescence	Fluorescent dyes, dark background	Specific detection, immunofluorescence	Stained
Confocal	2D/3D images, computer reconstruction	Detailed imaging, biofilms	Stained
TEM	Ultrathin sections, high resolution	Internal cell structures	Stained
SEM	Surface scanning, 3D images	Surface structures	Stained