9.2 Microorganism Demonstration: Bacteria, Fungi & Spirochetes

9.2 Microorganism Demonstration: Bacteria, Fungi & Spirochetes

ASCP HT Core Principle: Histological pathogen identification targets distinctive cell wall macromolecules—peptidoglycan thickness in Gram stains, mycolic acid lipids in acid-fast stains, 1,2-glycols in fungal GMS, and argyrophilic silver nucleation in spirochete stains.

Demonstrating infectious organisms in surgical pathology is an essential histotechnology competency. Special stains permit pathologists to confirm bacterial colonies, distinguish fungal hyphae from tissue debris, and detect fastidious spirochetes that resist routine culture.

---

Tissue Demonstration of Bacteria: Gram Stains

The Gram stain differentiates bacteria based on cell wall architecture:

• Gram-Positive Bacteria: Possess a thick (20 to 80 nm), cross-linked peptidoglycan wall with teichoic acids that traps the crystal violet-iodine complex.
• Gram-Negative Bacteria: Possess a thin (2 to 7 nm) peptidoglycan layer covered by an outer lipopolysaccharide membrane that is dissolved by organic decolorizers, allowing dye extraction.

Staining Steps & Tissue Modifications

1. Staining & Mordanting: Crystal violet followed by Gram's/Lugol's iodine forms a water-insoluble crystal violet-iodine (CV-I) coordination complex.
2. Decolorization: Acetone or alcohol dehydrates and shrinks Gram-positive peptidoglycan pores (retaining CV-I), while leaching CV-I from Gram-negative walls.
3. Counterstain: Basic fuchsin stains decolorized Gram-negative bacteria red-pink.

Tissue sections require specialized protocols to clear non-specifically bound crystal violet:

• Brown-Brenn: Differentiated with acetone, counterstained with basic fuchsin, and differentiated with picric acid-acetone. Results: Gram-positive bacteria stain blue-violet, Gram-negative bacteria stain red, and tissue background stains yellow.
• Brown-Hopps: Optimizes demonstration of delicate Gram-negative rods (Legionella, Pseudomonas). Uses Gallego's solution (mordant/differentiator), basic fuchsin, and tartrazine in cellosolve counterstain. Results: Gram-positive bacteria stain blue-violet, Gram-negative bacteria stain crisp red, and background stains yellow.

---

Demonstration of Mycobacteria: Acid-Fast Bacilli (AFB)

Mycobacteria (M. tuberculosis, M. leprae) have waxy cell walls containing mycolic acids—branched beta-hydroxy fatty acids (70–90 carbons) representing 60% of cell wall weight. This lipid barrier resists aqueous dyes, but lipophilic phenolic carbol-fuchsin penetrates readily. Once stained, mycobacteria resist mineral acid decolorization (acid-fastness).

Kinyoun (Cold) vs. Ziehl-Neelsen (Hot)

• Ziehl-Neelsen: Carbol-fuchsin is heated until steaming to soften waxy lipids and facilitate dye penetration.
• Kinyoun Method: Increases basic fuchsin (4%) and phenol (8%) concentrations, lowering surface tension to achieve penetration at room temperature without heat.
• Decolorization & Counterstain: Differentiated in 1% acid-alcohol (1% HCl in 70% ethanol) and counterstained with 0.5% aqueous methylene blue. Acid-fast bacilli stain bright magenta-red; background stains pale blue.

The Fite Method for Mycobacterium leprae

The waxy coat of M. leprae is fragile and contains less mycolic acid than M. tuberculosis. Standard xylene deparaffinization dissolves this lipid capsule, causing false-negative staining:

• Peanut Oil Deparaffinization: Deparaffinized in 2 parts xylene to 1 part peanut oil to insulate the waxy envelope from extraction.
• Gentle Decolorization: Weak 0.5%–1.0% aqueous sulfuric acid (H2SO4) replaces vigorous acid-alcohol.
• Results: M. leprae stain bright red against a pale blue background.

---

Demonstration of Fungi: GMS and PAS

Fungal cell walls consist of chitin, glucans, and mannans rich in 1,2-glycol hydroxyl groups.

Grocott's Methenamine Silver (GMS)

1. Oxidation: Sections are oxidized in 4%–5% chromic acid (CrO3), converting 1,2-glycols to reactive dialdehydes while over-oxidizing background aldehydes to non-reactive carboxylic acids.
2. Impregnation & Reduction: Methenamine silver nitrate buffered with sodium borate (pH 8.5 to 9.0) is heated to 58°C–60°C. Free dialdehydes reduce diamminesilver to black metallic silver (Ag0). Methenamine buffers and prevents solution precipitation.
3. Toning & Fixing: Toned with 0.1%–0.2% gold chloride; unreduced silver is dissolved in 2%–5% sodium thiosulfate.
4. Counterstain: 0.2% Light Green SF Yellowish.
5. Results: Fungal cell walls and Pneumocystis cysts stain sharp jet-black; background stains pale green.

Periodic Acid-Schiff (PAS) for Fungi

0.5% periodic acid oxidizes fungal walls to aldehydes, which react with Schiff reagent to stain bright magenta-rose.

---

Demonstration of Spirochetes: Warthin-Starry and Steiner

Spirochetes (Treponema pallidum, Borrelia, Helicobacter pylori) are thin (0.1–0.3 μm) and invisible on H&E.

• Mechanism: Spirochetes are argyrophilic, binding silver from 1% silver nitrate at pH 3.6 to 4.0.
• Physical Development: A developer with hydroquinone (reducer), gelatin (protective colloid), and silver nitrate deposits metallic silver onto sensitized spirochetes.
• Results: Spirochetes and H. pylori stain jet-black against a pale yellow-brown background.

---

Microorganism Special Stains Diagnostic Reference Table

---

Fluorochrome and Mucin-Blocking Methods

The ASCP BOC Summary of Stains lists auramine-rhodamine within the carbol-fuchsin group and alcian yellow among the carbohydrate dyes; both are organism-detection methods.

• Auramine-rhodamine (fluorochrome acid-fast stain). Auramine O and rhodamine B replace carbol-fuchsin as the primary fluorescent dyes, with acid-alcohol decolorization and a potassium permanganate quench that darkens the background. Acid-fast bacilli fluoresce bright yellow-orange against a black field and can be screened at 25x to 40x rather than under oil, which makes the method far more sensitive than Ziehl-Neelsen for scanty organisms. Because fluorescence fades, slides are read promptly and stored in the dark, and positives are conventionally confirmed with Ziehl-Neelsen or Kinyoun.
• Alcian yellow-toluidine blue (Leung method) for Helicobacter pylori. Alcian yellow first stains gastric mucin yellow, blocking the mucin background; toluidine blue then colors the organisms blue, so curved bacilli stand out along the surface epithelium and in the mucus layer.
• Other H. pylori options: Giemsa and Diff-Quik are inexpensive screens, Warthin-Starry and Steiner and Steiner silvers are highly sensitive but technically demanding, and immunohistochemistry for H. pylori is the most specific.
• Steiner and Steiner silver, listed in the guideline alongside Warthin-Starry, also demonstrates Legionella, Bartonella, and other argyrophil organisms as black rods on a golden background.

---

Clinical Scenarios & High-Yield Exam Traps

• Exam Trap: Over-Oxidation in GMS. Excessive chromic acid converts aldehydes to non-reducing carboxylic acids, yielding false-negative fungal staining.
• Exam Trap: Leprosy Deparaffinization. Deparaffinizing a leprosy specimen in pure xylene dissolves its fragile mycolic acids. Peanut oil must be blended with xylene.
• Exam Trap: Over-Decolorizing Gram Stains. Prolonged acetone decolorization strips crystal violet from Gram-positive organisms, making them stain red with basic fuchsin.