Culture media identification is among the most frequently tested short-answer and viva topics in 2nd Year MBBS Microbiology. This chart maps each major medium to the organisms it isolates, its key selective or differential ingredient, and the colony appearance students are expected to describe in exams.
| Medium | Type | Principal Use | Colony Appearance / Key Feature |
|---|---|---|---|
| Blood Agar | Enriched, differential | General pathogens; observing haemolysis in Streptococci, S. aureus | Alpha haemolysis (green partial zone): S. pneumoniae. Beta haemolysis (complete clear zone): S. pyogenes. Gamma (no haemolysis): Enterococci |
| MacConkey Agar | Selective, differential | Gram-negative enteric bacteria; separating lactose fermenters from non-fermenters | Lactose fermenters: pink/red colonies (E. coli, Klebsiella). Non-fermenters: colourless (Salmonella, Shigella, Pseudomonas). Crystal violet inhibits gram-positives |
| Chocolate Agar | Enriched | Fastidious organisms requiring X factor (haematin) and V factor (NAD) | Blood heated to 70-80°C releases factors. Used for H. influenzae, N. gonorrhoeae, N. meningitidis. No haemolysis (blood already lysed) |
| Lowenstein-Jensen (LJ) Medium | Selective | Mycobacterium tuberculosis isolation | Rough, dry, buff-coloured, non-pigmented colonies. Growth in 6-8 weeks (eugonic). Malachite green inhibits other organisms |
| Thayer-Martin Medium | Modified chocolate agar, selective | Neisseria gonorrhoeae isolation from clinical specimens | Chocolate agar base + antibiotics: Vancomycin (gram-positives), Colistin (gram-negatives), Nystatin (fungi), Trimethoprim (Proteus). Oxidase-positive colonies |
| Sabouraud's Dextrose Agar (SDA) | Selective | Fungi (Candida, Aspergillus, dermatophytes) | Acidic pH (5.4-5.6) inhibits bacteria. White/cream pasty yeast colonies (Candida). Mould forms branching hyphae. Chloramphenicol added to suppress bacterial contamination |
| Mueller-Hinton Agar | Non-selective | Antibiotic sensitivity testing (Kirby-Bauer disc diffusion) | Standard medium for antibiotic susceptibility testing (AST). Low thymine/thymidine (avoids false trimethoprim resistance). No specific colony appearance -- used for zone measurement |
| TCBS Agar | Selective, differential | Vibrio species (Cholera, food poisoning) | V. cholerae: large yellow colonies (sucrose fermenter). V. parahaemolyticus: blue-green colonies (non-sucrose fermenter). Thiosulphate-Citrate-Bile salts-Sucrose composition |
| Bordet-Gengou Medium | Enriched, selective | Bordetella pertussis (whooping cough) | 20-30% blood in glycerol-potato infusion agar. Colonies: round, opaque, grey, glistening -- described as "bisected pearl" or "mercury droplet" appearance |
| Loeffler's Serum Slope | Enriched | Initial isolation of Corynebacterium diphtheriae; demonstrating metachromatic granules | 3 parts animal serum + 1 part nutrient broth. C. diphtheriae grows rapidly (12-18 hours). Smear shows metachromatic granules (Babes-Ernst granules) with Albert's stain |
| Tellurite Blood Agar (McLeod's) | Selective | Corynebacterium diphtheriae (primary isolation) | Potassium tellurite selects for C. diphtheriae. Black/grey colonies (tellurite reduced to metallic tellurium). Other organisms inhibited |
| Salmonella-Shigella (SS) Agar | Highly selective | Salmonella and Shigella from faecal specimens | Salmonella: colourless colonies with black centre (H₂S production → FeS). Shigella: colourless, no black centre (no H₂S). Bile salts + brilliant green inhibit coliforms |
| Mannitol Salt Agar (MSA) | Selective, differential | Staphylococcus aureus (including MRSA screening) | 7.5% NaCl selects for Staphylococci. S. aureus: yellow colonies (mannitol fermented → acid → phenol red turns yellow). S. epidermidis: pink/unchanged |
| CLED Agar | Non-selective, differential | Urine culture (urinary tract infections) | Cystine Lactose Electrolyte-Deficient: electrolyte deficiency prevents Proteus swarming. E. coli: yellow colonies. Used in urine culture to allow all uropathogens to grow |
Medium
Blood Agar
Type
Enriched, differential
Principal Use
General pathogens; observing haemolysis in Streptococci, S. aureus
Colony Appearance / Key Feature
Alpha haemolysis (green partial zone): S. pneumoniae. Beta haemolysis (complete clear zone): S. pyogenes. Gamma (no haemolysis): Enterococci
Medium
MacConkey Agar
Type
Selective, differential
Principal Use
Gram-negative enteric bacteria; separating lactose fermenters from non-fermenters
Colony Appearance / Key Feature
Lactose fermenters: pink/red colonies (E. coli, Klebsiella). Non-fermenters: colourless (Salmonella, Shigella, Pseudomonas). Crystal violet inhibits gram-positives
Medium
Chocolate Agar
Type
Enriched
Principal Use
Fastidious organisms requiring X factor (haematin) and V factor (NAD)
Colony Appearance / Key Feature
Blood heated to 70-80°C releases factors. Used for H. influenzae, N. gonorrhoeae, N. meningitidis. No haemolysis (blood already lysed)
Medium
Lowenstein-Jensen (LJ) Medium
Type
Selective
Principal Use
Mycobacterium tuberculosis isolation
Colony Appearance / Key Feature
Rough, dry, buff-coloured, non-pigmented colonies. Growth in 6-8 weeks (eugonic). Malachite green inhibits other organisms
Medium
Thayer-Martin Medium
Type
Modified chocolate agar, selective
Principal Use
Neisseria gonorrhoeae isolation from clinical specimens
Colony Appearance / Key Feature
Chocolate agar base + antibiotics: Vancomycin (gram-positives), Colistin (gram-negatives), Nystatin (fungi), Trimethoprim (Proteus). Oxidase-positive colonies
Medium
Sabouraud's Dextrose Agar (SDA)
Type
Selective
Principal Use
Fungi (Candida, Aspergillus, dermatophytes)
Colony Appearance / Key Feature
Acidic pH (5.4-5.6) inhibits bacteria. White/cream pasty yeast colonies (Candida). Mould forms branching hyphae. Chloramphenicol added to suppress bacterial contamination
Medium
Mueller-Hinton Agar
Type
Non-selective
Principal Use
Antibiotic sensitivity testing (Kirby-Bauer disc diffusion)
Colony Appearance / Key Feature
Standard medium for antibiotic susceptibility testing (AST). Low thymine/thymidine (avoids false trimethoprim resistance). No specific colony appearance -- used for zone measurement
Medium
TCBS Agar
Type
Selective, differential
Principal Use
Vibrio species (Cholera, food poisoning)
Colony Appearance / Key Feature
V. cholerae: large yellow colonies (sucrose fermenter). V. parahaemolyticus: blue-green colonies (non-sucrose fermenter). Thiosulphate-Citrate-Bile salts-Sucrose composition
Medium
Bordet-Gengou Medium
Type
Enriched, selective
Principal Use
Bordetella pertussis (whooping cough)
Colony Appearance / Key Feature
20-30% blood in glycerol-potato infusion agar. Colonies: round, opaque, grey, glistening -- described as "bisected pearl" or "mercury droplet" appearance
Medium
Loeffler's Serum Slope
Type
Enriched
Principal Use
Initial isolation of Corynebacterium diphtheriae; demonstrating metachromatic granules
Colony Appearance / Key Feature
3 parts animal serum + 1 part nutrient broth. C. diphtheriae grows rapidly (12-18 hours). Smear shows metachromatic granules (Babes-Ernst granules) with Albert's stain
Medium
Tellurite Blood Agar (McLeod's)
Type
Selective
Principal Use
Corynebacterium diphtheriae (primary isolation)
Colony Appearance / Key Feature
Potassium tellurite selects for C. diphtheriae. Black/grey colonies (tellurite reduced to metallic tellurium). Other organisms inhibited
Medium
Salmonella-Shigella (SS) Agar
Type
Highly selective
Principal Use
Salmonella and Shigella from faecal specimens
Colony Appearance / Key Feature
Salmonella: colourless colonies with black centre (H₂S production → FeS). Shigella: colourless, no black centre (no H₂S). Bile salts + brilliant green inhibit coliforms
Medium
Mannitol Salt Agar (MSA)
Type
Selective, differential
Principal Use
Staphylococcus aureus (including MRSA screening)
Colony Appearance / Key Feature
7.5% NaCl selects for Staphylococci. S. aureus: yellow colonies (mannitol fermented → acid → phenol red turns yellow). S. epidermidis: pink/unchanged
Medium
CLED Agar
Type
Non-selective, differential
Principal Use
Urine culture (urinary tract infections)
Colony Appearance / Key Feature
Cystine Lactose Electrolyte-Deficient: electrolyte deficiency prevents Proteus swarming. E. coli: yellow colonies. Used in urine culture to allow all uropathogens to grow
Culture media questions appear in 2nd Year Microbiology theory and viva exams in several standard formats:
Viva examiners frequently pick a single medium and ask its use, colonial appearance, and selective ingredient in one question.
A selective medium contains ingredients that inhibit certain organisms while allowing others to grow (e.g., MacConkey agar uses bile salts and crystal violet to inhibit gram-positive bacteria, selecting for gram-negatives). A differential medium contains indicators that cause different organisms to produce visibly distinct colonies (e.g., MacConkey agar uses neutral red to distinguish lactose fermenters -- pink colonies -- from non-fermenters -- colourless colonies). Many media, including MacConkey, are both selective and differential simultaneously.
Lowenstein-Jensen (LJ) medium is the standard culture medium for M. tuberculosis. It contains eggs, glycerol, asparagine, and malachite green (to inhibit other organisms). M. tuberculosis grows as rough, dry, buff-coloured colonies after 6-8 weeks. This slow growth is due to the organism's waxy cell wall and is a key exam point.
Plain chocolate agar supports growth of N. gonorrhoeae but also allows many other organisms from genital specimens to grow, making isolation difficult. Thayer-Martin medium adds four antibiotics (Vancomycin, Colistin, Nystatin, and Trimethoprim) to chocolate agar, selectively inhibiting most competing organisms while N. gonorrhoeae remains susceptible only to those antibiotics in clinically relevant doses.
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