Preparing Slides

Part of Germ Theory

Step-by-step techniques for preparing microscope slides from clinical and environmental specimens for diagnostic observation.

Why This Matters

A microscope without well-prepared slides produces useless or misleading information. A slide that is too thick shows a confused tangle where nothing is identifiable. A slide with inadequate fixation allows organisms to wash off during staining. A contaminated slide introduces artifacts that mimic organisms. Poor technique wastes the microscope’s capability entirely.

Slide preparation is a craft skill — it requires practice and attention to detail. With practice, preparing a diagnostic-quality smear becomes quick and reliable. The skill directly enables everything else in diagnostic microbiology: Gram staining, acid-fast staining, malaria diagnosis, parasite identification, and cell morphology assessment. It is the gateway to seeing what cannot otherwise be seen.

All materials required for slide preparation can be produced or improvised from simple supplies. Glass slides and coverslips are the most critical specialized items; where glass is unavailable, polished transparent minerals (mica) or carefully prepared thin resin sheets can substitute, though with limitations.

Types of Preparations

1. Wet Mount

The simplest preparation — a fresh, unfixed specimen in liquid, viewed immediately.

Best for: Motile organisms (protozoa, live bacteria), fungal hyphae, yeast, helminth eggs in stool, urine microscopy.

Procedure:

1. Place one small drop of specimen on a clean slide. For stool: mix a pinhead-sized amount of fresh stool into a drop of physiological saline (0.9% salt water); do not use plain water — osmotic shock will lyse protozoa.
2. Lower a coverslip at an angle — tilt it and lower one edge first, then lay it down slowly to prevent bubbles.
3. Examine immediately at 10x, then 40x. Oil immersion is not used with coverslipped wet mounts (oil contaminates the lens if the specimen is covered, and the lens may crack the coverslip under pressure).
4. Look for movement (motile organisms), cell morphology, and recognizable structures.

Shelf life: Minutes. Wet mounts are single-use, immediate preparations. Organisms die, lyse, and become unrecognizable within 30-60 minutes.

Dark field variation: If the microscope has a dark-field condenser (or if a makeshift stop is placed in the condenser to block central light), organisms appear bright against a dark background. This allows visualization of spirochetes (too thin to see with standard bright-field at 40x) and improves contrast for unstained organisms generally.

2. Fixed Smear

A smear that has been dried and heat-fixed, suitable for staining. This is the standard preparation for bacterial identification.

Best for: Bacterial identification (Gram stain), acid-fast staining for tuberculosis, Giemsa staining for malaria.

Procedure:

Making a smear:

1. Label the slide (write patient identifier on the frosted end with a pencil, or score the glass, or tape a paper label to the bottom — ink can wash off during staining).
2. Apply specimen:
   • Liquid specimen (urine, broth culture, fluid): Place one small loop-full or drop on the center of the slide. Spread in a circle about 1 cm diameter.
   • Solid specimen (colonies from agar): Place a drop of distilled water on the slide; touch a sterile inoculating loop to a colony and mix into the water drop; spread thinly.
   • Wound swab: Roll the swab across the slide in one direction; do not rub back and forth.
   • Sputum: Select a purulent (yellowish-green) portion of the sputum sample. Place a small amount on the slide and use a second slide to spread it thinly.
   • Blood film (thin): Place a small drop of blood 1-2 cm from one end of the slide. Touch the edge of a second slide to the drop at 30-45°. Allow the blood to spread along the edge. Push the spreader slide smoothly and quickly toward the other end — this produces a thin film that thins to a feathered edge.

Critical thinness: The smear must be thin enough that organisms are in a single layer. A smear that is too thick appears dark under the microscope with overlapping material impossible to interpret. Bacteria should be spaced out, not crowded.

Air drying:

• Allow the smear to air dry completely at room temperature. Do not blow on it, wave it, or heat it before it has air-dried — wet smears rupture or distort cells.
• Drying time: 5-15 minutes depending on thickness and ambient conditions.

Heat fixation:

• Pass the dried slide (smear side up) through a gas flame or alcohol lamp flame 3-5 times. Each pass is 1-2 seconds.
• The slide should feel comfortably warm on the back of your hand — not painful hot.
• Overheating distorts cell morphology; underheating fails to fix organisms to the glass.
• Alternative: chemical fixation with 70% methanol (pour on, let stand 1 minute, pour off, allow to dry) — gentler than heat, better for blood films.

3. Blood Film (Thick and Thin)

Standard preparation for malaria diagnosis.

Thin film (morphology):

• Use the spreading technique described above
• Methanol-fix (do not heat fix — destroys red cell morphology needed for species identification)
• Stain with Giemsa

Thick film (sensitivity):

• Place a larger drop of blood on the slide
• Spread in a 1.5 cm circle using the corner of another slide or the drop of blood itself
• Allow to dry completely — thick films take longer (20-30 minutes)
• Do NOT fix with methanol — the thick film is designed to lyse in the aqueous Giemsa stain, concentrating parasites
• Stain with aqueous Giemsa for 30-45 minutes
• Examine: parasites appear dark against the background; 5-10x more concentrated than thin film

Making and Cleaning Slides

Cleaning slides: Dirty slides produce artifacts. Commercial pre-cleaned slides are preferable. To clean used slides:

1. Soak in 70% alcohol for 30+ minutes
2. Wipe clean with a lint-free cloth
3. Avoid touching the optical surface with fingers (finger grease is difficult to remove and degrades images)

Making slides from glass: If pre-made slides are unavailable, glass can be cut with a diamond stylus (a hard mineral point drawn against glass scores it; a gentle tap or bend along the score line breaks it cleanly). Bottle glass works; window glass is ideal. Edges must be smooth — file or stone the edges to prevent cuts.

Improvised coverslips: Thin glass is needed for coverslips. Broken microscope slide glass shards can be trimmed, or thin-walled glass containers carefully broken. Alternatively, mica (a mineral that splits into thin, transparent sheets) can be used as coverslip material and is optically superior to poor-quality glass.

Common Preparation Errors

Error	Result	Prevention
Smear too thick	Dark, uninterpretable image	Use smaller amount of specimen; spread further
Inadequate drying before fixation	Organisms wash off during staining	Always air-dry completely
Overheating during fixation	Distorted, unrecognizable cell morphology	Light passes through the flame; check heat with back of hand
Bubbles under coverslip	Obscures portion of preparation	Lower coverslip at angle, slowly
Smear too old (wet mount)	Dead, lysed organisms; motility lost	Prepare and examine wet mounts within 10 minutes
Wrong fixation for blood film	Thin film: air-dry then methanol; thick film: no fixation (lysed by stain)	Know which film type is being prepared
Dirty slides	Debris confused with organisms; oil contamination	Clean slides before use; store in sealed container

Storing and Filing Prepared Slides

Fixed and stained slides can be permanently archived if desired:

• Allow to dry completely after staining
• Apply a drop of mounting medium (clear nail varnish, Canada balsam from fir resin, or glycerol/gelatin) to the specimen area
• Lower a coverslip onto the mounting medium
• Allow to harden (1-24 hours depending on medium)
• Store in a flat slide box or wrapped in cloth

Properly mounted slides remain viewable for years to decades. A collection of identified slides serves as a reference library for training new practitioners and confirming identifications.

Label each stored slide with: patient identifier, date, specimen type, stain used, and identification result. This creates a documented record of the community’s diagnostic history.