Contents · 12 sections
- 1. History and author
- 2. Methodology (4 steps)
- 3. Cell Wall Structure
- 4. Gram-Positive Bacteria — Key Genera
- 5. Gram-Negative Bacteria — Key Genera
- 6. "Cannot Be Gram-Stained"
- 7. Clinical Applications
- 8. Empirical Antibiotic Therapy
- 9. Limitations
- 10. Common Technical Errors
- 11. Modern Alternatives & Additions
- 12. FAQ
In a nutshell. Gram staining (1884, Hans Christian Gram) is the foundational bacterial classification method: Gram-positive stain purple (thick peptidoglycan cell wall retains the crystal violet-iodine complex), Gram-negative stain pink (thin peptidoglycan + outer membrane with LPS, the complex washes out with alcohol, counterstained with safranin/fuchsin). Within 3–5 min, it provides initial orientation: Gram+ cocci — staphylo/strepto/enterococci; Gram− bacilli — enteric group, pseudomonads; Gram+ bacilli — clostridia, listeria, bacilli; Gram− cocci — Neisseria. Critical for selecting empirical antibiotic therapy prior to culture results.
1. History and author
- Developed in 1884 by Danish microbiologist Hans Christian Gram.
- Originally used to differentiate pneumococci from Klebsiella in lung tissue specimens.
- Since the 1920s, it has been the standard in medical microbiology.
- Based on differences in cell wall structure.
2. Methodology (4 steps)
| Step | Reagent | Time | Result |
|---|---|---|---|
| 1. Primary stain | Crystal violet | 1 min | All cells appear purple |
| 2. Mordant | Lugol's iodine (I₂-KI) | 1 min | Forms crystal violet-iodine complex |
| 3. Decolorization | 96% ethanol or acetone | 15–30 sec | Gram− lose the dye; Gram+ retain it |
| 4. Counterstain | Safranin or Pfeiffer's fuchsin | 1 min | Gram− appear pink; Gram+ remain purple |
- Rinse with distilled water between steps.
- Critical: do not over-decolorize, as this leads to false negatives (Gram+ appear pink).
- Specimen — native (sputum, urine, CSF, wound discharge) or from culture colonies on media.
3. Cell Wall Structure
Gram-positive bacteria
- Thick peptidoglycan layer (20–80 nm).
- Teichoic and lipoteichoic acids.
- No outer membrane.
- The peptidoglycan-iodine-crystal violet complex is large and resists alcohol wash-out.
- Susceptible to penicillins, vancomycin, and lysozyme.
Gram-negative bacteria
- Thin peptidoglycan layer (2–7 nm).
- Outer membrane containing lipopolysaccharide (LPS, endotoxin) and porins.
- Periplasmic space between the two membranes.
- Alcohol treatment dissolves the lipid layer → the complex washes out.
- Resistant to vancomycin (cannot penetrate the outer membrane).
4. Gram-Positive Bacteria — Key Genera
| Genus | Type | Morphology | Diseases |
|---|---|---|---|
| Staphylococcus | S. aureus | Clusters of cocci | Pyoderma, sepsis, endocarditis, pneumonia |
| Streptococcus | S. pyogenes (BGA) | Chains of cocci | Tonsillitis, scarlet fever, erysipelas, acute glomerulonephritis |
| Streptococcus | S. pneumoniae | Lancet-shaped diplococci | Pneumonia, meningitis, otitis media |
| Enterococcus | E. faecalis | Diplococci | UTIs, endocarditis |
| Bacillus | B. anthracis | Bacilli | Anthrax |
| Clostridium | C. difficile, tetani, botulinum, perfringens | Anaerobic bacilli | Pseudomembranous colitis, tetanus, botulism, gas gangrene |
| Listeria | L. monocytogenes | Bacilli | Listeriosis in pregnant women, newborns |
| Corynebacterium | C. diphtheriae | Bacilli | Diphtheria |
5. Gram-negative bacteria — major ones
| Genus | Representatives | Diseases |
|---|---|---|
| Enterobacteriaceae | E. coli, Klebsiella, Enterobacter, Proteus, Serratia | UTI, sepsis, pneumonia, peritonitis |
| Salmonella | S. typhi, S. enteritidis | Typhoid fever, salmonellosis |
| Shigella | S. dysenteriae, etc. | Shigellosis |
| Pseudomonas | P. aeruginosa | Nosocomial infections, VAP, burn wounds |
| Acinetobacter | A. baumannii | Nosocomial, MDR |
| Neisseria | N. meningitidis, N. gonorrhoeae | Meningitis, gonorrhea |
| Haemophilus | H. influenzae | Epiglottitis, meningitis, otitis |
| Bordetella | B. pertussis | Whooping cough |
| Campylobacter | C. jejuni | Enteritis |
| Helicobacter | H. pylori | Gastritis, ulcer, gastric cancer |
6. "Do not stain with Gram"
- Mycobacteria (M. tuberculosis, M. leprae) — waxy wall; Ziehl-Neelsen stain (AFB).
- Mycoplasmas (M. pneumoniae) — no cell wall; not visible on Gram stain.
- Spirochetes (T. pallidum, Borrelia) — thin, poor staining; dark-field microscopy, silver impregnation.
- Chlamydia, rickettsiae — intracellular parasites.
- Legionella — requires silver impregnation or special media (BCYE).
7. Clinical application
- Sputum — rapid orientation in community-acquired and nosocomial pneumonia.
- CSF — emergency in meningitis: Gram-positive diplococci — Streptococcus pneumoniae; Gram-negative diplococci — Neisseria meningitidis; Gram-negative bacilli — H. influenzae.
- Urine — in urosepsis; Gram-negative bacilli — predominantly enteric group.
- Blood (from blood culture bottles) — in sepsis.
- Wound discharge, pus.
- Urethral/cervical swab — gonorrhea (Gram-negative diplococci inside neutrophils).
8. Empiric antibiotic therapy
- Gram-positive cocci in sputum in community-acquired pneumonia — ceftriaxone ± macrolide; Streptococcus pneumoniae suspected.
- Gram-negative bacilli in urine — ceftriaxone, fluoroquinolones.
- Gram-positive diplococci in CSF — ceftriaxone + vancomycin (pneumococcus, resistance possible).
- Gram-negative diplococci in CSF — ceftriaxone (meningococcus); contact isolation.
- Mixed flora in peritoneal discharge — β-lactam + anti-anaerobic (metronidazole) or carbapenem.
- Anaerobic Gram-positive spore-forming bacilli — suspect clostridial infection.
9. Limitations of the method
- Sensitivity ~60–80% (requires ≥ 10⁵ cells per high-power field for reliable evaluation).
- Sample quality: in sputum — control by criteria (< 10 epithelial cells and > 25 neutrophils per field at ×100).
- Does not differentiate viable and dead cells.
- Does not provide species diagnosis (culture + biochemistry + MALDI-TOF mass spectrometry required).
- Does not replace antibiotic susceptibility testing — it is determined only after culture.
10. Diagnostic errors
- Over-decolorization in alcohol — false negatives (Gram-positive appear as Gram-negative).
- Under-decolorization — false positives.
- Old cultures — Gram-positive may show gram-variable pattern (due to cell wall thinning).
- Excessively thick smear — poor decolorization.
- Contamination — multiple morphotypes.
11. Modern alternatives and additions
- Culture with identification via MALDI-TOF (mass spectrometry) — species diagnosis in 15 min.
- PCR panels (Septi-Fast, FilmArray BCID/GI/RP) — rapid pathogen detection in blood, CSF, respiratory samples.
- 16S rRNA sequencing — identification with negative culture.
- Antigen tests (pneumococcal, legionella urinary antigen).
- Gram stain remains the primary screening due to speed and low cost.
12. FAQ
Why do mycobacteria not stain with Gram?
Their cell wall contains high-molecular-weight mycolic acids and wax → the hydrophobic surface prevents aqueous dyes from penetrating. Acid-fast staining is used (Ziehl-Neelsen, auramine fluorochrome).
Who are gram-variable organisms?
Bacteria that show a different picture under various conditions: old Gram-positive bacteria (wall thinning), some clostridia, listeria. Usually correspond to true Gram-positive.
Why are Gram-negative bacilli in urine most often E. coli?
Escherichia coli is a normal commensal of the GI tract and the main cause of community-acquired UTIs (70–80%). This is the primary hypothesis when Gram-negative bacilli appear in urine.
Can S. aureus and S. epidermidis be differentiated by Gram stain?
No. Both are Gram-positive cocci in clusters. They are differentiated via coagulase (S. aureus +), catalase (both +), and biochemistry.
Why doesn't vancomycin work against Gram-negative bacteria?
Vancomycin is a large molecule that binds to D-Ala-D-Ala in peptidoglycan. Gram-negative bacteria have an outer membrane with small porins on the outside, through which vancomycin cannot penetrate.
See also: Antibiotics · Sepsis · Meningitis · Tuberculosis · Microbiology Catalog