Staining Techniques in Microbiology
Microbiology · General Microbiology · lean revision notes
Staining Techniques in Microbiology
Bacteria are colourless and nearly invisible under the light microscope because their refractive index is close to that of the surrounding medium. Stains add contrast, reveal morphology, and, in many cases, give an immediate provisional diagnosis from a clinical specimen — which is exactly why examiners love linking stain results to vignettes, specimen types, and organisms.
Why we stain — basic principles
A stain is a salt with a chromophore (colour-bearing group) and an auxochrome (binding group). Most bacterial cytoplasm and surface are acidic, so they bind basic (cationic) dyes such as methylene blue, crystal violet, basic fuchsin, and safranin. Acidic (anionic) dyes like eosin, nigrosin, and India ink are repelled by the bacterial surface and are therefore used for negative (background) staining.
Stains are classified by mechanism:
| Type of staining | Definition | Examples |
|---|---|---|
| Simple stain | Single dye, uniform colour, shows morphology only | Methylene blue, basic fuchsin, crystal violet |
| Negative stain | Background stained, organism stands out unstained | India ink, nigrosin (for capsules) |
| Differential stain | Two or more dyes separate organisms into groups | Gram stain, Ziehl–Neelsen (acid-fast) |
| Special/structural stain | Demonstrates a specific structure | Capsule, spore, flagella, metachromatic granule stains |
| Impregnation methods | Heavy-metal deposition thickens thin structures | Silver stains for spirochaetes, flagella |
High-yield: Most bacteria are stained by basic dyes because the bacterial cell surface and nucleic acids are negatively charged (acidic). India ink/nigrosin are acidic dyes used for negative staining of capsules.
Gram stain — the cornerstone
Devised by Hans Christian Gram (1884), it is the single most important and most frequently asked stain. It divides bacteria into Gram-positive (purple/violet) and Gram-negative (pink/red).
Reagents and sequence (the four steps)
Crystal violet (primary, 1 min) → Gram's iodine (mordant, 1 min) → Decolouriser: acetone/alcohol (10–30 s — the critical step) → Safranin / dilute carbol fuchsin (counterstain, 30 s)
- Crystal violet stains all cells violet.
- Gram's iodine is the mordant — it forms a large crystal-violet–iodine (CV–I) complex within the cell.
- Decolouriser (acetone faster, alcohol slower) is the step that differentiates. Over-decolourisation makes Gram-positives appear false Gram-negative.
- Counterstain colours the now-colourless Gram-negative cells pink.
Mechanism — why the difference?
The key is the thick peptidoglycan (20–80 nm, 40–80 layers) of Gram-positive cells. On exposure to alcohol the peptidoglycan dehydrates, pores close, and the bulky CV–I complex is trapped → stays violet. Gram-negative cells have thin peptidoglycan (2–3 layers) and an outer membrane rich in lipid; alcohol dissolves the lipid, increases permeability, and washes the CV–I complex out → cell decolourises and takes up safranin → pink. Higher lipid content of the Gram-negative wall is the classical reason quoted.
High-yield: The mordant in Gram stain is Gram's iodine; the step that differentiates is decolourisation. Gram positivity depends on an intact, thick peptidoglycan layer.
| Feature | Gram positive | Gram negative |
|---|---|---|
| Colour after staining | Violet/purple | Pink/red |
| Peptidoglycan | Thick (multilayer) | Thin (1–3 layers) |
| Outer membrane / LPS | Absent | Present (endotoxin) |
| Teichoic acid | Present | Absent |
| Periplasmic space | Poorly developed | Well developed |
| Lipid content of wall | Low (1–4%) | High (11–22%) |
| Susceptibility to lysozyme/penicillin | More susceptible | More resistant |
| Exotoxin vs endotoxin | Mainly exotoxin | Mainly endotoxin (LPS) |
Organisms that don't Gram-stain well
- Mycobacteria — high mycolic acid/lipid wall (use acid-fast).
- Mycoplasma — no cell wall.
- Treponema, Leptospira — too thin (use dark-ground/silver).
- Chlamydia, Rickettsia — intracellular (use Giemsa).
- Legionella — stains poorly (silver/Dieterle, or Gram with prolonged safranin).
High-yield: Gram-variable / "Gram-ghost" appearance is classic with old cultures and with Clostridium, Bacillus. Always Gram-stain from a young (log-phase) culture.
Ziehl–Neelsen (acid-fast) stain
Used for organisms with a waxy, mycolic-acid–rich cell wall that resists decolourisation by acid-alcohol → acid-fast bacilli (AFB) appear bright red on a blue/green background.
Method (hot ZN)
- Strong (concentrated) carbol fuchsin flooded on the smear.
- Heat until steaming, 5–7 min (heat drives dye through the lipid wall) — do not boil.
- Decolourise with 20–25% sulphuric acid (and 3% alcohol for partially acid-fast).
- Counterstain with methylene blue / malachite green.
Result: AFB = red; background and non-acid-fast cells = blue/green.
High-yield: Acid-fastness is due to mycolic acid (and the integrity of the cell wall). The primary stain is carbol fuchsin; heat is the mordant in the hot method.
Modifications and grades of acid-fastness
| Concentration of acid for decolourisation | Used for |
|---|---|
| 20–25% H₂SO₄ | M. tuberculosis (strongly acid-fast) |
| 5% H₂SO₄ | M. leprae |
| 1% H₂SO₄ (and weak acid) | Nocardia, Legionella micdadei |
| 0.5–1% (modified ZN, Kinyoun cold) | Oocysts of Cryptosporidium, Cyclospora, Isospora (Cystoisospora) |
- Kinyoun stain = "cold" acid-fast method (more phenol, no heat).
- Auramine–rhodamine (fluorochrome) stain → AFB fluoresce yellow-gold under UV; more sensitive, used for screening high loads (TB).
- Spores and the head of Nocardia are weakly/partially acid-fast.
High-yield: Cryptosporidium oocysts in stool = modified (cold) ZN, pink/red round bodies. Nocardia is partially acid-fast (1% acid) — differentiates it from Actinomyces (not acid-fast).
The cut-off for grading sputum smear positivity (per ml/field) and the WHO/RNTCP scale (Scanty, 1+, 2+, 3+) is examinable: ≥10 AFB per field = 3+.
Albert stain — metachromatic granules
The classic stain for Corynebacterium diphtheriae, demonstrating volutin / metachromatic granules (Babes–Ernst / polar bodies) of polymerised inorganic polyphosphate.
- Reagents: Albert I (toluidine blue + malachite green) then Albert II (Lugol's iodine) as mordant.
- Result: bluish-black/green granules at the poles, green bacillary body, arranged in Chinese-letter / cuneiform / V–L patterns.
- Other granule stains: Neisser's and Ponder's.
High-yield: C. diphtheriae → Albert stain → bluish-black metachromatic granules; growth on Loeffler's serum slope and black colonies with brown-black halo on potassium tellurite (Tinsdale) media.
Leishman & Giemsa stains — Romanowsky group
These are Romanowsky stains (eosin + methylene blue/azure) used mainly on blood and tissue films.
- Leishman stain — methanol-fixed; used for peripheral blood films, malarial parasites, LD bodies of Leishmania (amastigotes in macrophages).
- Giemsa stain — the workhorse for blood parasites and intracellular organisms.
Giemsa demonstrates (memorise the list):
- Malaria parasites (Plasmodium) — ring forms, schizonts, gametocytes.
- Leishmania (LD bodies), Trypanosoma, Toxoplasma, Babesia.
- Inclusion bodies / intracellular bacteria: Chlamydia (Halberstaedter–Prowazek bodies), Rickettsia, Borrelia (relapsing fever — also seen on Leishman/Wright), Histoplasma capsulatum (intracellular yeast in macrophages).
High-yield: Giemsa = parasite & intracellular-organism stain. Malaria, Leishmania, Toxoplasma, Chlamydia inclusions, Histoplasma, Borrelia — all best on Giemsa. Wayson / methylene blue shows the bipolar "safety-pin" appearance of Yersinia pestis.
India ink — capsule (negative stain)
A negative stain: the acidic ink particles cannot penetrate the capsule, so the capsule appears as a clear unstained halo against a dark grey/black background.
- Classic use: Cryptococcus neoformans in CSF of immunocompromised/HIV patients — a round budding yeast with a wide refractile capsular halo.
- Also used for bacterial capsules (e.g., Klebsiella, pneumococcus). Nigrosin is an alternative negative stain.
High-yield: India ink preparation of CSF → Cryptococcus neoformans (capsular halo, narrow-based budding). Antigen detection (CrAg / latex agglutination) is now the more sensitive test of choice, but the India ink image remains a favourite MCQ.
PAS (Periodic acid–Schiff) stain
Stains polysaccharides, mucopolysaccharides, glycogen, and fungal cell walls magenta/bright pink.
- Fungi (cell-wall glucan/mannan) → magenta. Used for Candida, dermatophytes, Histoplasma, Tropheryma whipplei.
- Whipple's disease → PAS-positive, diastase-resistant foamy macrophages in lamina propria (Tropheryma whipplei).
- Other fungal stains: GMS (Gomori/Grocott methenamine silver) → fungi appear black; calcofluor white → fungi fluoresce (chitin-binding, very sensitive); mucicarmine → stains Cryptococcus capsule rose-red.
High-yield: PAS-positive macrophages in small-bowel biopsy = Whipple's disease (Tropheryma whipplei). GMS = fungi black; PAS = fungi magenta; calcofluor white = fungi fluoresce.
Special structural stains (quick reference)
| Structure | Stain(s) | Note |
|---|---|---|
| Capsule | India ink, nigrosin (negative); Maneval's; Welch | Halo around cell |
| Spore | Schaeffer–Fulton (malachite green + safranin); Dorner | Spore green, body red |
| Flagella | Leifson, Ryu | Tannic acid mordant thickens flagella |
| Metachromatic granules | Albert, Neisser, Ponder | C. diphtheriae |
| Spirochaetes | Fontana (silver impregnation), Levaditi (tissue) | Treponema, Leptospira |
| AFB | Ziehl–Neelsen, Kinyoun, auramine | Mycobacteria, Nocardia |
| Lipid / PHB granules | Sudan black | Fat inclusions |
High-yield: Schaeffer–Fulton = spore stain (heat drives malachite green into the spore → spore green, vegetative cell red/pink with safranin). Fontana silver = spirochaetes (they are too thin for Gram stain).
Stain → Organism rapid map (must-know)
| Stain | Best demonstrates |
|---|---|
| Gram | General morphology; Gram +ve cocci/bacilli, Gram −ve rods |
| Ziehl–Neelsen (hot) | M. tuberculosis, M. leprae |
| Modified/cold ZN | Cryptosporidium, Cyclospora, Isospora oocysts; Nocardia (1%) |
| Auramine–rhodamine | TB screening (fluorescent AFB) |
| Albert / Neisser | Corynebacterium diphtheriae (granules) |
| Leishman | Malaria, Leishmania (LD bodies), blood film |
| Giemsa | Malaria, Leishmania, Toxoplasma, Chlamydia, Histoplasma, Borrelia |
| India ink | Cryptococcus neoformans capsule |
| PAS | Fungi, Whipple's (T. whipplei) |
| GMS (silver) | Pneumocystis jirovecii, fungi (black) |
| Calcofluor white | Fungi, Acanthamoeba, Pneumocystis (fluorescence) |
| Mucicarmine | Cryptococcus capsule (rose-red), Rhinosporidium |
| Fontana / Dieterle silver | Spirochaetes, Legionella, cat-scratch Bartonella |
| Warthin–Starry silver | H. pylori, Bartonella, spirochaetes |
High-yield: Pneumocystis jirovecii cysts → GMS / toluidine blue / calcofluor / immunofluorescence; trophozoites → Giemsa. It does not stain with routine fungal culture (cannot be cultured).
Clinical correlation — what a smear tells you immediately
A Gram-stained smear gives a provisional diagnosis before culture:
- Gram −ve diplococci, intracellular, in urethral pus → Neisseria gonorrhoeae.
- Gram −ve diplococci in CSF → N. meningitidis; Gram +ve diplococci (lanceolate) in CSF/sputum → Pneumococcus; Gram −ve coccobacilli → H. influenzae.
- Gram +ve cocci in clusters → Staphylococcus; in chains → Streptococcus.
- "School-of-fish" Gram −ve coccobacilli in genital ulcer → Haemophilus ducreyi (chancroid).
- Boxcar Gram +ve bacilli → Bacillus anthracis; drumstick spore → Clostridium tetani.
- Sulphur granules, Gram +ve branching filaments, NOT acid-fast → Actinomyces; branching filaments, partially acid-fast → Nocardia.
Common errors & pitfalls
- Over-decolourisation in Gram stain → Gram-positives read false-negative (commonest error). Thick smears → incomplete decolourisation → Gram-negatives read false-positive.
- Using an old culture → Gram-variable artefacts.
- For ZN, insufficient heating or over-decolourisation → missed AFB.
- India ink: leucocytes can mimic capsule — look for the budding yeast inside the halo to confirm Cryptococcus.
Recently asked / exam angle
- Image-based MCQs are increasingly common: identify the stain from a photomicrograph (red bacilli on blue = ZN; halo on dark background = India ink; ring forms in RBCs = Giemsa malaria).
- "Mordant in Gram stain" → Gram's iodine; "step that differentiates Gram +ve and −ve" → decolourisation; "chemical basis of Gram positivity" → thick peptidoglycan retaining CV–I complex.
- "Stain for Cryptosporidium" → modified/cold (Kinyoun) ZN. "Stain for metachromatic granules" → Albert.
- "Most sensitive stain for AFB" → auramine–rhodamine fluorescent stain. "Stain for Pneumocystis" → GMS / immunofluorescence.
- Whipple's disease + PAS-positive macrophages is a recurring integrative question across Medicine and Microbiology.
- "Negative staining acidic dye" → India ink/nigrosin; "why bacteria take basic dyes" → acidic (negatively charged) cell surface.
- Match-the-following / assertion-reason formats linking specimen (CSF, sputum, stool, blood film, genital ulcer) to the correct stain.
Rapid revision
- Basic (cationic) dyes stain most bacteria; acidic dyes (India ink, nigrosin) are for negative/capsule staining.
- Gram stain order: crystal violet → iodine (mordant) → acetone/alcohol (decolouriser = differentiating step) → safranin.
- Gram positive = violet = thick peptidoglycan retaining CV–iodine complex; Gram negative = pink = thin wall + lipid outer membrane.
- ZN: carbol fuchsin + heat → red AFB on blue background; acid-fastness due to mycolic acid.
- Cold/modified ZN for Cryptosporidium, Cyclospora, Isospora; 1% acid for partially-acid-fast Nocardia.
- Auramine–rhodamine = most sensitive (fluorescent) AFB stain for TB screening.
- Albert stain → metachromatic (volutin) granules of Corynebacterium diphtheriae; Chinese-letter arrangement.
- Giemsa = malaria, Leishmania, Toxoplasma, Chlamydia inclusions, Histoplasma, Borrelia; Leishman for blood films/LD bodies.
- India ink → Cryptococcus neoformans capsule (halo) in CSF; CrAg latex test is more sensitive.
- PAS → fungi & Whipple's disease (PAS-positive macrophages, T. whipplei); GMS = fungi/Pneumocystis black.
- Schaeffer–Fulton = spore stain (spore green, body red); Fontana silver = spirochaetes.
- Wayson/methylene blue = bipolar safety-pin Yersinia pestis; mucicarmine = Cryptococcus capsule rose-red.